The Descent of Man and Selection in Relation to Sex
by
Charles Darwin
Part 7 out of 17
pear-tree in my garden produced about fifty larvae of Cic. pruinosa; and I
several times noticed the females to alight near a male while he was
uttering his clanging notes." Fritz Muller writes to me from S. Brazil
that he has often listened to a musical contest between two or three males
of a species with a particularly loud voice, seated at a considerable
distance from each other: as soon as one had finished his song, another
immediately began, and then another. As there is so much rivalry between
the males, it is probable that the females not only find them by their
sounds, but that, like female birds, they are excited or allured by the
male with the most attractive voice.
I have not heard of any well-marked cases of ornamental differences between
the sexes of the Homoptera. Mr. Douglas informs me that there are three
British species, in which the male is black or marked with black bands,
whilst the females are pale-coloured or obscure.
ORDER, ORTHOPTERA (CRICKETS AND GRASSHOPPERS).
The males in the three saltatorial families in this Order are remarkable
for their musical powers, namely the Achetidae or crickets, the Locustidae
for which there is no equivalent English name, and the Acridiidae or
grasshoppers. The stridulation produced by some of the Locustidae is so
loud that it can be heard during the night at the distance of a mile (27.
L. Guilding, 'Transactions of the Linnean Society,' vol. xv. p. 154.); and
that made by certain species is not unmusical even to the human ear, so
that the Indians on the Amazons keep them in wicker cages. All observers
agree that the sounds serve either to call or excite the mute females.
With respect to the migratory locusts of Russia, Korte has given (28. I
state this on the authority of Koppen, 'Uber die Heuschrecken in
Sudrussland,' 1866, p. 32, for I have in vain endeavoured to procure
Korte's work.) an interesting case of selection by the female of a male.
The males of this species (Pachytylus migratorius) whilst coupled with the
female stridulate from anger or jealousy, if approached by other males.
The house-cricket when surprised at night uses its voice to warn its
fellows. (29. Gilbert White, 'Natural History of Selborne,' vol. ii.
1825, p. 262.) In North America the Katy-did (Platyphyllum concavum, one
of the Locustidae) is described (30. Harris, 'Insects of New England,'
1842, p. 128.) as mounting on the upper branches of a tree, and in the
evening beginning "his noisy babble, while rival notes issue from the
neighbouring trees, and the groves resound with the call of Katy-did-she-
did the live-long night." Mr. Bates, in speaking of the European field-
cricket (one of the Achetidae), says "the male has been observed to place
himself in the evening at the entrance of his burrow, and stridulate until
a female approaches, when the louder notes are succeeded by a more subdued
tone, whilst the successful musician caresses with his antennae the mate he
has won." (31. 'The Naturalist on the Amazons,' vol. i. 1863, p. 252. Mr.
Bates gives a very interesting discussion on the gradations in the musical
apparatus of the three families. See also Westwood, 'Modern Classification
of Insects,' vol. ii. pp. 445 and 453.) Dr. Scudder was able to excite one
of these insects to answer him, by rubbing on a file with a quill. (32.
'Proceedings of the Boston Society of Natural History,' vol. xi. April
1868.) In both sexes a remarkable auditory apparatus has been discovered
by Von Siebold, situated in the front legs. (33. 'Nouveau Manuel d'Anat.
Comp.' (French translat.), tom. 1, 1850, p. 567.)
[Fig.11. Gryllus campestris (from Landois).
Right-hand figure, under side of part of a wing-nervure, much magnified,
showing the teeth, st.
Left-hand figure, upper surface of wing-cover, with the projecting, smooth
nervure, r, across which the teeth (st) are scraped.
Fig.12. Teeth of Nervure of Gryllus domesticus (from Landois).]
In the three Families the sounds are differently produced. In the males of
the Achetidae both wing-covers have the same apparatus; and this in the
field-cricket (see Gryllus campestris, Fig. 11) consists, as described by
Landois (34. 'Zeitschrift fur wissenschaft. Zoolog.' B. xvii. 1867, s.
117.), of from 131 to 138 sharp, transverse ridges or teeth (st) on the
under side of one of the nervures of the wing-cover. This toothed nervure
is rapidly scraped across a projecting, smooth, hard nervure (r) on the
upper surface of the opposite wing. First one wing is rubbed over the
other, and then the movement is reversed. Both wings are raised a little
at the same time, so as to increase the resonance. In some species the
wing-covers of the males are furnished at the base with a talc-like plate.
(35. Westwood, 'Modern Classification of Insects,' vol. i. p. 440.) I
here give a drawing (Fig. 12) of the teeth on the under side of the nervure
of another species of Gryllus, viz., G. domesticus. With respect to the
formation of these teeth, Dr. Gruber has shewn (36. 'Ueber der Tonapparat
der Locustiden, ein Beitrag zum Darwinismus,' 'Zeitschrift fur
wissenschaft. Zoolog.' B. xxii. 1872, p. 100.) that they have been
developed by the aid of selection, from the minute scales and hairs with
which the wings and body are covered, and I came to the same conclusion
with respect to those of the Coleoptera. But Dr. Gruber further shews that
their development is in part directly due to the stimulus from the friction
of one wing over the other.
[Fig.13. Chlorocoelus Tanana (from Bates).
a,b. Lobes of opposite wing-covers.]
In the Locustidae the opposite wing-covers differ from each other in
structure (Fig. 13), and the action cannot, as in the last family, be
reversed. The left wing, which acts as the bow, lies over the right wing
which serves as the fiddle. One of the nervures (a) on the under surface
of the former is finely serrated, and is scraped across the prominent
nervures on the upper surface of the opposite or right wing. In our
British Phasgonura viridissima it appeared to me that the serrated nervure
is rubbed against the rounded hind-corner of the opposite wing, the edge of
which is thickened, coloured brown, and very sharp. In the right wing, but
not in the left, there is a little plate, as transparent as talc,
surrounded by nervures, and called the speculum. In Ephippiger vitium, a
member of this same family, we have a curious subordinate modification; for
the wing-covers are greatly reduced in size, but "the posterior part of the
pro-thorax is elevated into a kind of dome over the wing-covers, and which
has probably the effect of increasing the sound." (37. Westwood 'Modern
Classification of Insects,' vol. i. p. 453.)
We thus see that the musical apparatus is more differentiated or
specialised in the Locustidae (which include, I believe, the most powerful
performers in the Order), than in the Achetidae, in which both wing-covers
have the same structure and the same function. (38. Landois, 'Zeitschrift
fur wissenschaft Zoolog.' B. xvii. 1867, ss. 121, 122.) Landois, however,
detected in one of the Locustidae, namely in Decticus, a short and narrow
row of small teeth, mere rudiments, on the inferior surface of the right
wing-cover, which underlies the other and is never used as the bow. I
observed the same rudimentary structure on the under side of the right
wing-cover in Phasgonura viridissima. Hence we may infer with confidence
that the Locustidae are descended from a form, in which, as in the existing
Achetidae, both wing-covers had serrated nervures on the under surface, and
could be indifferently used as the bow; but that in the Locustidae the two
wing-covers gradually became differentiated and perfected, on the principle
of the division of labour, the one to act exclusively as the bow, and the
other as the fiddle. Dr. Gruber takes the same view, and has shewn that
rudimentary teeth are commonly found on the inferior surface of the right
wing. By what steps the more simple apparatus in the Achetidae originated,
we do not know, but it is probable that the basal portions of the wing-
covers originally overlapped each other as they do at present; and that the
friction of the nervures produced a grating sound, as is now the case with
the wing-covers of the females. (39. Mr. Walsh also informs me that he
has noticed that the female of the Platyphyllum concavum, "when captured
makes a feeble grating noise by shuffling her wing-covers together.") A
grating sound thus occasionally and accidentally made by the males, if it
served them ever so little as a love-call to the females, might readily
have been intensified through sexual selection, by variations in the
roughness of the nervures having been continually preserved.
[Fig.14. Hind-leg of Stenobothrus pratorum:
r, the stridulating ridge;
lower figure, the teeth forming the ridge, much magnified (from Landois).
Fig.15. Pneumora (from specimens in the British Museum).
Upper figure, male;
lower figure, female.]
In the last and third family, namely the Acridiidae or grasshoppers, the
stridulation is produced in a very different manner, and according to Dr.
Scudder, is not so shrill as in the preceding Families. The inner surface
of the femur (Fig. 14, r) is furnished with a longitudinal row of minute,
elegant, lancet-shaped, elastic teeth, from 85 to 93 in number (40.
Landois, ibid. s. 113.); and these are scraped across the sharp, projecting
nervures on the wing-covers, which are thus made to vibrate and resound.
Harris (41. 'Insects of New England,' 1842, p. 133.) says that when one of
the males begins to play, he first "bends the shank of the hind-leg beneath
the thigh, where it is lodged in a furrow designed to receive it, and then
draws the leg briskly up and down. He does not play both fiddles together,
but alternately, first upon one and then on the other." In many species,
the base of the abdomen is hollowed out into a great cavity which is
believed to act as a resounding board. In Pneumora (Fig. 15), a S. African
genus belonging to the same family, we meet with a new and remarkable
modification; in the males a small notched ridge projects obliquely from
each side of the abdomen, against which the hind femora are rubbed. (42.
Westwood, 'Modern Classification,' vol i. p. 462.) As the male is
furnished with wings (the female being wingless), it is remarkable that the
thighs are not rubbed in the usual manner against the wing-covers; but this
may perhaps be accounted for by the unusually small size of the hind-legs.
I have not been able to examine the inner surface of the thighs, which,
judging from analogy, would be finely serrated. The species of Pneumora
have been more profoundly modified for the sake of stridulation than any
other orthopterous insect; for in the male the whole body has been
converted into a musical instrument, being distended with air, like a great
pellucid bladder, so as to increase the resonance. Mr. Trimen informs me
that at the Cape of Good Hope these insects make a wonderful noise during
the night.
In the three foregoing families, the females are almost always destitute of
an efficient musical apparatus. But there are a few exceptions to this
rule, for Dr. Gruber has shewn that both sexes of Ephippiger vitium are
thus provided; though the organs differ in the male and female to a certain
extent. Hence we cannot suppose that they have been transferred from the
male to the female, as appears to have been the case with the secondary
sexual characters of many other animals. They must have been independently
developed in the two sexes, which no doubt mutually call to each other
during the season of love. In most other Locustidae (but not according to
Landois in Decticus) the females have rudiments of the stridulatory organs
proper to the male; from whom it is probable that these have been
transferred. Landois also found such rudiments on the under surface of the
wing-covers of the female Achetidae, and on the femora of the female
Acridiidae. In the Homoptera, also, the females have the proper musical
apparatus in a functionless state; and we shall hereafter meet in other
divisions of the animal kingdom with many instances of structures proper to
the male being present in a rudimentary condition of the female.
Landois has observed another important fact, namely, that in the females of
the Acridiidae, the stridulating teeth on the femora remain throughout life
in the same condition in which they first appear during the larval state in
both sexes. In the males, on the other hand, they become further
developed, and acquire their perfect structure at the last moult, when the
insect is mature and ready to breed.
From the facts now given, we see that the means by which the males of the
Orthoptera produce their sounds are extremely diversified, and are
altogether different from those employed by the Homoptera. (43. Landois
has recently found in certain Orthoptera rudimentary structures closely
similar to the sound-producing organs in the Homoptera; and this is a
surprising fact. See 'Zeitschrift fur wissenschaft Zoolog.' B. xxii. Heft
3, 1871, p. 348.) But throughout the animal kingdom we often find the same
object gained by the most diversified means; this seems due to the whole
organisation having undergone multifarious changes in the course of ages,
and as part after part varied different variations were taken advantage of
for the same general purpose. The diversity of means for producing sound
in the three families of the Orthoptera and in the Homoptera, impresses the
mind with the high importance of these structures to the males, for the
sake of calling or alluring the females. We need feel no surprise at the
amount of modification which the Orthoptera have undergone in this respect,
as we now know, from Dr. Scudder's remarkable discovery (44.
'Transactions, Entomological Society,' 3rd series, vol. ii. ('Journal of
Proceedings,' p. 117).), that there has been more than ample time. This
naturalist has lately found a fossil insect in the Devonian formation of
New Brunswick, which is furnished with "the well-known tympanum or
stridulating apparatus of the male Locustidae." The insect, though in most
respects related to the Neuroptera, appears, as is so often the case with
very ancient forms, to connect the two related Orders of the Neuroptera and
Orthoptera.
I have but little more to say on the Orthoptera. Some of the species are
very pugnacious: when two male field-crickets (Gryllus campestris) are
confined together, they fight till one kills the other; and the species of
Mantis are described as manoeuvring with their sword-like front-limbs, like
hussars with their sabres. The Chinese keep these insects in little bamboo
cages, and match them like game-cocks. (45. Westwood, 'Modern
Classification of Insects,' vol. i. p. 427; for crickets, p. 445.) With
respect to colour, some exotic locusts are beautifully ornamented; the
posterior wings being marked with red, blue, and black; but as throughout
the Order the sexes rarely differ much in colour, it is not probable that
they owe their bright tints to sexual selection. Conspicuous colours may
be of use to these insects, by giving notice that they are unpalatable.
Thus it has been observed (46. Mr. Ch. Horne, in 'Proceedings of the
Entomological Society,' May 3, 1869, p. xii.) that a bright-coloured Indian
locust was invariably rejected when offered to birds and lizards. Some
cases, however, are known of sexual differences in colour in this Order.
The male of an American cricket (47. The Oecanthus nivalis, Harris,
'Insects of New England,' 1842, p. 124. The two sexes of OE. pellucidus of
Europe differ, as I hear from Victor Carus, in nearly the same manner.) is
described as being as white as ivory, whilst the female varies from almost
white to greenish-yellow or dusky. Mr. Walsh informs me that the adult
male of Spectrum femoratum (one of the Phasmidae) "is of a shining
brownish-yellow colour; the adult female being of a dull, opaque, cinereous
brown; the young of both sexes being green." Lastly, I may mention that
the male of one curious kind of cricket (48. Platyblemnus: Westwood,
'Modern Classification,' vol. i. p. 447.) is furnished with "a long
membranous appendage, which falls over the face like a veil;" but what its
use may be, is not known.
ORDER, NEUROPTERA.
Little need here be said, except as to colour. In the Ephemeridae the
sexes often differ slightly in their obscure tints (49. B.D. Walsh, the
'Pseudo-neuroptera of Illinois,' in 'Proceedings of the Entomological
Society of Philadelphia,' 1862, p. 361.); but it is not probable that the
males are thus rendered attractive to the females. The Libellulidae, or
dragon-flies, are ornamented with splendid green, blue, yellow, and
vermilion metallic tints; and the sexes often differ. Thus, as Prof.
Westwood remarks (50. 'Modern Classification,' vol. ii. p. 37.), the males
of some of the Agrionidae, "are of a rich blue with black wings, whilst the
females are fine green with colourless wings." But in Agrion Ramburii
these colours are exactly reversed in the two sexes. (51. Walsh, ibid. p.
381. I am indebted to this naturalist for the following facts on
Hetaerina, Anax, and Gomphus.) In the extensive N. American genus of
Hetaerina, the males alone have a beautiful carmine spot at the base of
each wing. In Anax junius the basal part of the abdomen in the male is a
vivid ultramarine blue, and in the female grass-green. In the allied genus
Gomphus, on the other hand, and in some other genera, the sexes differ but
little in colour. In closely-allied forms throughout the animal kingdom,
similar cases of the sexes differing greatly, or very little, or not at
all, are of frequent occurrence. Although there is so wide a difference in
colour between the sexes of many Libellulidae, it is often difficult to say
which is the more brilliant; and the ordinary coloration of the two sexes
is reversed, as we have just seen, in one species of Agrion. It is not
probable that their colours in any case have been gained as a protection.
Mr. MacLachlan, who has closely attended to this family, writes to me that
dragon-flies--the tyrants of the insect-world--are the least liable of any
insect to be attacked by birds or other enemies, and he believes that their
bright colours serve as a sexual attraction. Certain dragon-flies
apparently are attracted by particular colours: Mr. Patterson observed
(52. 'Transactions, Ent. Soc.' vol. i. 1836, p. lxxxi.) that the
Agrionidae, of which the males are blue, settled in numbers on the blue
float of a fishing line; whilst two other species were attracted by shining
white colours.
It is an interesting fact, first noticed by Schelver, that, in several
genera belonging to two sub-families, the males on first emergence from the
pupal state, are coloured exactly like the females; but that their bodies
in a short time assume a conspicuous milky-blue tint, owing to the
exudation of a kind of oil, soluble in ether and alcohol. Mr. MacLachlan
believes that in the male of Libellula depressa this change of colour does
not occur until nearly a fortnight after the metamorphosis, when the sexes
are ready to pair.
Certain species of Neurothemis present, according to Brauer (53. See
abstract in the 'Zoological Record' for 1867, p. 450.), a curious case of
dimorphism, some of the females having ordinary wings, whilst others have
them "very richly netted, as in the males of the same species." Brauer
"explains the phenomenon on Darwinian principles by the supposition that
the close netting of the veins is a secondary sexual character in the
males, which has been abruptly transferred to some of the females, instead
of, as generally occurs, to all of them." Mr. MacLachlan informs me of
another instance of dimorphism in several species of Agrion, in which some
individuals are of an orange colour, and these are invariably females.
This is probably a case of reversion; for in the true Libellulae, when the
sexes differ in colour, the females are orange or yellow; so that supposing
Agrion to be descended from some primordial form which resembled the
typical Libellulae in its sexual characters, it would not be surprising
that a tendency to vary in this manner should occur in the females alone.
Although many dragon-flies are large, powerful, and fierce insects, the
males have not been observed by Mr. MacLachlan to fight together,
excepting, as he believes, in some of the smaller species of Agrion. In
another group in this Order, namely, the Termites or white ants, both sexes
at the time of swarming may be seen running about, "the male after the
female, sometimes two chasing one female, and contending with great
eagerness who shall win the prize." (54. Kirby and Spence, 'Introduction
to Entomology,' vol. ii. 1818, p. 35.) The Atropos pulsatorius is said to
make a noise with its jaws, which is answered by other individuals. (55.
Houzeau, 'Les Facultes Mentales,' etc. Tom. i. p. 104.)
ORDER, HYMENOPTERA.
That inimitable observer, M. Fabre (56. See an interesting article, 'The
Writings of Fabre,' in 'Nat. Hist. Review,' April 1862, p. 122.), in
describing the habits of Cerceris, a wasp-like insect, remarks that "fights
frequently ensue between the males for the possession of some particular
female, who sits an apparently unconcerned beholder of the struggle for
supremacy, and when the victory is decided, quietly flies away in company
with the conqueror." Westwood (57. 'Journal of Proceedings of
Entomological Society,' Sept. 7, 1863, p. 169.) says that the males of one
of the saw-flies (Tenthredinae) "have been found fighting together, with
their mandibles locked." As M. Fabre speaks of the males of Cerceris
striving to obtain a particular female, it may be well to bear in mind that
insects belonging to this Order have the power of recognising each other
after long intervals of time, and are deeply attached. For instance,
Pierre Huber, whose accuracy no one doubts, separated some ants, and when,
after an interval of four months, they met others which had formerly
belonged to the same community, they recognised and caressed one another
with their antennae. Had they been strangers they would have fought
together. Again, when two communities engage in a battle, the ants on the
same side sometimes attack each other in the general confusion, but they
soon perceive their mistake, and the one ant soothes the other. (58. P.
Huber, 'Recherches sur les Moeurs des Fourmis,' 1810, pp. 150, 165.)
In this Order slight differences in colour, according to sex, are common,
but conspicuous differences are rare except in the family of Bees; yet both
sexes of certain groups are so brilliantly coloured--for instance in
Chrysis, in which vermilion and metallic greens prevail--that we are
tempted to attribute the result to sexual selection. In the Ichneumonidae,
according to Mr. Walsh (59. 'Proceedings of the Entomological Society of
Philadelphia,' 1866, pp. 238, 239.), the males are almost universally
lighter-coloured than the females. On the other hand, in the
Tenthredinidae the males are generally darker than the females. In the
Siricidae the sexes frequently differ; thus the male of Sirex juvencus is
banded with orange, whilst the female is dark purple; but it is difficult
to say which sex is the more ornamented. In Tremex columbae the female is
much brighter coloured than the male. I am informed by Mr. F. Smith, that
the male ants of several species are black, the females being testaceous.
In the family of Bees, especially in the solitary species, as I hear from
the same entomologist, the sexes often differ in colour. The males are
generally the brighter, and in Bombus as well as in Apathus, much more
variable in colour than the females. In Anthophora retusa the male is of a
rich fulvous-brown, whilst the female is quite black: so are the females
of several species of Xylocopa, the males being bright yellow. On the
other hand the females of some species, as of Andraena fulva, are much
brighter coloured than the males. Such differences in colour can hardly be
accounted for by the males being defenceless and thus requiring protection,
whilst the females are well defended by their stings. H. Muller (60.
'Anwendung der Darwinschen Lehre auf Bienen,' Verh. d. n. V. Jahrg. xxix.),
who has particularly attended to the habits of bees, attributes these
differences in colour in chief part to sexual selection. That bees have a
keen perception of colour is certain. He says that the males search
eagerly and fight for the possession of the females; and he accounts
through such contests for the mandibles of the males being in certain
species larger than those of the females. In some cases the males are far
more numerous than the females, either early in the season, or at all times
and places, or locally; whereas the females in other cases are apparently
in excess. In some species the more beautiful males appear to have been
selected by the females; and in others the more beautiful females by the
males. Consequently in certain genera (Muller, p. 42), the males of the
several species differ much in appearance, whilst the females are almost
indistinguishable; in other genera the reverse occurs. H. Muller believes
(p. 82) that the colours gained by one sex through sexual selection have
often been transferred in a variable degree to the other sex, just as the
pollen-collecting apparatus of the female has often been transferred to the
male, to whom it is absolutely useless. (61. M. Perrier in his article
'la Selection sexuelle d'apres Darwin' ('Revue Scientifique,' Feb. 1873, p.
868), without apparently having reflected much on the subject, objects that
as the males of social bees are known to be produced from unfertilised ova,
they could not transmit new characters to their male offspring. This is an
extraordinary objection. A female bee fertilised by a male, which
presented some character facilitating the union of the sexes, or rendering
him more attractive to the female, would lay eggs which would produce only
females; but these young females would next year produce males; and will it
be pretended that such males would not inherit the characters of their male
grandfathers? To take a case with ordinary animals as nearly parallel as
possible: if a female of any white quadruped or bird were crossed by a
male of a black breed, and the male and female offspring were paired
together, will it be pretended that the grandchildren would not inherit a
tendency to blackness from their male grandfather? The acquirement of new
characters by the sterile worker-bees is a much more difficult case, but I
have endeavoured to shew in my 'Origin of Species,' how these sterile
beings are subjected to the power of natural selection.)
Mutilla Europaea makes a stridulating noise; and according to Goureau (62.
Quoted by Westwood, 'Modern Classification of Insects,' vol. ii. p. 214.)
both sexes have this power. He attributes the sound to the friction of the
third and preceding abdominal segments, and I find that these surfaces are
marked with very fine concentric ridges; but so is the projecting thoracic
collar into which the head articulates, and this collar, when scratched
with the point of a needle, emits the proper sound. It is rather
surprising that both sexes should have the power of stridulating, as the
male is winged and the female wingless. It is notorious that Bees express
certain emotions, as of anger, by the tone of their humming; and according
to H. Muller (p. 80), the males of some species make a peculiar singing
noise whilst pursuing the females.
ORDER, COLEOPTERA (BEETLES).
Many beetles are coloured so as to resemble the surfaces which they
habitually frequent, and they thus escape detection by their enemies.
Other species, for instance diamond-beetles, are ornamented with splendid
colours, which are often arranged in stripes, spots, crosses, and other
elegant patterns. Such colours can hardly serve directly as a protection,
except in the case of certain flower-feeding species; but they may serve as
a warning or means of recognition, on the same principle as the
phosphorescence of the glow-worm. As with beetles the colours of the two
sexes are generally alike, we have no evidence that they have been gained
through sexual selection; but this is at least possible, for they have been
developed in one sex and then transferred to the other; and this view is
even in some degree probable in those groups which possess other well-
marked secondary sexual characters. Blind beetles, which cannot of course
behold each other's beauty, never, as I hear from Mr. Waterhouse, jun.,
exhibit bright colours, though they often have polished coats; but the
explanation of their obscurity may be that they generally inhabit caves and
other obscure stations.
Some Longicorns, especially certain Prionidae, offer an exception to the
rule that the sexes of beetles do not differ in colour. Most of these
insects are large and splendidly coloured. The males in the genus Pyrodes
(63. Pyrodes pulcherrimus, in which the sexes differ conspicuously, has
been described by Mr. Bates in 'Transact. Ent. Soc.' 1869, p. 50. I will
specify the few other cases in which I have heard of a difference in colour
between the sexes of beetles. Kirby and Spence ('Introduct. to
Entomology,' vol. iii. p. 301) mention a Cantharis, Meloe, Rhagium, and the
Leptura testacea; the male of the latter being testaceous, with a black
thorax, and the female of a dull red all over. These two latter beetles
belong to the family of Longicorns. Messrs. R. Trimen and Waterhouse,
jun., inform me of two Lamellicorns, viz., a Peritrichia and Trichius, the
male of the latter being more obscurely coloured than the female. In
Tillus elongatus the male is black, and the female always, as it is
believed, of a dark blue colour, with a red thorax. The male, also, of
Orsodacna atra, as I hear from Mr. Walsh, is black, the female (the so-
called O. ruficollis) having a rufous thorax.), which I saw in Mr. Bates's
collection, are generally redder but rather duller than the females, the
latter being coloured of a more or less splendid golden-green. On the
other hand, in one species the male is golden-green, the female being
richly tinted with red and purple. In the genus Esmeralda the sexes differ
so greatly in colour that they have been ranked as distinct species; in one
species both are of a beautiful shining green, but the male has a red
thorax. On the whole, as far as I could judge, the females of those
Prionidae, in which the sexes differ, are coloured more richly than the
males, and this does not accord with the common rule in regard to colour,
when acquired through sexual selection.
[Fig.16. Chalcosoma atlas.
Upper figure, male (reduced);
lower figure, female (nat. size).
Fig. 17. Copris isidis.
Fig. 18. Phanaeus faunus.
Fig. 19. Dipelicus cantori.
Fig. 20. Onthophagus rangifer, enlarged.
(In Figs. 17 to 20 the left-hand figures are males.)]
A most remarkable distinction between the sexes of many beetles is
presented by the great horns which rise from the head, thorax, and clypeus
of the males; and in some few cases from the under surface of the body.
These horns, in the great family of the Lamellicorns, resemble those of
various quadrupeds, such as stags, rhinoceroses, etc., and are wonderful
both from their size and diversified shapes. Instead of describing them, I
have given figures of the males and females of some of the more remarkable
forms. (Figs. 16 to 20.) The females generally exhibit rudiments of the
horns in the form of small knobs or ridges; but some are destitute of even
the slightest rudiment. On the other hand, the horns are nearly as well
developed in the female as in the male Phanaeus lancifer; and only a little
less well developed in the females of some other species of this genus and
of Copris. I am informed by Mr. Bates that the horns do not differ in any
manner corresponding with the more important characteristic differences
between the several subdivisions of the family: thus within the same
section of the genus Onthophagus, there are species which have a single
horn, and others which have two.
In almost all cases, the horns are remarkable from their excessive
variability; so that a graduated series can be formed, from the most highly
developed males to others so degenerate that they can barely be
distinguished from the females. Mr. Walsh (64. 'Proceedings of the
Entomological Society of Philadephia,' 1864, p. 228.) found that in
Phanaeus carnifex the horns were thrice as long in some males as in others.
Mr. Bates, after examining above a hundred males of Onthophagus rangifer
(Fig. 20), thought that he had at last discovered a species in which the
horns did not vary; but further research proved the contrary.
The extraordinary size of the horns, and their widely different structure
in closely-allied forms, indicate that they have been formed for some
purpose; but their excessive variability in the males of the same species
leads to the inference that this purpose cannot be of a definite nature.
The horns do not shew marks of friction, as if used for any ordinary work.
Some authors suppose (65. Kirby and Spence, 'Introduction to Entomology,'
vol. iii. P. 300.) that as the males wander about much more than the
females, they require horns as a defence against their enemies; but as the
horns are often blunt, they do not seem well adapted for defence. The most
obvious conjecture is that they are used by the males for fighting
together; but the males have never been observed to fight; nor could Mr.
Bates, after a careful examination of numerous species, find any sufficient
evidence, in their mutilated or broken condition, of their having been thus
used. If the males had been habitual fighters, the size of their bodies
would probably have been increased through sexual selection, so as to have
exceeded that of the females; but Mr. Bates, after comparing the two sexes
in above a hundred species of the Copridae, did not find any marked
difference in this respect amongst well-developed individuals. In Lethrus,
moreover, a beetle belonging to the same great division of the
Lamellicorns, the males are known to fight, but are not provided with
horns, though their mandibles are much larger than those of the female.
The conclusion that the horns have been acquired as ornaments is that which
best agrees with the fact of their having been so immensely, yet not
fixedly, developed,--as shewn by their extreme variability in the same
species, and by their extreme diversity in closely-allied species. This
view will at first appear extremely improbable; but we shall hereafter find
with many animals standing much higher in the scale, namely fishes,
amphibians, reptiles and birds, that various kinds of crests, knobs, horns
and combs have been developed apparently for this sole purpose.
[Fig.21. Onitis furcifer, male viewed from beneath.
Fig.22. Onitis furcifer.
Left-hand figure, male, viewed laterally.
Right-hand figure, female.
a. Rudiment of cephalic horn.
b. Trace of thoracic horn or crest.]
The males of Onitis furcifer (Fig. 21), and of some other species of the
genus, are furnished with singular projections on their anterior femora,
and with a great fork or pair of horns on the lower surface of the thorax.
Judging from other insects, these may aid the male in clinging to the
female. Although the males have not even a trace of a horn on the upper
surface of the body, yet the females plainly exhibit a rudiment of a single
horn on the head (Fig. 22, a), and of a crest (b) on the thorax. That the
slight thoracic crest in the female is a rudiment of a projection proper to
the male, though entirely absent in the male of this particular species, is
clear: for the female of Bubas bison (a genus which comes next to Onitis)
has a similar slight crest on the thorax, and the male bears a great
projection in the same situation. So, again, there can hardly be a doubt
that the little point (a) on the head of the female Onitis furcifer, as
well as on the head of the females of two or three allied species, is a
rudimentary representative of the cephalic horn, which is common to the
males of so many Lamellicorn beetles, as in Phanaeus (Fig. 18).
The old belief that rudiments have been created to complete the scheme of
nature is here so far from holding good, that we have a complete inversion
of the ordinary state of things in the family. We may reasonably suspect
that the males originally bore horns and transferred them to the females in
a rudimentary condition, as in so many other Lamellicorns. Why the males
subsequently lost their horns, we know not; but this may have been caused
through the principle of compensation, owing to the development of the
large horns and projections on the lower surface; and as these are confined
to the males, the rudiments of the upper horns on the females would not
have been thus obliterated.
[Fig. 23. Bledius taurus, magnified.
Left-hand figure, male;
right-hand figure, female.]
The cases hitherto given refer to the Lamellicorns, but the males of some
few other beetles, belonging to two widely distinct groups, namely, the
Curculionidae and Staphylinidae, are furnished with horns--in the former on
the lower surface of the body (66. Kirby and Spence, 'Introduction to
Entomology,' vol. iii. p. 329.), in the latter on the upper surface of the
head and thorax. In the Staphylinidae, the horns of the males are
extraordinarily variable in the same species, just as we have seen with the
Lamellicorns. In Siagonium we have a case of dimorphism, for the males can
be divided into two sets, differing greatly in the size of their bodies and
in the development of their horns, without intermediate gradations. In a
species of Bledius (Fig. 23), also belonging to the Staphylinidae,
Professor Westwood states that, "male specimens can be found in the same
locality in which the central horn of the thorax is very large, but the
horns of the head quite rudimental; and others, in which the thoracic horn
is much shorter, whilst the protuberances on the head are long." (67.
'Modern Classification of Insects,' vol. i. p. 172: Siagonium, p. 172. In
the British Museum I noticed one male specimen of Siagonium in an
intermediate condition, so that the dimorphism is not strict.) Here we
apparently have a case of compensation, which throws light on that just
given, of the supposed loss of the upper horns by the males of Onitis.
LAW OF BATTLE.
Some male beetles, which seem ill-fitted for fighting, nevertheless engage
in conflicts for the possession of the females. Mr. Wallace (68. 'The
Malay Archipelago,' vol. ii. 1869, p. 276. Riley, Sixth 'Report on Insects
of Missouri,' 1874, p. 115.) saw two males of Leptorhynchus angustatus, a
linear beetle with a much elongated rostrum, "fighting for a female, who
stood close by busy at her boring. They pushed at each other with their
rostra, and clawed and thumped, apparently in the greatest rage." The
smaller male, however, "soon ran away, acknowledging himself vanquished."
In some few cases male beetles are well adapted for fighting, by possessing
great toothed mandibles, much larger than those of the females. This is
the case with the common stag-beetle (Lucanus cervus), the males of which
emerge from the pupal state about a week before the other sex, so that
several may often be seen pursuing the same female. At this season they
engage in fierce conflicts. When Mr. A.H. Davis (69. 'Entomological
Magazine,' vol. i. 1833, p. 82. See also on the conflicts of this species,
Kirby and Spence, ibid. vol. iii. p. 314; and Westwood, ibid. vol. i. p.
187.) enclosed two males with one female in a box, the larger male severely
pinched the smaller one, until he resigned his pretensions. A friend
informs me that when a boy he often put the males together to see them
fight, and he noticed that they were much bolder and fiercer than the
females, as with the higher animals. The males would seize hold of his
finger, if held in front of them, but not so the females, although they
have stronger jaws. The males of many of the Lucanidae, as well as of the
above-mentioned Leptorhynchus, are larger and more powerful insects than
the females. The two sexes of Lethrus cephalotes (one of the Lamellicorns)
inhabit the same burrow; and the male has larger mandibles than the female.
If, during the breeding-season, a strange male attempts to enter the
burrow, he is attacked; the female does not remain passive, but closes the
mouth of the burrow, and encourages her mate by continually pushing him on
from behind; and the battle lasts until the aggressor is killed or runs
away. (70. Quoted from Fischer, in 'Dict. Class. d'Hist. Nat.' tom. x. p.
324.) The two sexes of another Lamellicorn beetle, the Ateuchus
cicatricosus, live in pairs, and seem much attached to each other; the male
excites the females to roll the balls of dung in which the ova are
deposited; and if she is removed, he becomes much agitated. If the male is
removed the female ceases all work, and as M. Brulerie believes, would
remain on the same spot until she died. (71. 'Ann. Soc. Entomolog.
France,' 1866, as quoted in 'Journal of Travel,' by A. Murray, 1868, p.
135.)
[Fig. 24. Chiasognathus Grantii, reduced.
Upper figure, male;
lower figure, female.]
The great mandibles of the male Lucanidae are extremely variable both in
size and structure, and in this respect resemble the horns on the head and
thorax of many male Lamellicorns and Staphylinidae. A perfect series can
be formed from the best-provided to the worst-provided or degenerate males.
Although the mandibles of the common stag-beetle, and probably of many
other species, are used as efficient weapons for fighting, it is doubtful
whether their great size can thus be accounted for. We have seen that they
are used by the Lucanus elaphus of N. America for seizing the female. As
they are so conspicuous and so elegantly branched, and as owing to their
great length they are not well adapted for pinching, the suspicion has
crossed my mind that they may in addition serve as an ornament, like the
horns on the head and thorax of the various species above described. The
male Chiasognathus grantii of S. Chile--a splendid beetle belonging to the
same family--has enormously developed mandibles (Fig. 24); he is bold and
pugnacious; when threatened he faces round, opens his great jaws, and at
the same time stridulates loudly. But the mandibles were not strong enough
to pinch my finger so as to cause actual pain.
Sexual selection, which implies the possession of considerable perceptive
powers and of strong passions, seems to have been more effective with the
Lamellicorns than with any other family of beetles. With some species the
males are provided with weapons for fighting; some live in pairs and shew
mutual affection; many have the power of stridulating when excited; many
are furnished with the most extraordinary horns, apparently for the sake of
ornament; and some, which are diurnal in their habits, are gorgeously
coloured. Lastly, several of the largest beetles in the world belong to
this family, which was placed by Linnaeus and Fabricius as the head of the
Order. (72. Westwood, 'Modern Classification,' vol. i. p. 184.)
STRIDULATING ORGANS.
Beetles belonging to many and widely distinct families possess these
organs. The sound thus produced can sometimes be heard at the distance of
several feet or even yards (73. Wollaston, 'On Certain Musical
Curculionidae,' 'Annals and Mag. of Nat. Hist.' vol. vi. 1860, p. 14.), but
it is not comparable with that made by the Orthoptera. The rasp generally
consists of a narrow, slightly-raised surface, crossed by very fine,
parallel ribs, sometimes so fine as to cause iridescent colours, and having
a very elegant appearance under the microscope. In some cases, as with
Typhoeus, minute, bristly or scale-like prominences, with which the whole
surrounding surface is covered in approximately parallel lines, could be
traced passing into the ribs of the rasp. The transition takes place by
their becoming confluent and straight, and at the same time more prominent
and smooth. A hard ridge on an adjoining part of the body serves as the
scraper for the rasp, but this scraper in some cases has been specially
modified for the purpose. It is rapidly moved across the rasp, or
conversely the rasp across the scraper.
[Fig.25. Necrophorus (from Landois).
r. The two rasps.
Left-hand figure, part of the rasp highly magnified.]
These organs are situated in widely different positions. In the carrion-
beetles (Necrophorus) two parallel rasps (r, Fig. 25) stand on the dorsal
surface of the fifth abdominal segment, each rasp (74. Landois,
'Zeitschrift fur wissenschaft Zoolog.' B. xvii. 1867, s. 127.) consisting
of 126 to 140 fine ribs. These ribs are scraped against the posterior
margins of the elytra, a small portion of which projects beyond the general
outline. In many Crioceridae, and in Clythra 4-punctata (one of the
Chrysomelidae), and in some Tenebrionidae, etc. (75. I am greatly indebted
to Mr. G.R. Crotch for having sent me many prepared specimens of various
beetles belonging to these three families and to others, as well as for
valuable information. He believes that the power of stridulation in the
Clythra has not been previously observed. I am also much indebted to Mr.
E.W. Janson, for information and specimens. I may add that my son, Mr. F.
Darwin, finds that Dermestes murinus stridulates, but he searched in vain
for the apparatus. Scolytus has lately been described by Dr. Chapman as a
stridulator, in the 'Entomologist's Monthly Magazine,' vol. vi. p. 130.),
the rasp is seated on the dorsal apex of the abdomen, on the pygidium or
pro-pygidium, and is scraped in the same manner by the elytra. In
Heterocerus, which belongs to another family, the rasps are placed on the
sides of the first abdominal segment, and are scraped by ridges on the
femora. (76. Schiodte, translated, in 'Annals and Magazine of Natural
History,' vol. xx. 1867, p. 37.) In certain Curculionidae and Carabidae
(77. Westring has described (Kroyer, 'Naturhist. Tidskrift,' B. ii. 1848-
49, p. 334) the stridulating organs in these two, as well as in other
families. In the Carabidae I have examined Elaphrus uliginosus and
Blethisa multipunctata, sent to me by Mr. Crotch. In Blethisa the
transverse ridges on the furrowed border of the abdominal segment do not,
as far as I could judge, come into play in scraping the rasps on the
elytra.), the parts are completely reversed in position, for the rasps are
seated on the inferior surface of the elytra, near their apices, or along
their outer margins, and the edges of the abdominal segments serve as the
scrapers. In Pelobius Hermanni (one of Dytiscidae or water-beetles) a
strong ridge runs parallel and near to the sutural margin of the elytra,
and is crossed by ribs, coarse in the middle part, but becoming gradually
finer at both ends, especially at the upper end; when this insect is held
under water or in the air, a stridulating noise is produced by the extreme
horny margin of the abdomen being scraped against the rasps. In a great
number of long-horned beetles (Longicornia) the organs are situated quite
otherwise, the rasp being on the meso-thorax, which is rubbed against the
pro-thorax; Landois counted 238 very fine ribs on the rasp of Cerambyx
heros.
[Fig.26. Hind-leg of Geotrupes stercorarius (from Landois).
r. Rasp. c. Coxa. f. Femur. t. Tibia. tr. Tarsi.]
Many Lamellicorns have the power of stridulating, and the organs differ
greatly in position. Some species stridulate very loudly, so that when Mr.
F. Smith caught a Trox sabulosus, a gamekeeper, who stood by, thought he
had caught a mouse; but I failed to discover the proper organs in this
beetle. In Geotrupes and Typhoeus, a narrow ridge runs obliquely across
(r, Fig. 26) the coxa of each hind-leg (having in G. stercorarius 84 ribs),
which is scraped by a specially projecting part of one of the abdominal
segments. In the nearly allied Copris lunaris, an excessively narrow fine
rasp runs along the sutural margin of the elytra, with another short rasp
near the basal outer margin; but in some other Coprini the rasp is seated,
according to Leconte (78. I am indebted to Mr. Walsh, of Illinois, for
having sent me extracts from Leconte's 'Introduction to Entomology,' pp.
101, 143.), on the dorsal surface of the abdomen. In Oryctes it is seated
on the pro-pygidium; and, according to the same entomologist, in some other
Dynastini, on the under surface of the elytra. Lastly, Westring states
that in Omaloplia brunnea the rasp is placed on the pro-sternum, and the
scraper on the meta-sternum, the parts thus occupying the under surface of
the body, instead of the upper surface as in the Longicorns.
We thus see that in the different coleopterous families the stridulating
organs are wonderfully diversified in position, but not much in structure.
Within the same family some species are provided with these organs, and
others are destitute of them. This diversity is intelligible, if we
suppose that originally various beetles made a shuffling or hissing noise
by the rubbing together of any hard and rough parts of their bodies, which
happened to be in contact; and that from the noise thus produced being in
some way useful, the rough surfaces were gradually developed into regular
stridulating organs. Some beetles as they move, now produce, either
intentionally or unintentionally, a shuffling noise, without possessing any
proper organs for the purpose. Mr. Wallace informs me that the Euchirus
longimanus (a Lamellicorn, with the anterior legs wonderfully elongated in
the male) "makes, whilst moving, a low hissing sound by the protrusion and
contraction of the abdomen; and when seized it produces a grating sound by
rubbing its hind-legs against the edges of the elytra." The hissing sound
is clearly due to a narrow rasp running along the sutural margin of each
elytron; and I could likewise make the grating sound by rubbing the
shagreened surface of the femur against the granulated margin of the
corresponding elytron; but I could not here detect any proper rasp; nor is
it likely that I could have overlooked it in so large an insect. After
examining Cychrus, and reading what Westring has written about this beetle,
it seems very doubtful whether it possesses any true rasp, though it has
the power of emitting a sound.
From the analogy of the Orthoptera and Homoptera, I expected to find the
stridulating organs in the Coleoptera differing according to sex; but
Landois, who has carefully examined several species, observed no such
difference; nor did Westring; nor did Mr. G.R. Crotch in preparing the many
specimens which he had the kindness to send me. Any difference in these
organs, if slight, would, however, be difficult to detect, on account of
their great variability. Thus, in the first pair of specimens of
Necrophorus humator and of Pelobius which I examined, the rasp was
considerably larger in the male than in the female; but not so with
succeeding specimens. In Geotrupes stercorarius the rasp appeared to me
thicker, opaquer, and more prominent in three males than in the same number
of females; in order, therefore, to discover whether the sexes differed in
their power of stridulating, my son, Mr. F. Darwin, collected fifty-seven
living specimens, which he separated into two lots, according as they made
a greater or lesser noise, when held in the same manner. He then examined
all these specimens, and found that the males were very nearly in the same
proportion to the females in both the lots. Mr. F. Smith has kept alive
numerous specimens of Monoynchus pseudacori (Curculionidae), and is
convinced that both sexes stridulate, and apparently in an equal degree.
Nevertheless, the power of stridulating is certainly a sexual character in
some few Coleoptera. Mr. Crotch discovered that the males alone of two
species of Heliopathes (Tenebrionidae) possess stridulating organs. I
examined five males of H. gibbus, and in all these there was a well-
developed rasp, partially divided into two, on the dorsal surface of the
terminal abdominal segment; whilst in the same number of females there was
not even a rudiment of the rasp, the membrane of this segment being
transparent, and much thinner than in the male. In H. cribratostriatus the
male has a similar rasp, excepting that it is not partially divided into
two portions, and the female is completely destitute of this organ; the
male in addition has on the apical margins of the elytra, on each side of
the suture, three or four short longitudinal ridges, which are crossed by
extremely fine ribs, parallel to and resembling those on the abdominal
rasp; whether these ridges serve as an independent rasp, or as a scraper
for the abdominal rasp, I could not decide: the female exhibits no trace
of this latter structure.
Again, in three species of the Lamellicorn genus Oryctes, we have a nearly
parallel case. In the females of O. gryphus and nasicornis the ribs on the
rasp of the pro-pygidium are less continuous and less distinct than in the
males; but the chief difference is that the whole upper surface of this
segment, when held in the proper light, is seen to be clothed with hairs,
which are absent or are represented by excessively fine down in the males.
It should be noticed that in all Coleoptera the effective part of the rasp
is destitute of hairs. In O. senegalensis the difference between the sexes
is more strongly marked, and this is best seen when the proper abdominal
segment is cleaned and viewed as a transparent object. In the female the
whole surface is covered with little separate crests, bearing spines;
whilst in the male these crests in proceeding towards the apex, become more
and more confluent, regular, and naked; so that three-fourths of the
segment is covered with extremely fine parallel ribs, which are quite
absent in the female. In the females, however, of all three species of
Oryctes, a slight grating or stridulating sound is produced, when the
abdomen of a softened specimen is pushed backwards and forwards.
In the case of the Heliopathes and Oryctes there can hardly be a doubt that
the males stridulate in order to call or to excite the females; but with
most beetles the stridulation apparently serves both sexes as a mutual
call. Beetles stridulate under various emotions, in the same manner as
birds use their voices for many purposes besides singing to their mates.
The great Chiasognathus stridulates in anger or defiance; many species do
the same from distress or fear, if held so that they cannot escape; by
striking the hollow stems of trees in the Canary Islands, Messrs. Wollaston
and Crotch were able to discover the presence of beetles belonging to the
genus Acalles by their stridulation. Lastly, the male Ateuchus stridulates
to encourage the female in her work, and from distress when she is removed.
(79. M. P. de la Brulerie, as quoted in 'Journal of Travel,' A. Murray,
vol. i. 1868, p. 135.) Some naturalists believe that beetles make this
noise to frighten away their enemies; but I cannot think that a quadruped
or bird, able to devour a large beetle, would be frightened by so slight a
sound. The belief that the stridulation serves as a sexual call is
supported by the fact that death-ticks (Anobium tessellatum) are well known
to answer each other's ticking, and, as I have myself observed, a tapping
noise artificially made. Mr. Doubleday also informs me that he has
sometimes observed a female ticking (80. According to Mr. Doubleday, "the
noise is produced by the insect raising itself on its legs as high as it
can, and then striking its thorax five or six times, in rapid succession,
against the substance upon which it is sitting." For references on this
subject see Landois, 'Zeitschrift fur wissen. Zoolog.' B. xvii. s. 131.
Olivier says (as quoted by Kirby and Spence, 'Introduction to Entomology,'
vol. ii. p. 395) that the female of Pimelia striata produces a rather loud
sound by striking her abdomen against any hard substance, "and that the
male, obedient to this call, soon attends her, and they pair."), and in an
hour or two afterwards has found her united with a male, and on one
occasion surrounded by several males. Finally, it is probable that the two
sexes of many kinds of beetles were at first enabled to find each other by
the slight shuffling noise produced by the rubbing together of the
adjoining hard parts of their bodies; and that as those males or females
which made the greatest noise succeeded best in finding partners,
rugosities on various parts of their bodies were gradually developed by
means of sexual selection into true stridulating organs.
CHAPTER XI.
INSECTS, continued.
ORDER LEPIDOPTERA. (BUTTERFLIES AND MOTHS.)
Courtship of butterflies--Battles--Ticking noise--Colours common to both
sexes, or more brilliant in the males--Examples--Not due to the direct
action of the conditions of life--Colours adapted for protection--Colours
of moths--Display--Perceptive powers of the Lepidoptera--Variability--
Causes of the difference in colour between the males and females--Mimicry,
female butterflies more brilliantly coloured than the males--Bright colours
of caterpillars--Summary and concluding remarks on the secondary sexual
characters of insects--Birds and insects compared.
In this great Order the most interesting points for us are the differences
in colour between the sexes of the same species, and between the distinct
species of the same genus. Nearly the whole of the following chapter will
be devoted to this subject; but I will first make a few remarks on one or
two other points. Several males may often be seen pursuing and crowding
round the same female. Their courtship appears to be a prolonged affair,
for I have frequently watched one or more males pirouetting round a female
until I was tired, without seeing the end of the courtship. Mr. A.G.
Butler also informs me that he has several times watched a male courting a
female for a full quarter of an hour; but she pertinaciously refused him,
and at last settled on the ground and closed her wings, so as to escape
from his addresses.
Although butterflies are weak and fragile creatures, they are pugnacious,
and an emperor butterfly (1. Apatura Iris: 'The Entomologist's Weekly
Intelligence,' 1859, p. 139. For the Bornean Butterflies, see C.
Collingwood, 'Rambles of a Naturalist,' 1868, p. 183.) has been captured
with the tips of its wings broken from a conflict with another male. Mr.
Collingwood, in speaking of the frequent battles between the butterflies of
Borneo, says, "They whirl round each other with the greatest rapidity, and
appear to be incited by the greatest ferocity."
The Ageronia feronia makes a noise like that produced by a toothed wheel
passing under a spring catch, and which can be heard at the distance of
several yards: I noticed this sound at Rio de Janeiro, only when two of
these butterflies were chasing each other in an irregular course, so that
it is probably made during the courtship of the sexes. (2. See my
'Journal of Researches,' 1845, p. 33. Mr. Doubleday has detected ('Proc.
Ent. Soc.' March 3, 1845, p. 123) a peculiar membranous sac at the base of
the front wings, which is probably connected with the production of the
sound. For the case of Thecophora, see 'Zoological Record,' 1869, p. 401.
For Mr. Buchanan White's observations, the Scottish Naturalist, July 1872,
p. 214.)
Some moths also produce sounds; for instance, the males Theocophora fovea.
On two occasions Mr. F. Buchanan White (3. 'The Scottish Naturalist,' July
1872, p. 213.) heard a sharp quick noise made by the male of Hylophila
prasinana, and which he believes to be produced, as in Cicada, by an
elastic membrane, furnished with a muscle. He quotes, also, Guenee, that
Setina produces a sound like the ticking of a watch, apparently by the aid
of "two large tympaniform vesicles, situated in the pectoral region"; and
these "are much more developed in the male than in the female." Hence the
sound-producing organs in the Lepidoptera appear to stand in some relation
with the sexual functions. I have not alluded to the well-known noise made
by the Death's Head Sphinx, for it is generally heard soon after the moth
has emerged from its cocoon.
Giard has always observed that the musky odour, which is emitted by two
species of Sphinx moths, is peculiar to the males (4. 'Zoological Record,'
1869, p. 347.); and in the higher classes we shall meet with many instances
of the males alone being odoriferous.
Every one must have admired the extreme beauty of many butterflies and of
some moths; and it may be asked, are their colours and diversified patterns
the result of the direct action of the physical conditions to which these
insects have been exposed, without any benefit being thus derived? Or have
successive variations been accumulated and determined as a protection, or
for some unknown purpose, or that one sex may be attractive to the other?
And, again, what is the meaning of the colours being widely different in
the males and females of certain species, and alike in the two sexes of
other species of the same genus? Before attempting to answer these
questions a body of facts must be given.
With our beautiful English butterflies, the admiral, peacock, and painted
lady (Vanessae), as well as many others, the sexes are alike. This is also
the case with the magnificent Heliconidae, and most of the Danaidae in the
tropics. But in certain other tropical groups, and in some of our English
butterflies, as the purple emperor, orange-tip, etc. (Apatura Iris and
Anthocharis cardamines), the sexes differ either greatly or slightly in
colour. No language suffices to describe the splendour of the males of
some tropical species. Even within the same genus we often find species
presenting extraordinary differences between the sexes, whilst others have
their sexes closely alike. Thus in the South American genus Epicalia, Mr.
Bates, to whom I am indebted for most of the following facts, and for
looking over this whole discussion, informs me that he knows twelve
species, the two sexes of which haunt the same stations (and this is not
always the case with butterflies), and which, therefore, cannot have been
differently affected by external conditions. (5. See also Mr. Bates's
paper in 'Proc. Ent. Soc. of Philadelphia,' 1865, p. 206. Also Mr. Wallace
on the same subject, in regard to Diadema, in 'Transactions, Entomological
Society of London,' 1869, p. 278.) In nine of these twelve species the
males rank amongst the most brilliant of all butterflies, and differ so
greatly from the comparatively plain females that they were formerly placed
in distinct genera. The females of these nine species resemble each other
in their general type of coloration; and they likewise resemble both sexes
of the species in several allied genera found in various parts of the
world. Hence we may infer that these nine species, and probably all the
others of the genus, are descended from an ancestral form which was
coloured in nearly the same manner. In the tenth species the female still
retains the same general colouring, but the male resembles her, so that he
is coloured in a much less gaudy and contrasted manner than the males of
the previous species. In the eleventh and twelfth species, the females
depart from the usual type, for they are gaily decorated almost like the
males, but in a somewhat less degree. Hence in these two latter species
the bright colours of the males seem to have been transferred to the
females; whilst in the tenth species the male has either retained or
recovered the plain colours of the female, as well as of the parent-form of
the genus. The sexes in these three cases have thus been rendered nearly
alike, though in an opposite manner. In the allied genus Eubagis, both
sexes of some of the species are plain-coloured and nearly alike; whilst
with the greater number the males are decorated with beautiful metallic
tints in a diversified manner, and differ much from their females. The
females throughout the genus retain the same general style of colouring, so
that they resemble one another much more closely than they resemble their
own males.
In the genus Papilio, all the species of the Aeneas group are remarkable
for their conspicuous and strongly contrasted colours, and they illustrate
the frequent tendency to gradation in the amount of difference between the
sexes. In a few species, for instance in P. ascanius, the males and
females are alike; in others the males are either a little brighter, or
very much more superb than the females. The genus Junonia, allied to our
Vanessae, offers a nearly parallel case, for although the sexes of most of
the species resemble each other, and are destitute of rich colours, yet in
certain species, as in J. oenone, the male is rather more bright-coloured
than the female, and in a few (for instance J. andremiaja) the male is so
different from the female that he might be mistaken for an entirely
distinct species.
Another striking case was pointed out to me in the British Museum by Mr. A.
Butler, namely, one of the tropical American Theclae, in which both sexes
are nearly alike and wonderfully splendid; in another species the male is
coloured in a similarly gorgeous manner, whilst the whole upper surface of
the female is of a dull uniform brown. Our common little English blue
butterflies of the genus Lycaena, illustrate the various differences in
colour between the sexes, almost as well, though not in so striking a
manner, as the above exotic genera. In Lycaena agestis both sexes have
wings of a brown colour, bordered with small ocellated orange spots, and
are thus alike. In L. oegon the wings of the males are of a fine blue,
bordered with black, whilst those of the female are brown, with a similar
border, closely resembling the wings of L. agestis. Lastly, in L. arion
both sexes are of a blue colour and are very like, though in the female the
edges of the wings are rather duskier, with the black spots plainer; and in
a bright blue Indian species both sexes are still more alike.
I have given the foregoing details in order to shew, in the first place,
that when the sexes of butterflies differ, the male as a general rule is
the more beautiful, and departs more from the usual type of colouring of
the group to which the species belongs. Hence in most groups the females
of the several species resemble each other much more closely than do the
males. In some cases, however, to which I shall hereafter allude, the
females are coloured more splendidly than the males. In the second place,
these details have been given to bring clearly before the mind that within
the same genus, the two sexes frequently present every gradation from no
difference in colour, to so great a difference that it was long before the
two were placed by entomologists in the same genus. In the third place, we
have seen that when the sexes nearly resemble each other, this appears due
either to the male having transferred his colours to the female, or to the
male having retained, or perhaps recovered, the primordial colours of the
group. It also deserves notice that in those groups in which the sexes
differ, the females usually somewhat resemble the males, so that when the
males are beautiful to an extraordinary degree, the females almost
invariably exhibit some degree of beauty. From the many cases of gradation
in the amount of difference between the sexes, and from the prevalence of
the same general type of coloration throughout the whole of the same group,
we may conclude that the causes have generally been the same which have
determined the brilliant colouring of the males alone of some species, and
of both sexes of other species.
As so many gorgeous butterflies inhabit the tropics, it has often been
supposed that they owe their colours to the great heat and moisture of
these zones; but Mr. Bates (6. 'The Naturalist on the Amazons,' vol. i.
1863, p. 19.) has shown by the comparison of various closely-allied groups
of insects from the temperate and tropical regions, that this view cannot
be maintained; and the evidence becomes conclusive when brilliantly-
coloured males and plain-coloured females of the same species inhabit the
same district, feed on the same food, and follow exactly the same habits of
life. Even when the sexes resemble each other, we can hardly believe that
their brilliant and beautifully-arranged colours are the purposeless result
of the nature of the tissues and of the action of the surrounding
conditions.
With animals of all kinds, whenever colour has been modified for some
special purpose, this has been, as far as we can judge, either for direct
or indirect protection, or as an attraction between the sexes. With many
species of butterflies the upper surfaces of the wings are obscure; and
this in all probability leads to their escaping observation and danger.
But butterflies would be particularly liable to be attacked by their
enemies when at rest; and most kinds whilst resting raise their wings
vertically over their backs, so that the lower surface alone is exposed to
view. Hence it is this side which is often coloured so as to imitate the
objects on which these insects commonly rest. Dr. Rossler, I believe,
first noticed the similarity of the closed wings of certain Vanessae and
other butterflies to the bark of trees. Many analogous and striking facts
could be given. The most interesting one is that recorded by Mr. Wallace
(7. See the interesting article in the 'Westminster Review,' July 1867, p.
10. A woodcut of the Kallima is given by Mr. Wallace in 'Hardwicke's
Science Gossip,' September 1867, p. 196.) of a common Indian and Sumatran
butterfly (Kallima) which disappears like magic when it settles on a bush;
for it hides its head and antennae between its closed wings, which, in
form, colour and veining, cannot be distinguished from a withered leaf with
its footstalk. In some other cases the lower surfaces of the wings are
brilliantly coloured, and yet are protective; thus in Thecla rubi the wings
when closed are of an emerald green, and resemble the young leaves of the
bramble, on which in spring this butterfly may often be seen seated. It is
also remarkable that in very many species in which the sexes differ greatly
in colour on their upper surface, the lower surface is closely similar or
identical in both sexes, and serves as a protection. (8. Mr. G. Fraser,
in 'Nature,' April 1871, p. 489.)
Although the obscure tints both of the upper and under sides of many
butterflies no doubt serve to conceal them, yet we cannot extend this view
to the brilliant and conspicuous colours on the upper surface of such
species as our admiral and peacock Vanessae, our white cabbage-butterflies
(Pieris), or the great swallow-tail Papilio which haunts the open fens--for
these butterflies are thus rendered visible to every living creature. In
these species both sexes are alike; but in the common brimstone butterfly
(Gonepteryx rhamni), the male is of an intense yellow, whilst the female is
much paler; and in the orange-tip (Anthocharis cardamines) the males alone
have their wings tipped with bright orange. Both the males and females in
these cases are conspicuous, and it is not credible that their difference
in colour should stand in any relation to ordinary protection. Prof.
Weismann remarks (9. 'Einfluss der Isolirung auf die Artbildung,' 1872, p.
58.), that the female of one of the Lycaenae expands her brown wings when
she settles on the ground, and is then almost invisible; the male, on the
other hand, as if aware of the danger incurred from the bright blue of the
upper surface of his wings, rests with them closed; and this shows that the
blue colour cannot be in any way protective. Nevertheless, it is probable
that conspicuous colours are indirectly beneficial to many species, as a
warning that they are unpalatable. For in certain other cases, beauty has
been gained through the imitation of other beautiful species, which inhabit
the same district and enjoy an immunity from attack by being in some way
offensive to their enemies; but then we have to account for the beauty of
the imitated species.
As Mr. Walsh has remarked to me, the females of our orange-tip butterfly,
above referred to, and of an American species (Anth. genutia) probably shew
us the primordial colours of the parent-species of the genus; for both
sexes of four or five widely-distributed species are coloured in nearly the
same manner. As in several previous cases, we may here infer that it is
the males of Anth. cardamines and genutia which have departed from the
usual type of the genus. In the Anth. sara from California, the orange-
tips to the wings have been partially developed in the female; but they are
paler than in the male, and slightly different in some other respects. In
an allied Indian form, the Iphias glaucippe, the orange-tips are fully
developed in both sexes. In this Iphias, as pointed out to me by Mr. A.
Butler, the under surface of the wings marvellously resembles a pale-
coloured leaf; and in our English orange-tip, the under surface resembles
the flower-head of the wild parsley, on which the butterfly often rests at
night. (10. See the interesting observations by T.W. Wood, 'The Student,'
Sept. 1868, p. 81.) The same reason which compels us to believe that the
lower surfaces have here been coloured for the sake of protection, leads us
to deny that the wings have been tipped with bright orange for the same
purpose, especially when this character is confined to the males.
Most Moths rest motionless during the whole or greater part of the day with
their wings depressed; and the whole upper surface is often shaded and
coloured in an admirable manner, as Mr. Wallace has remarked, for escaping
detection. The front-wings of the Bombycidae and Noctuidae (11. Mr.
Wallace in 'Hardwicke's Science Gossip,' September 1867, p. 193.), when at
rest, generally overlap and conceal the hind-wings; so that the latter
might be brightly coloured without much risk; and they are in fact often
thus coloured. During flight, moths would often be able to escape from
their enemies; nevertheless, as the hind-wings are then fully exposed to
view, their bright colours must generally have been acquired at some little
risk. But the following fact shews how cautious we ought to be in drawing
conclusions on this head. The common Yellow Under-wings (Triphaena) often
fly about during the day or early evening, and are then conspicuous from
the colour of their hind-wings. It would naturally be thought that this
would be a source of danger; but Mr. J. Jenner Weir believes that it
actually serves them as a means of escape, for birds strike at these
brightly coloured and fragile surfaces, instead of at the body. For
instance, Mr. Weir turned into his aviary a vigorous specimen of Triphaena
pronuba, which was instantly pursued by a robin; but the bird's attention
being caught by the coloured wings, the moth was not captured until after
about fifty attempts, and small portions of the wings were repeatedly
broken off. He tried the same experiment, in the open air, with a swallow
and T. fimbria; but the large size of this moth probably interfered with
its capture. (12. See also, on this subject, Mr. Weir's paper in
'Transactions, Entomological Society,' 1869, p. 23.) We are thus reminded
of a statement made by Mr. Wallace (13. 'Westminster Review,' July 1867,
p. 16.), namely, that in the Brazilian forests and Malayan islands, many
common and highly-decorated butterflies are weak flyers, though furnished
with a broad expanse of wing; and they "are often captured with pierced and
broken wings, as if they had been seized by birds, from which they had
escaped: if the wings had been much smaller in proportion to the body, it
seems probable that the insect would more frequently have been struck or
pierced in a vital part, and thus the increased expanse of the wings may
have been indirectly beneficial."
DISPLAY.
The bright colours of many butterflies and of some moths are specially
arranged for display, so that they may be readily seen. During the night
colours are not visible, and there can be no doubt that the nocturnal
moths, taken as a body, are much less gaily decorated than butterflies, all
of which are diurnal in their habits. But the moths of certain families,
such as the Zygaenidae, several Sphingidae, Uraniidae, some Arctiidae and
Saturniidae, fly about during the day or early evening, and many of these
are extremely beautiful, being far brighter coloured than the strictly
nocturnal kinds. A few exceptional cases, however, of bright-coloured
nocturnal species have been recorded. (14. For instance, Lithosia; but
Prof. Westwood ('Modern Class. of Insects,' vol. ii. p. 390) seems
surprised at this case. On the relative colours of diurnal and nocturnal
Lepidoptera, see ibid. pp. 333 and 392; also Harris, 'Treatise on the
Insects of New England,' 1842, p. 315.)
There is evidence of another kind in regard to display. Butterflies, as
before remarked, elevate their wings when at rest, but whilst basking in
the sunshine often alternately raise and depress them, thus exposing both
surfaces to full view; and although the lower surface is often coloured in
an obscure manner as a protection, yet in many species it is as highly
decorated as the upper surface, and sometimes in a very different manner.
In some tropical species the lower surface is even more brilliantly
coloured than the upper. (15. Such differences between the upper and
lower surfaces of the wings of several species of Papilio may be seen in
the beautiful plates to Mr. Wallace's 'Memoir on the Papilionidae of the
Malayan Region,' in 'Transactions of the Linnean Society,' vol. xxv. part
i. 1865.) In the English fritillaries (Argynnis) the lower surface alone
is ornamented with shining silver. Nevertheless, as a general rule, the
upper surface, which is probably more fully exposed, is coloured more
brightly and diversely than the lower. Hence the lower surface generally
affords to entomologists the more useful character for detecting the
affinities of the various species. Fritz Muller informs me that three
species of Castnia are found near his house in S. Brazil: of two of them
the hind-wings are obscure, and are always covered by the front-wings when
these butterflies are at rest; but the third species has black hind-wings,
beautifully spotted with red and white, and these are fully expanded and
displayed whenever the butterfly rests. Other such cases could be added.
If we now turn to the enormous group of moths, which, as I hear from Mr.
Stainton, do not habitually expose the under surface of their wings to full
view, we find this side very rarely coloured with a brightness greater
than, or even equal to, that of the upper side. Some exceptions to the
rule, either real or apparent, must be noticed, as the case of Hypopyra.
(16. See Mr. Wormald on this moth: 'Proceedings of the Entomological
Society,' March 2, 1868.) Mr. Trimen informs me that in Guenee's great
work, three moths are figured, in which the under surface is much the more
brilliant. For instance, in the Australian Gastrophora the upper surface
of the fore-wing is pale greyish-ochreous, while the lower surface is
magnificently ornamented by an ocellus of cobalt-blue, placed in the midst
of a black mark, surrounded by orange-yellow, and this by bluish-white.
But the habits of these three moths are unknown; so that no explanation can
be given of their unusual style of colouring. Mr. Trimen also informs me
that the lower surface of the wings in certain other Geometrae (17. See
also an account of the S. American genus Erateina (one of the Geometrae) in
'Transactions, Ent. Soc.' new series, vol. v. pl. xv. and xvi.) and
quadrifid Noctuae are either more variegated or more brightly-coloured than
the upper surface; but some of these species have the habit of "holding
their wings quite erect over their backs, retaining them in this position
for a considerable time," and thus exposing the under surface to view.
Other species, when settled on the ground or herbage, now and then suddenly
and slightly lift up their wings. Hence the lower surface of the wings
being brighter than the upper surface in certain moths is not so anomalous
as it at first appears. The Saturniidae include some of the most beautiful
of all moths, their wings being decorated, as in our British Emperor moth,
with fine ocelli; and Mr. T.W. Wood (18. 'Proc Ent. Soc. of London,' July
6, 1868, p. xxvii.) observes that they resemble butterflies in some of
their movements; "for instance, in the gentle waving up and down of the
wings as if for display, which is more characteristic of diurnal than of
nocturnal Lepidoptera."
It is a singular fact that no British moths which are brilliantly coloured,
and, as far as I can discover, hardly any foreign species, differ much in
colour according to sex; though this is the case with many brilliant
butterflies. The male, however, of one American moth, the Saturnia Io, is
described as having its fore-wings deep yellow, curiously marked with
purplish-red spots; whilst the wings of the female are purple-brown, marked
with grey lines. (19. Harris, 'Treatise,' etc., edited by Flint, 1862, p.
395.) The British moths which differ sexually in colour are all brown, or
of various dull yellow tints, or nearly white. In several species the
males are much darker than the females (20. For instance, I observe in my
son's cabinet that the males are darker than the females in the Lasiocampa
quercus, Odonestis potatoria, Hypogymna dispar, Dasychira pudibunda, and
Cycnia mendica. In this latter species the difference in colour between
the two sexes is strongly marked; and Mr. Wallace informs me that we here
have, as he believes, an instance of protective mimicry confined to one
sex, as will hereafter be more fully explained. The white female of the
Cycnia resembles the very common Spilosoma menthrasti, both sexes of which
are white; and Mr. Stainton observed that this latter moth was rejected
with utter disgust by a whole brood of young turkeys, which were fond of
eating other moths; so that if the Cycnia was commonly mistaken by British
birds for the Spilosoma, it would escape being devoured, and its white
deceptive colour would thus be highly beneficial.), and these belong to
groups which generally fly about during the afternoon. On the other hand,
in many genera, as Mr. Stainton informs me, the males have the hind-wings
whiter than those of the female--of which fact Agrotis exclamationis offers
a good instance. In the Ghost Moth (Hepialus humuli) the difference is
more strongly marked; the males being white, and the females yellow with
darker markings. (21. It is remarkable, that in the Shetland Islands the
male of this moth, instead of differing widely from the female, frequently
resembles her closely in colour (see Mr. MacLachlan, 'Transactions,
Entomological Society,' vol. ii. 1866, p. 459). Mr. G. Fraser suggests
('Nature,' April 1871, p. 489) that at the season of the year when the
ghost-moth appears in these northern islands, the whiteness of the males
would not be needed to render them visible to the females in the twilight
night.) It is probable that in these cases the males are thus rendered
more conspicuous, and more easily seen by the females whilst flying about
in the dusk.
From the several foregoing facts it is impossible to admit that the
brilliant colours of butterflies, and of some few moths, have commonly been
acquired for the sake of protection. We have seen that their colours and
elegant patterns are arranged and exhibited as if for display. Hence I am
led to believe that the females prefer or are most excited by the more
brilliant males; for on any other supposition the males would, as far as we
can see, be ornamented to no purpose. We know that ants and certain
Lamellicorn beetles are capable of feeling an attachment for each other,
and that ants recognise their fellows after an interval of several months.
Hence there is no abstract improbability in the Lepidoptera, which probably
stand nearly or quite as high in the scale as these insects, having
sufficient mental capacity to admire bright colours. They certainly
discover flowers by colour. The Humming-bird Sphinx may often be seen to
swoop down from a distance on a bunch of flowers in the midst of green
foliage; and I have been assured by two persons abroad, that these moths
repeatedly visit flowers painted on the walls of a room, and vainly
endeavour to insert their proboscis into them. Fritz Muller informs me
that several kinds of butterflies in S. Brazil shew an unmistakable
preference for certain colours over others: he observed that they very
often visited the brilliant red flowers of five or six genera of plants,
but never the white or yellow flowering species of the same and other
genera, growing in the same garden; and I have received other accounts to
the same effect. As I hear from Mr. Doubleday, the common white butterfly
often flies down to a bit of paper on the ground, no doubt mistaking it for
one of its own species. Mr. Collingwood (22. 'Rambles of a Naturalist in
the Chinese Seas,' 1868, p. 182.) in speaking of the difficulty in
collecting certain butterflies in the Malay Archipelago, states that "a
dead specimen pinned upon a conspicuous twig will often arrest an insect of
the same species in its headlong flight, and bring it down within easy
reach of the net, especially if it be of the opposite sex."
The courtship of butterflies is, as before remarked, a prolonged affair.
The males sometimes fight together in rivalry; and many may be seen
pursuing or crowding round the same female. Unless, then, the females
prefer one male to another, the pairing must be left to mere chance, and
this does not appear probable. If, on the other band, the females
habitually, or even occasionally, prefer the more beautiful males, the
colours of the latter will have been rendered brighter by degrees, and will
have been transmitted to both sexes or to one sex, according to the law of
inheritance which has prevailed. The process of sexual selection will have
been much facilitated, if the conclusion can be trusted, arrived at from
various kinds of evidence in the supplement to the ninth chapter; namely,
that the males of many Lepidoptera, at least in the imago state, greatly
exceed the females in number.
Some facts, however, are opposed to the belief that female butterflies
prefer the more beautiful males; thus, as I have been assured by several
collectors, fresh females may frequently be seen paired with battered,
faded, or dingy males; but this is a circumstance which could hardly fail
often to follow from the males emerging from their cocoons earlier than the
females. With moths of the family of the Bombycidae, the sexes pair
immediately after assuming the imago state; for they cannot feed, owing to
the rudimentary condition of their mouths. The females, as several
entomologists have remarked to me, lie in an almost torpid state, and
appear not to evince the least choice in regard to their partners. This is
the case with the common silk-moth (B. mori), as I have been told by some
continental and English breeders. Dr. Wallace, who has had great
experience in breeding Bombyx cynthia, is convinced that the females evince
no choice or preference. He has kept above 300 of these moths together,
and has often found the most vigorous females mated with stunted males.
The reverse appears to occur seldom; for, as he believes, the more vigorous
males pass over the weakly females, and are attracted by those endowed with
most vitality. Nevertheless, the Bombycidae, though obscurely-coloured,
are often beautiful to our eyes from their elegant and mottled shades.
I have as yet only referred to the species in which the males are brighter
coloured than the females, and I have attributed their beauty to the
females for many generations having chosen and paired with the more
attractive males. But converse cases occur, though rarely, in which the
females are more brilliant than the males; and here, as I believe, the
males have selected the more beautiful females, and have thus slowly added
to their beauty. We do not know why in various classes of animals the
males of some few species have selected the more beautiful females instead
of having gladly accepted any female, as seems to be the general rule in
the animal kingdom: but if, contrary to what generally occurs with the
Lepidoptera, the females were much more numerous than the males, the latter
would be likely to pick out the more beautiful females. Mr. Butler shewed
me several species of Callidryas in the British Museum, in some of which
the females equalled, and in others greatly surpassed the males in beauty;
for the females alone have the borders of their wings suffused with crimson
and orange, and spotted with black. The plainer males of these species
closely resemble each other, shewing that here the females have been
modified; whereas in those cases, where the males are the more ornate, it
is these which have been modified, the females remaining closely alike.
In England we have some analogous cases, though not so marked. The females
alone of two species of Thecla have a bright-purple or orange patch on
their fore-wings. In Hipparchia the sexes do not differ much; but it is
the female of H. janira which has a conspicuous light-brown patch on her
wings; and the females of some of the other species are brighter coloured
than their males. Again, the females of Colias edusa and hyale have
"orange or yellow spots on the black marginal border, represented in the
males only by thin streaks"; and in Pieris it is the females which "are
ornamented with black spots on the fore-wings, and these are only partially
present in the males." Now the males of many butterflies are known to
support the females during their marriage flight; but in the species just
named it is the females which support the males; so that the part which the
two sexes play is reversed, as is their relative beauty. Throughout the
animal kingdom the males commonly take the more active share in wooing, and
their beauty seems to have been increased by the females having accepted
the more attractive individuals; but with these butterflies, the females
take the more active part in the final marriage ceremony, so that we may
suppose that they likewise do so in the wooing; and in this case we can
understand how it is that they have been rendered the more beautiful. Mr.
Meldola, from whom the foregoing statements have been taken, says in
conclusion: "Though I am not convinced of the action of sexual selection
in producing the colours of insects, it cannot be denied that these facts
are strikingly corroborative of Mr. Darwin's views." (23. 'Nature,' April
27, 1871, p. 508. Mr. Meldola quotes Donzel, in 'Soc. Ent. de France,'
1837, p. 77, on the flight of butterflies whilst pairing. See also Mr. G.
Fraser, in 'Nature,' April 20, 1871, p. 489, on the sexual differences of
several British butterflies.)
As sexual selection primarily depends on variability, a few words must be
added on this subject. In respect to colour there is no difficulty, for
any number of highly variable Lepidoptera could be named. One good
instance will suffice. Mr. Bates shewed me a whole series of specimens of
Papilio sesostris and P. childrenae; in the latter the males varied much in
the extent of the beautifully enamelled green patch on the fore-wings, and
in the size of the white mark, and of the splendid crimson stripe on the
hind-wings; so that there was a great contrast amongst the males between
the most and the least gaudy. The male of Papilio sesostris is much less
beautiful than of P. childrenae; and it likewise varies a little in the
size of the green patch on the fore-wings, and in the occasional appearance
of the small crimson stripe on the hind-wings, borrowed, as it would seem,
from its own female; for the females of this and of many other species in
the Aeneas group possess this crimson stripe. Hence between the brightest
specimens of P. sesostris and the dullest of P. childrenae, there was but a
small interval; and it was evident that as far as mere variability is
concerned, there would be no difficulty in permanently increasing the
beauty of either species by means of selection. The variability is here
almost confined to the male sex; but Mr. Wallace and Mr. Bates have shewn
(24. Wallace on the Papilionidae of the Malayan Region, in 'Transact.
Linn. Soc.' vol. xxv. 1865, pp. 8, 36. A striking case of a rare variety,
strictly intermediate between two other well-marked female varieties, is
given by Mr. Wallace. See also Mr. Bates, in 'Proc. Entomolog. Soc.' Nov.
19, 1866, p. xl.) that the females of some species are extremely variable,
the males being nearly constant. In a future chapter I shall have occasion
to shew that the beautiful eye-like spots, or ocelli, found on the wings of
many Lepidoptera, are eminently variable. I may here add that these ocelli
offer a difficulty on the theory of sexual selection; for though appearing
to us so ornamental, they are never present in one sex and absent in the
other, nor do they ever differ much in the two sexes. (25. Mr. Bates was
so kind as to lay this subject before the Entomological Society, and I have
received answers to this effect from several entomologists.) This fact is
at present inexplicable; but if it should hereafter be found that the
formation of an ocellus is due to some change in the tissues of the wings,
for instance, occurring at a very early period of development, we might
expect, from what we know of the laws of inheritance, that it would be
transmitted to both sexes, though arising and perfected in one sex alone.
On the whole, although many serious objections may be urged, it seems
probable that most of the brilliantly-coloured species of Lepidoptera owe
their colours to sexual selection, excepting in certain cases, presently to
be mentioned, in which conspicuous colours have been gained through mimicry
as a protection. From the ardour of the male throughout the animal
kingdom, he is generally willing to accept any female; and it is the female
which usually exerts a choice. Hence, if sexual selection has been
efficient with the Lepidoptera, the male, when the sexes differ, ought to
be the more brilliantly coloured, and this undoubtedly is the case. When
both sexes are brilliantly coloured and resemble each other, the characters
acquired by the males appear to have been transmitted to both. We are led
to this conclusion by cases, even within the same genus, of gradation from
an extraordinary amount of difference to identity in colour between the two
sexes.
But it may be asked whether the difference in colour between the sexes may
not be accounted for by other means besides sexual selection. Thus the
males and females of the same species of butterfly are in several cases
known (26. H.W. Bates, 'The Naturalist on the Amazons,' vol. ii. 1863, p.
228. A.R. Wallace, in 'Transactions, Linnean Society,' vol. xxv. 1865, p.
10.) to inhabit different stations, the former commonly basking in the
sunshine, the latter haunting gloomy forests. It is therefore possible
that different conditions of life may have acted directly on the two sexes;
but this is not probable (27. On this whole subject see 'The Variation of
Animals and Plants under Domestication,' 1868, vol. ii. chap. xxiii.) as in
the adult state they are exposed to different conditions during a very
short period; and the larvae of both are exposed to the same conditions.
Mr. Wallace believes that the difference between the sexes is due not so
much to the males having been modified, as to the females having in all or
almost all cases acquired dull colours for the sake of protection. It
seems to me, on the contrary, far more probable that it is the males which
have been chiefly modified through sexual selection, the females having
been comparatively little changed. We can thus understand how it is that
the females of allied species generally resemble one another so much more
closely than do the males. They thus shew us approximately the primordial
colouring of the parent-species of the group to which they belong. They
have, however, almost always been somewhat modified by the transfer to them
of some of the successive variations, through the accumulation of which the
males were rendered beautiful. But I do not wish to deny that the females
alone of some species may have been specially modified for protection. In
most cases the males and females of distinct species will have been exposed
during their prolonged larval state to different conditions, and may have
been thus affected; though with the males any slight change of colour thus
caused will generally have been masked by the brilliant tints gained
through sexual selection. When we treat of Birds, I shall have to discuss
the whole question, as to how far the differences in colour between the
sexes are due to the males having been modified through sexual selection
for ornamental purposes, or to the females having been modified through
natural selection for the sake of protection, so that I will here say but
little on the subject.
In all the cases in which the more common form of equal inheritance by both
sexes has prevailed, the selection of bright-coloured males would tend to
make the females bright-coloured; and the selection of dull-coloured
females would tend to make the males dull. If both processes were carried
on simultaneously, they would tend to counteract each other; and the final
result would depend on whether a greater number of females from being well
protected by obscure colours, or a greater number of males by being
brightly-coloured and thus finding partners, succeeded in leaving more
numerous offspring.
In order to account for the frequent transmission of characters to one sex
alone, Mr. Wallace expresses his belief that the more common form of equal
inheritance by both sexes can be changed through natural selection into
inheritance by one sex alone, but in favour of this view I can discover no
evidence. We know from what occurs under domestication that new characters
often appear, which from the first are transmitted to one sex alone; and by
the selection of such variations there would not be the slightest
difficulty in giving bright colours to the males alone, and at the same
time or subsequently, dull colours to the females alone. In this manner
the females of some butterflies and moths have, it is probable, been
rendered inconspicuous for the sake of protection, and widely different
from their males.
I am, however, unwilling without distinct evidence to admit that two
complex processes of selection, each requiring the transference of new
characters to one sex alone, have been carried on with a multitude of
species,--that the males have been rendered more brilliant by beating their
rivals, and the females more dull-coloured by having escaped from their
enemies. The male, for instance, of the common brimstone butterfly
(Gonepteryx), is of a far more intense yellow than the female, though she
is equally conspicuous; and it does not seem probable that she specially
acquired her pale tints as a protection, though it is probable that the
male acquired his bright colours as a sexual attraction. The female of
Anthocharis cardamines does not possess the beautiful orange wing-tips of
the male; consequently she closely resembles the white butterflies (Pieris)
so common in our gardens; but we have no evidence that this resemblance is
beneficial to her. As, on the other hand, she resembles both sexes of
several other species of the genus inhabiting various quarters of the
world, it is probable that she has simply retained to a large extent her
primordial colours.
Finally, as we have seen, various considerations lead to the conclusion
that with the greater number of brilliantly-coloured Lepidoptera it is the
male which has been chiefly modified through sexual selection; the amount
of difference between the sexes mostly depending on the form of inheritance
which has prevailed. Inheritance is governed by so many unknown laws or
conditions, that it seems to us to act in a capricious manner (28. The
'Variation of Animals and Plants under Domestication,' vol. ii. chap. xii.
p. 17.); and we can thus, to a certain extent, understand how it is that
with closely allied species the sexes either differ to an astonishing
degree, or are identical in colour. As all the successive steps in the
process of variation are necessarily transmitted through the female, a
greater or less number of such steps might readily become developed in her;
and thus we can understand the frequent gradations from an extreme
difference to none at all between the sexes of allied species. These cases
of gradation, it may be added, are much too common to favour the
supposition that we here see females actually undergoing the process of
transition and losing their brightness for the sake of protection; for we
have every reason to conclude that at any one time the greater number of
species are in a fixed condition.
MIMICRY.
This principle was first made clear in an admirable paper by Mr. Bates (29.
'Transact. Linn. Soc.' vol. xxiii. 1862, p. 495.), who thus threw a flood
of light on many obscure problems. It had previously been observed that
certain butterflies in S. America belonging to quite distinct families,
resembled the Heliconidae so closely in every stripe and shade of colour,
that they could not be distinguished save by an experienced entomologist.
As the Heliconidae are coloured in their usual manner, whilst the others
depart from the usual colouring of the groups to which they belong, it is
clear that the latter are the imitators, and the Heliconidae the imitated.
Mr. Bates further observed that the imitating species are comparatively
rare, whilst the imitated abound, and that the two sets live mingled
together. From the fact of the Heliconidae being conspicuous and beautiful
insects, yet so numerous in individuals and species, he concluded that they
must be protected from the attacks of enemies by some secretion or odour;
and this conclusion has now been amply confirmed (30. 'Proc. Entomological
Soc.' Dec. 3, 1866, p. xlv.), especially by Mr. Belt. Hence Mr. Bates
inferred that the butterflies which imitate the protected species have
acquired their present marvellously deceptive appearance through variation
and natural selection, in order to be mistaken for the protected kinds, and
thus to escape being devoured. No explanation is here attempted of the
brilliant colours of the imitated, but only of the imitating butterflies.
We must account for the colours of the former in the same general manner,
as in the cases previously discussed in this chapter. Since the
publication of Mr. Bates' paper, similar and equally striking facts have
been observed by Mr. Wallace in the Malayan region, by Mr. Trimen in South
Africa, and by Mr. Riley in the United States. (31. Wallace, 'Transact.
Linn. Soc.' vol. xxv. 1865 p. i.; also, 'Transact. Ent. Soc.' vol. iv. (3rd
series), 1867, p. 301. Trimen, 'Linn. Transact.' vol. xxvi. 1869, p. 497.
Riley, 'Third Annual Report on the Noxious Insects of Missouri,' 1871, pp.
163-168. This latter essay is valuable, as Mr. Riley here discusses all
the objections which have been raised against Mr. Bates's theory.)
As some writers have felt much difficulty in understanding how the first
steps in the process of mimicry could have been effected through natural
selection, it may be well to remark that the process probably commenced
long ago between forms not widely dissimilar in colour. In this case even
a slight variation would be beneficial, if it rendered the one species more
like the other; and afterwards the imitated species might be modified to an
extreme degree through sexual selection or other means, and if the changes
were gradual, the imitators might easily be led along the same track, until
they differed to an equally extreme degree from their original condition;
and they would thus ultimately assume an appearance or colouring wholly
unlike that of the other members of the group to which they belonged. It
should also be remembered that many species of Lepidoptera are liable to
considerable and abrupt variations in colour. A few instances have been
given in this chapter; and many more may be found in the papers of Mr.
Bates and Mr. Wallace.
With several species the sexes are alike, and imitate the two sexes of
another species. But Mr. Trimen gives, in the paper already referred to,
three cases in which the sexes of the imitated form differ from each other
in colour, and the sexes of the imitating form differ in a like manner.
Several cases have also been recorded where the females alone imitate
brilliantly-coloured and protected species, the males retaining "the normal
aspect of their immediate congeners." It is here obvious that the
successive variations by which the female has been modified have been
transmitted to her alone. It is, however, probable that some of the many
successive variations would have been transmitted to, and developed in, the
males had not such males been eliminated by being thus rendered less
attractive to the females; so that only those variations were preserved
which were from the first strictly limited in their transmission to the
female sex. We have a partial illustration of these remarks in a statement
by Mr. Belt (32. 'The Naturalist in Nicaragua,' 1874, p. 385.); that the
males of some of the Leptalides, which imitate protected species, still
retain in a concealed manner some of their original characters. Thus in
the males "the upper half of the lower wing is of a pure white, whilst all
the rest of the wings is barred and spotted with black, red and yellow,
like the species they mimic. The females have not this white patch, and
the males usually conceal it by covering it with the upper wing, so that I
cannot imagine its being of any other use to them than as an attraction in
courtship, when they exhibit it to the females, and thus gratify their
deep-seated preference for the normal colour of the Order to which the
Leptalides belong."
BRIGHT COLOURS OF CATERPILLARS.
Whilst reflecting on the beauty of many butterflies, it occurred to me that
some caterpillars were splendidly coloured; and as sexual selection could
not possibly have here acted, it appeared rash to attribute the beauty of
the mature insect to this agency, unless the bright colours of their larvae
could be somehow explained. In the first place, it may be observed that
the colours of caterpillars do not stand in any close correlation with
those of the mature insect. Secondly, their bright colours do not serve in
any ordinary manner as a protection. Mr. Bates informs me, as an instance
of this, that the most conspicuous caterpillar which he ever beheld (that
of a Sphinx) lived on the large green leaves of a tree on the open llanos
of South America; it was about four inches in length, transversely banded
with black and yellow, and with its head, legs, and tail of a bright red.
Hence it caught the eye of any one who passed by, even at the distance of
many yards, and no doubt that of every passing bird.
I then applied to Mr. Wallace, who has an innate genius for solving
difficulties. After some consideration he replied: "Most caterpillars
require protection, as may be inferred from some kinds being furnished with
spines or irritating hairs, and from many being coloured green like the
leaves on which they feed, or being curiously like the twigs of the trees
on which they live." Another instance of protection, furnished me by Mr.
J. Mansel Weale, may be added, namely, that there is a caterpillar of a
moth which lives on the mimosas in South Africa, and fabricates for itself
a case quite indistinguishable from the surrounding thorns. From such
considerations Mr. Wallace thought it probable that conspicuously coloured
caterpillars were protected by having a nauseous taste; but as their skin
is extremely tender, and as their intestines readily protrude from a wound,
a slight peck from the beak of a bird would be as fatal to them as if they
had been devoured. Hence, as Mr. Wallace remarks, "distastefulness alone
would be insufficient to protect a caterpillar unless some outward sign
indicated to its would-be destroyer that its prey was a disgusting morsel."
Under these circumstances it would be highly advantageous to a caterpillar
to be instantaneously and certainly recognised as unpalatable by all birds
and other animals. Thus the most gaudy colours would be serviceable, and
might have been gained by variation and the survival of the most easily-
recognised individuals.
This hypothesis appears at first sight very bold, but when it was brought
before the Entomological Society (33. 'Proceedings, Entomological
Society,' Dec. 3, 1866, p. xlv. and March 4, 1867, p. lxxx.) it was
supported by various statements; and Mr. J. Jenner Weir, who keeps a large
number of birds in an aviary, informs me that he has made many trials, and
finds no exception to the rule, that all caterpillars of nocturnal and
retiring habits with smooth skins, all of a green colour, and all which
imitate twigs, are greedily devoured by his birds. The hairy and spinose
kinds are invariably rejected, as were four conspicuously-coloured species.
When the birds rejected a caterpillar, they plainly shewed, by shaking
their heads, and cleansing their beaks, that they were disgusted by the
taste. (34. See Mr. J. Jenner Weir's paper on Insects and Insectivorous
Birds, in 'Transact. Ent. Soc.' 1869, p. 21; also Mr. Butler's paper, ibid.
p. 27. Mr. Riley has given analogous facts in the 'Third Annual Report on
the Noxious Insects of Missouri,' 1871, p. 148. Some opposed cases are,
however, given by Dr. Wallace and M. H. d'Orville; see 'Zoological Record,'
1869, p. 349.) Three conspicuous kinds of caterpillars and moths were also
given to some lizards and frogs, by Mr. A. Butler, and were rejected,
though other kinds were eagerly eaten. Thus the probability of Mr.
Wallace's view is confirmed, namely, that certain caterpillars have been
made conspicuous for their own good, so as to be easily recognised by their
enemies, on nearly the same principle that poisons are sold in coloured
bottles by druggists for the good of man. We cannot, however, at present
thus explain the elegant diversity in the colours of many caterpillars; but
any species which had at some former period acquired a dull, mottled, or
striped appearance, either in imitation of surrounding objects, or from the
direct action of climate, etc., almost certainly would not become uniform
in colour, when its tints were rendered intense and bright; for in order to
make a caterpillar merely conspicuous, there would be no selection in any
definite direction.
SUMMARY AND CONCLUDING REMARKS ON INSECTS.
Looking back to the several Orders, we see that the sexes often differ in
various characters, the meaning of which is not in the least understood.
The sexes, also, often differ in their organs of sense and means of
locomotion, so that the males may quickly discover and reach the females.
They differ still oftener in the males possessing diversified contrivances
for retaining the females when found. We are, however, here concerned only
in a secondary degree with sexual differences of these kinds.
In almost all the Orders, the males of some species, even of weak and
delicate kinds, are known to be highly pugnacious; and some few are
furnished with special weapons for fighting with their rivals. But the law
of battle does not prevail nearly so widely with insects as with the higher
animals. Hence it probably arises, that it is in only a few cases that the
males have been rendered larger and stronger than the females. On the
contrary, they are usually smaller, so that they may be developed within a
shorter time, to be ready in large numbers for the emergence of the
females.
In two families of the Homoptera and in three of the Orthoptera, the males
alone possess sound-producing organs in an efficient state. These are used
incessantly during the breeding-season, not only for calling the females,
but apparently for charming or exciting them in rivalry with other males.
No one who admits the agency of selection of any kind, will, after reading
the above discussion, dispute that these musical instruments have been
acquired through sexual selection. In four other Orders the members of one
sex, or more commonly of both sexes, are provided with organs for producing
various sounds, which apparently serve merely as call-notes. When both
sexes are thus provided, the individuals which were able to make the
loudest or most continuous noise would gain partners before those which
were less noisy, so that their organs have probably been gained through
sexual selection. It is instructive to reflect on the wonderful diversity
of the means for producing sound, possessed by the males alone, or by both
sexes, in no less than six Orders. We thus learn how effectual sexual
selection has been in leading to modifications which sometimes, as with the
Homoptera, relate to important parts of the organisation.
From the reasons assigned in the last chapter, it is probable that the
great horns possessed by the males of many Lamellicorn, and some other
beetles, have been acquired as ornaments. From the small size of insects,
we are apt to undervalue their appearance. If we could imagine a male
Chalcosoma (Fig. 16), with its polished bronzed coat of mail, and its vast
complex horns, magnified to the size of a horse, or even of a dog, it would
be one of the most imposing animals in the world.
The colouring of insects is a complex and obscure subject. When the male
differs slightly from the female, and neither are brilliantly-coloured, it
is probable that the sexes have varied in a slightly different manner, and
that the variations have been transmitted by each sex to the same without
any benefit or evil thus accruing. When the male is brilliantly-coloured
and differs conspicuously from the female, as with some dragon-flies and
many butterflies, it is probable that he owes his colours to sexual
selection; whilst the female has retained a primordial or very ancient type
of colouring, slightly modified by the agencies before explained. But in
some cases the female has apparently been made obscure by variations
transmitted to her alone, as a means of direct protection; and it is almost
certain that she has sometimes been made brilliant, so as to imitate other
protected species inhabiting the same district. When the sexes resemble
each other and both are obscurely coloured, there is no doubt that they
have been in a multitude of cases so coloured for the sake of protection.
So it is in some instances when both are brightly-coloured, for they thus
imitate protected species, or resemble surrounding objects such as flowers;
or they give notice to their enemies that they are unpalatable. In other
cases in which the sexes resemble each other and are both brilliant,
especially when the colours are arranged for display, we may conclude that
they have been gained by the male sex as an attraction, and have been
transferred to the female. We are more especially led to this conclusion
whenever the same type of coloration prevails throughout a whole group, and
we find that the males of some species differ widely in colour from the
females, whilst others differ slightly or not at all with intermediate
gradations connecting these extreme states.
In the same manner as bright colours have often been partially transferred
from the males to the females, so it has been with the extraordinary horns
of many Lamellicorn and some other beetles. So again, the sound-producing
organs proper to the males of the Homoptera and Orthoptera have generally
been transferred in a rudimentary, or even in a nearly perfect condition,
to the females; yet not sufficiently perfect to be of any use. It is also
an interesting fact, as bearing on sexual selection, that the stridulating
organs of certain male Orthoptera are not fully developed until the last
moult; and that the colours of certain male dragon-flies are not fully
developed until some little time after their emergence from the pupal
state, and when they are ready to breed.
Sexual selection implies that the more attractive individuals are preferred
by the opposite sex; and as with insects, when the sexes differ, it is the
male which, with some rare exceptions, is the more ornamented, and departs
more from the type to which the species belongs;--and as it is the male
which searches eagerly for the female, we must suppose that the females
habitually or occasionally prefer the more beautiful males, and that these
have thus acquired their beauty. That the females in most or all the
Orders would have the power of rejecting any particular male, is probable
from the many singular contrivances possessed by the males, such as great
jaws, adhesive cushions, spines, elongated legs, etc., for seizing the
female; for these contrivances show that there is some difficulty in the
act, so that her concurrence would seem necessary. Judging from what we
know of the perceptive powers and affections of various insects, there is
no antecedent improbability in sexual selection having come largely into
play; but we have as yet no direct evidence on this head, and some facts
are opposed to the belief. Nevertheless, when we see many males pursuing
the same female, we can hardly believe that the pairing is left to blind
chance--that the female exerts no choice, and is not influenced by the
gorgeous colours or other ornaments with which the male is decorated.
If we admit that the females of the Homoptera and Orthoptera appreciate the
musical tones of their male partners, and that the various instruments have
been perfected through sexual selection, there is little improbability in
the females of other insects appreciating beauty in form or colour, and
consequently in such characters having been thus gained by the males. But
from the circumstance of colour being so variable, and from its having been
so often modified for the sake of protection, it is difficult to decide in
how large a proportion of cases sexual selection has played a part. This
is more especially difficult in those Orders, such as Orthoptera,
Hymenoptera, and Coleoptera, in which the two sexes rarely differ much in
colour; for we are then left to mere analogy. With the Coleoptera,
however, as before remarked, it is in the great Lamellicorn group, placed
by some authors at the head of the Order, and in which we sometimes see a
mutual attachment between the sexes, that we find the males of some species
possessing weapons for sexual strife, others furnished with wonderful
horns, many with stridulating organs, and others ornamented with splendid
metallic tints. Hence it seems probable that all these characters have
been gained through the same means, namely sexual selection. With
butterflies we have the best evidence, as the males sometimes take pains to
display their beautiful colours; and we cannot believe that they would act
thus, unless the display was of use to them in their courtship.
When we treat of Birds, we shall see that they present in their secondary
sexual characters the closest analogy with insects. Thus, many male birds
are highly pugnacious, and some are furnished with special weapons for
fighting with their rivals. They possess organs which are used during the
breeding-season for producing vocal and instrumental music. They are
frequently ornamented with combs, horns, wattles and plumes of the most
diversified kinds, and are decorated with beautiful colours, all evidently
for the sake of display. We shall find that, as with insects, both sexes
in certain groups are equally beautiful, and are equally provided with
ornaments which are usually confined to the male sex. In other groups both
sexes are equally plain-coloured and unornamented. Lastly, in some few
anomalous cases, the females are more beautiful than the males. We shall
often find, in the same group of birds, every gradation from no difference
between the sexes, to an extreme difference. We shall see that female
birds, like female insects, often possess more or less plain traces or
rudiments of characters which properly belong to the males and are of use
only to them. The analogy, indeed, in all these respects between birds and
insects is curiously close. Whatever explanation applies to the one class
probably applies to the other; and this explanation, as we shall hereafter
attempt to shew in further detail, is sexual selection.
CHAPTER XII.
SECONDARY SEXUAL CHARACTERS OF FISHES, AMPHIBIANS, AND REPTILES.
FISHES: Courtship and battles of the males--Larger size of the females--
Males, bright colours and ornamental appendages; other strange characters--
Colours and appendages acquired by the males during the breeding-season
alone--Fishes with both sexes brilliantly coloured--Protective colours--The
less conspicuous colours of the female cannot be accounted for on the
principle of protection--Male fishes building nests, and taking charge of
the ova and young.
AMPHIBIANS: Differences in structure and colour between the sexes--Vocal
organs.
REPTILES: Chelonians--Crocodiles--Snakes, colours in some cases
protective--Lizards, battles of--Ornamental appendages--Strange differences
in structure between the sexes--Colours--Sexual differences almost as great
as with birds.
We have now arrived at the great sub-kingdom of the Vertebrata, and will
commence with the lowest class, that of fishes. The males of Plagiostomous
fishes (sharks, rays) and of Chimaeroid fishes are provided with claspers
which serve to retain the female, like the various structures possessed by
many of the lower animals. Besides the claspers, the males of many rays
have clusters of strong sharp spines on their heads, and several rows along
"the upper outer surface of their pectoral fins." These are present in the
males of some species, which have other parts of their bodies smooth. They
are only temporarily developed during the breeding-season; and Dr. Gunther
suspects that they are brought into action as prehensile organs by the
doubling inwards and downwards of the two sides of the body. It is a
remarkable fact that the females and not the males of some species, as of
Raia clavata, have their backs studded with large hook-formed spines. (1.
Yarrell's 'Hist. of British Fishes,' vol. ii. 1836, pp 417, 425, 436. Dr.
Gunther informs me that the spines in R. clavata are peculiar to the
female.)
The males alone of the capelin (Mallotus villosus, one of Salmonidae), are
provided with a ridge of closely-set, brush-like scales, by the aid of
which two males, one on each side, hold the female, whilst she runs with
great swiftness on the sandy beach, and there deposits her spawn. (2. The
'American Naturalist,' April 1871, p. 119.) The widely distinct
Monacanthus scopas presents a somewhat analogous structure. The male, as
Dr. Gunther informs me, has a cluster of stiff, straight spines, like those
of a comb, on the sides of the tail; and these in a specimen six inches
long were nearly one and a half inches in length; the female has in the
same place a cluster of bristles, which may be compared with those of a
tooth-brush. In another species, M. peronii, the male has a brush like
that possessed by the female of the last species, whilst the sides of the
tail in the female are smooth. In some other species of the same genus the
tail can be perceived to be a little roughened in the male and perfectly
smooth in the female; and lastly in others, both sexes have smooth sides.
The males of many fish fight for the possession of the females. Thus the
male stickleback (Gasterosteus leiurus) has been described as "mad with
delight," when the female comes out of her hiding-place and surveys the
nest which he has made for her. "He darts round her in every direction,
then to his accumulated materials for the nest, then back again in an
instant; and as she does not advance he endeavours to push her with his
snout, and then tries to pull her by the tail and side-spine to the nest."
(3. See Mr. R. Warington's interesting articles in 'Annals and Magazine of
Natural History,' October 1852, and November 1855.) The males are said to
be polygamists (4. Noel Humphreys, 'River Gardens,' 1857.); they are
extraordinarily bold and pugnacious, whilst "the females are quite
pacific." Their battles are at times desperate; "for these puny combatants
fasten tight on each other for several seconds, tumbling over and over
again until their strength appears completely exhausted." With the rough-
tailed stickleback (G. trachurus) the males whilst fighting swim round and
round each other, biting and endeavouring to pierce each other with their
raised lateral spines. The same writer adds (5. Loudon's 'Magazine of
Natural History,' vol. iii. 1830, p. 331.), "the bite of these little
furies is very severe. They also use their lateral spines with such fatal
effect, that I have seen one during a battle absolutely rip his opponent
quite open, so that he sank to the bottom and died." When a fish is
conquered, "his gallant bearing forsakes him; his gay colours fade away;
and he hides his disgrace among his peaceable companions, but is for some
time the constant object of his conqueror's persecution."
The male salmon is as pugnacious as the little stickleback; and so is the
male trout, as I hear from Dr. Gunther. Mr. Shaw saw a violent contest
between two male salmon which lasted the whole day; and Mr. R. Buist,
Superintendent of Fisheries, informs me that he has often watched from the
bridge at Perth the males driving away their rivals, whilst the females
were spawning. The males "are constantly fighting and tearing each other
on the spawning-beds, and many so injure each other as to cause the death
of numbers, many being seen swimming near the banks of the river in a state
of exhaustion, and apparently in a dying state." (6. The 'Field,' June
29, 1867. For Mr. Shaw's Statement, see 'Edinburgh Review,' 1843. Another
experienced observer (Scrope's 'Days of Salmon Fishing,' p. 60) remarks
that like the stag, the male would, if he could, keep all other males
away.) Mr. Buist informs me, that in June 1868, the keeper of the
Stormontfield breeding-ponds visited the northern Tyne and found about 300
dead salmon, all of which with one exception were males; and he was
convinced that they had lost their lives by fighting.
[Fig. 27. Head of male common salmon (Salmo salar) during the breeding-
season.
[This drawing, as well as all the others in the present chapter, have been
executed by the well-known artist, Mr. G. Ford, from specimens in the
British Museum, under the kind superintendence of Dr. Gunther.]
Fig. 28. Head of female salmon.]
The most curious point about the male salmon is that during the breeding-
season, besides a slight change in colour, "the lower jaw elongates, and a
cartilaginous projection turns upwards from the point, which, when the jaws
are closed, occupies a deep cavity between the intermaxillary bones of the
upper jaw." (7. Yarrell, 'History of British Fishes,' vol. ii. 1836, p.
10.) (Figs. 27 and 28.) In our salmon this change of structure lasts only
during the breeding-season; but in the Salmo lycaodon of N.W. America the
change, as Mr. J.K. Lord (8. 'The Naturalist in Vancouver's Island,' vol.
i. 1866, p. 54.) believes, is permanent, and best marked in the older males
which have previously ascended the rivers. In these old males the jaw
becomes developed into an immense hook-like projection, and the teeth grow
into regular fangs, often more than half an inch in length. With the
European salmon, according to Mr. Lloyd (9. 'Scandinavian Adventures,'
vol. i. 1854, pp. 100, 104.), the temporary hook-like structure serves to
strengthen and protect the jaws, when one male charges another with
wonderful violence; but the greatly developed teeth of the male American
salmon may be compared with the tusks of many male mammals, and they
indicate an offensive rather than a protective purpose.
The salmon is not the only fish in which the teeth differ in the two sexes;
as this is the case with many rays. In the thornback (Raia clavata) the
adult male has sharp, pointed teeth, directed backwards, whilst those of
the female are broad and flat, and form a pavement; so that these teeth
differ in the two sexes of the same species more than is usual in distinct
genera of the same family. The teeth of the male become sharp only when he
is adult: whilst young they are broad and flat like those of the female.
As so frequently occurs with secondary sexual characters, both sexes of
some species of rays (for instance R. batis), when adult, possess sharp
pointed teeth; and here a character, proper to and primarily gained by the
male, appears to have been transmitted to the offspring of both sexes. The
teeth are likewise pointed in both sexes of R. maculata, but only when
quite adult; the males acquiring them at an earlier age than the females.
We shall hereafter meet with analogous cases in certain birds, in which the
male acquires the plumage common to both sexes when adult, at a somewhat
earlier age than does the female. With other species of rays the males
even when old never possess sharp teeth, and consequently the adults of
both sexes are provided with broad, flat teeth like those of the young, and
like those of the mature females of the above-mentioned species. (10. See
Yarrell's account of the rays in his 'History of British Fishes,' vol. ii.
1836, p. 416, with an excellent figure, and pp. 422, 432.) As the rays are
bold, strong and voracious fish, we may suspect that the males require
their sharp teeth for fighting with their rivals; but as they possess many
parts modified and adapted for the prehension of the female, it is possible
that their teeth may be used for this purpose.
In regard to size, M. Carbonnier (11. As quoted in 'The Farmer,' 1868, p.
369.) maintains that the female of almost all fishes is larger than the
male; and Dr. Gunther does not know of a single instance in which the male
is actually larger than the female. With some Cyprinodonts the male is not
even half as large. As in many kinds of fishes the males habitually fight
together, it is surprising that they have not generally become larger and
stronger than the females through the effects of sexual selection. The
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