SECONDARY SEXUAL CHARACTERS OF INSECTS.
Diversified structures possessed by the males for seizing the
females—Differences between the sexes, of which the meaning is not
understood—Difference in size between the
sexes—Thysanura—Diptera—Hemiptera—Homoptera, musical powers possessed
by the males alone—Orthoptera, musical instruments of the males, much
diversified in structure; pugnacity; colours—Neuroptera, sexual
differences in colour—Hymenoptera, pugnacity and odours—Coleoptera,
colours; furnished with great horns, apparently as an ornament;
battles, stridulating organs generally common to both sexes.
In the immense class of insects the sexes sometimes differ in their
locomotive-organs, and often in their sense-organs, as in the
pectinated and beautifully plumose antennae of the males of many
species. In Chloeon, one of the Ephemerae, the male has great pillared
eyes, of which the female is entirely destitute. (1. Sir J. Lubbock,
‘Transact. Linnean Soc.’ vol. xxv, 1866, p. 484. With respect to the
Mutillidae see Westwood, ‘Modern Class. of Insects,’ vol. ii. p. 213.)
The ocelli are absent in the females of certain insects, as in the
Mutillidae; and here the females are likewise wingless. But we are
chiefly concerned with structures by which one male is enabled to
conquer another, either in battle or courtship, through his strength,
pugnacity, ornaments, or music. The innumerable contrivances,
therefore, by which the male is able to seize the female, may be
briefly passed over. Besides the complex structures at the apex of the
abdomen, which ought perhaps to be ranked as primary organs (2. These
organs in the male often differ in closely-allied species, and afford
excellent specific characters. But their importance, from a functional
point of view, as Mr. R. MacLachlan has remarked to me, has probably
been overrated. It has been suggested, that slight differences in these
organs would suffice to prevent the intercrossing of well-marked
varieties or incipient species, and would thus aid in their
development. That this can hardly be the case, we may infer from the
many recorded cases (see, for instance, Bronn, ‘Geschichte der Natur,’
B. ii. 1843, s. 164; and Westwood, ‘Transact. Ent. Soc.’ vol. iii.
1842, p. 195) of distinct species having been observed in union. Mr.
MacLachlan informs me (vide ‘Stett. Ent. Zeitung,’ 1867, s. 155) that
when several species of Phryganidae, which present strongly-pronounced
differences of this kind, were confined together by Dr. Aug. Meyer,
THEY COUPLED, and one pair produced fertile ova.), “it is astonishing,”
as Mr. B.D. Walsh (3. ‘The Practical Entomologist,’ Philadelphia, vol.
ii. May 1867, p. 88.) has remarked, “how many different organs are
worked in by nature for the seemingly insignificant object of enabling
the male to grasp the female firmly.” The mandibles or jaws are
sometimes used for this purpose; thus the male Corydalis cornutus (a
neuropterous insect in some degree allied to the Dragon flies, etc.)
has immense curved jaws, many times longer than those of the female;
and they are smooth instead of being toothed, so that he is thus
enabled to seize her without injury. (4. Mr. Walsh, ibid. p. 107.) One
of the stag-beetles of North America (Lucanus elaphus) uses his jaws,
which are much larger than those of the female, for the same purpose,
but probably likewise for fighting. In one of the sand-wasps
(Ammophila) the jaws in the two sexes are closely alike, but are used
for widely different purposes: the males, as Professor Westwood
observes, “are exceedingly ardent, seizing their partners round the
neck with their sickle-shaped jaws” (5. ‘Modern Classification of
Insects,’ vol. ii. 1840, pp. 205, 206. Mr. Walsh, who called my
attention to the double use of the jaws, says that he has repeatedly
observed this fact.); whilst the females use these organs for burrowing
in sand-banks and making their nests.
[Fig. 9. Crabro cribrarius. Upper figure, male; lower figure, female.]
The tarsi of the front-legs are dilated in many male beetles, or are
furnished with broad cushions of hairs; and in many genera of
water-beetles they are armed with a round flat sucker, so that the male
may adhere to the slippery body of the female. It is a much more
unusual circumstance that the females of some water-beetles (Dytiscus)
have their elytra deeply grooved, and in Acilius sulcatus thickly set
with hairs, as an aid to the male. The females of some other
water-beetles (Hydroporus) have their elytra punctured for the same
purpose. (6. We have here a curious and inexplicable case of
dimorphism, for some of the females of four European species of
Dytiscus, and of certain species of Hydroporus, have their elytra
smooth; and no intermediate gradations between the sulcated or
punctured, and the quite smooth elytra have been observed. See Dr. H.
Schaum, as quoted in the ‘Zoologist,’ vols. v.-vi. 1847-48, p. 1896.
Also Kirby and Spence, ‘Introduction to Entomology,’ vol. iii. 1826, p.
305.) In the male of Crabro cribrarius (Fig. 9), it is the tibia which
is dilated into a broad horny plate, with minute membraneous dots,
giving to it a singular appearance like that of a riddle. (7. Westwood,
‘Modern Class.’ vol. ii. p. 193. The following statement about Penthe,
and others in inverted commas, are taken from Mr. Walsh, ‘Practical
Entomologist,’ Philadelphia, vol. iii. p. 88.) In the male of Penthe (a
genus of beetles) a few of the middle joints of the antennae are
dilated and furnished on the inferior surface with cushions of hair,
exactly like those on the tarsi of the Carabidae, “and obviously for
the same end.” In male dragon-flies, “the appendages at the tip of the
tail are modified in an almost infinite variety of curious patterns to
enable them to embrace the neck of the female.” Lastly, in the males of
many insects, the legs are furnished with peculiar spines, knobs or
spurs; or the whole leg is bowed or thickened, but this is by no means
invariably a sexual character; or one pair, or all three pairs are
elongated, sometimes to an extravagant length. (8. Kirby and Spence,
‘Introduct.’ etc., vol. iii. pp. 332-336.)
[Fig. 10. Taphroderes distortus (much enlarged). Upper figure, male;
lower figure, female.]
The sexes of many species in all the orders present differences, of
which the meaning is not understood. One curious case is that of a
beetle (Fig. 10), the male of which has left mandible much enlarged; so
that the mouth is greatly distorted. In another Carabidous beetle,
Eurygnathus (9. ‘Insecta Maderensia,’ 1854, page 20.), we have the
case, unique as far as known to Mr. Wollaston, of the head of the
female being much broader and larger, though in a variable degree, than
that of the male. Any number of such cases could be given. They abound
in the Lepidoptera: one of the most extraordinary is that certain male
butterflies have their fore-legs more or less atrophied, with the
tibiae and tarsi reduced to mere rudimentary knobs. The wings, also, in
the two sexes often differ in neuration (10. E. Doubleday, ‘Annals and
Mag. of Nat. Hist.’ vol. i. 1848, p. 379. I may add that the wings in
certain Hymenoptera (see Shuckard, ‘Fossorial Hymenoptera,’ 1837, pp.
39-43) differ in neuration according to sex.), and sometimes
considerably in outline, as in the Aricoris epitus, which was shewn to
me in the British Museum by Mr. A. Butler. The males of certain South
American butterflies have tufts of hair on the margins of the wings,
and horny excrescences on the discs of the posterior pair. (11. H.W.
Bates, in ‘Journal of Proc. Linn. Soc.’ vol. vi. 1862, p. 74. Mr.
Wonfor’s observations are quoted in ‘Popular Science Review,’ 1868, p.
343.) In several British butterflies, as shewn by Mr. Wonfor, the males
alone are in parts clothed with peculiar scales.
The use of the bright light of the female glow-worm has been subject to
much discussion. The male is feebly luminous, as are the larvae and
even the eggs. It has been supposed by some authors that the light
serves to frighten away enemies, and by others to guide the male to the
female. At last, Mr. Belt (12. ‘The Naturalist in Nicaragua,’ 1874, pp.
316-320. On the phosphorescence of the eggs, see ‘Annals and Magazine
of Natural History,’ Nov. 1871, p. 372.) appears to have solved the
difficulty: he finds that all the Lampyridae which he has tried are
highly distasteful to insectivorous mammals and birds. Hence it is in
accordance with Mr. Bates’ view, hereafter to be explained, that many
insects mimic the Lampyridae closely, in order to be mistaken for them,
and thus to escape destruction. He further believes that the luminous
species profit by being at once recognised as unpalatable. It is
probable that the same explanation may be extended to the Elaters, both
sexes of which are highly luminous. It is not known why the wings of
the female glow-worm have not been developed; but in her present state
she closely resembles a larva, and as larvae are so largely preyed on
by many animals, we can understand why she has been rendered so much
more luminous and conspicuous than the male; and why the larvae
themselves are likewise luminous.
DIFFERENCE IN SIZE BETWEEN THE SEXES.
With insects of all kinds the males are commonly smaller than the
females; and this difference can often be detected even in the larval
state. So considerable is the difference between the male and female
cocoons of the silk-moth (Bombyx mori), that in France they are
separated by a particular mode of weighing. (13. Robinet, ‘Vers a
Soie,’ 1848, p. 207.) In the lower classes of the animal kingdom, the
greater size of the females seems generally to depend on their
developing an enormous number of ova; and this may to a certain extent
hold good with insects. But Dr. Wallace has suggested a much more
probable explanation. He finds, after carefully attending to the
development of the caterpillars of Bombyx cynthia and yamamai, and
especially to that of some dwarfed caterpillars reared from a second
brood on unnatural food, “that in proportion as the individual moth is
finer, so is the time required for its metamorphosis longer; and for
this reason the female, which is the larger and heavier insect, from
having to carry her numerous eggs, will be preceded by the male, which
is smaller and has less to mature.” (14. ‘Transact. Ent. Soc.’ 3rd
series, vol. v. p. 486.) Now as most insects are short-lived, and as
they are exposed to many dangers, it would manifestly be advantageous
to the female to be impregnated as soon as possible. This end would be
gained by the males being first matured in large numbers ready for the
advent of the females; and this again would naturally follow, as Mr.
A.R. Wallace has remarked (15. ‘Journal of Proc. Ent. Soc.’ Feb. 4,
1867, p. lxxi.), through natural selection; for the smaller males would
be first matured, and thus would procreate a large number of offspring
which would inherit the reduced size of their male parents, whilst the
larger males from being matured later would leave fewer offspring.
There are, however, exceptions to the rule of male insects being
smaller than the females: and some of these exceptions are
intelligible. Size and strength would be an advantage to the males,
which fight for the possession of the females; and in these cases, as
with the stag-beetle (Lucanus), the males are larger than the females.
There are, however, other beetles which are not known to fight
together, of which the males exceed the females in size; and the
meaning of this fact is not known; but in some of these cases, as with
the huge Dynastes and Megasoma, we can at least see that there would be
no necessity for the males to be smaller than the females, in order to
be matured before them, for these beetles are not short-lived, and
there would be ample time for the pairing of the sexes. So again, male
dragon-flies (Libellulidae) are sometimes sensibly larger, and never
smaller, than the females (16. For this and other statements on the
size of the sexes, see Kirby and Spence, ibid. vol. iii. p. 300; on the
duration of life in insects, see p. 344.); and as Mr. MacLachlan
believes, they do not generally pair with the females until a week or
fortnight has elapsed, and until they have assumed their proper
masculine colours. But the most curious case, shewing on what complex
and easily-overlooked relations, so trifling a character as difference
in size between the sexes may depend, is that of the aculeate
Hymenoptera; for Mr. F. Smith informs me that throughout nearly the
whole of this large group, the males, in accordance with the general
rule, are smaller than the females, and emerge about a week before
them; but amongst the Bees, the males of Apis mellifica, Anthidium
manicatum, and Anthophora acervorum, and amongst the Fossores, the
males of the Methoca ichneumonides, are larger than the females. The
explanation of this anomaly is that a marriage flight is absolutely
necessary with these species, and the male requires great strength and
size in order to carry the female through the air. Increased size has
here been acquired in opposition to the usual relation between size and
the period of development, for the males, though larger, emerge before
the smaller females.
We will now review the several Orders, selecting such facts as more
particularly concern us. The Lepidoptera (Butterflies and Moths) will
be retained for a separate chapter.
ORDER, THYSANURA.
The members of this lowly organised order are wingless, dull-coloured,
minute insects, with ugly, almost misshapen heads and bodies. Their
sexes do not differ, but they are interesting as shewing us that the
males pay sedulous court to the females even low down in the animal
scale. Sir J. Lubbock (17. ‘Transact. Linnean Soc.’ vol. xxvi. 1868, p.
296.) says: “it is very amusing to see these little creatures
(Smynthurus luteus) coquetting together. The male, which is much
smaller than the female, runs round her, and they butt one another,
standing face to face and moving backward and forward like two playful
lambs. Then the female pretends to run away and the male runs after her
with a queer appearance of anger, gets in front and stands facing her
again; then she turns coyly round, but he, quicker and more active,
scuttles round too, and seems to whip her with his antennae; then for a
bit they stand face to face, play with their antennae, and seem to be
all in all to one another.”
ORDER, DIPTERA (FLIES).
The sexes differ little in colour. The greatest difference, known to
Mr. F. Walker, is in the genus Bibio, in which the males are blackish
or quite black, and the females obscure brownish-orange. The genus
Elaphomyia, discovered by Mr. Wallace (18. ‘The Malay Archipelago,’
vol. ii. 1869, p. 313.) in New Guinea, is highly remarkable, as the
males are furnished with horns, of which the females are quite
destitute. The horns spring from beneath the eyes, and curiously
resemble those of a stag, being either branched or palmated. In one of
the species, they equal the whole body in length. They might be thought
to be adapted for fighting, but as in one species they are of a
beautiful pink colour, edged with black, with a pale central stripe,
and as these insects have altogether a very elegant appearance, it is
perhaps more probable that they serve as ornaments. That the males of
some Diptera fight together is certain; Prof. Westwood (19. ‘Modern
Classification of Insects,’ vol. ii. 1840, p. 526.) has several times
seen this with the Tipulae. The males of other Diptera apparently try
to win the females by their music: H. Müller (20. ‘Anwendung,’ etc.,
‘Verh. d. n. V. Jahrg.’ xxix. p. 80. Mayer, in ‘American Naturalist,’
1874, p. 236.) watched for some time two males of an Eristalis courting
a female; they hovered above her, and flew from side to side, making a
high humming noise at the same time. Gnats and mosquitoes (Culicidae)
also seem to attract each other by humming; and Prof. Mayer has
recently ascertained that the hairs on the antennae of the male vibrate
in unison with the notes of a tuning-fork, within the range of the
sounds emitted by the female. The longer hairs vibrate sympathetically
with the graver notes, and the shorter hairs with the higher ones.
Landois also asserts that he has repeatedly drawn down a whole swarm of
gnats by uttering a particular note. It may be added that the mental
faculties of the Diptera are probably higher than in most other
insects, in accordance with their highly-developed nervous system. (21.
See Mr. B.T. Lowne’s interesting work, ‘On the Anatomy of the Blow-fly,
Musca vomitoria,’ 1870, p. 14. He remarks (p. 33) that, “the captured
flies utter a peculiar plaintive note, and that this sound causes other
flies to disappear.”)
ORDER, HEMIPTERA (FIELD-BUGS).
Mr. J.W. Douglas, who has particularly attended to the British species,
has kindly given me an account of their sexual differences. The males
of some species are furnished with wings, whilst the females are
wingless; the sexes differ in the form of their bodies, elytra,
antennae and tarsi; but as the signification of these differences are
unknown, they may be here passed over. The females are generally larger
and more robust than the males. With British, and, as far as Mr.
Douglas knows, with exotic species, the sexes do not commonly differ
much in colour; but in about six British species the male is
considerably darker than the female, and in about four other species
the female is darker than the male. Both sexes of some species are
beautifully coloured; and as these insects emit an extremely nauseous
odour, their conspicuous colours may serve as a signal that they are
unpalatable to insectivorous animals. In some few cases their colours
appear to be directly protective: thus Prof. Hoffmann informs me that
he could hardly distinguish a small pink and green species from the
buds on the trunks of lime-trees, which this insect frequents.
Some species of Reduvidae make a stridulating noise; and, in the case
of Pirates stridulus, this is said (22. Westwood, ‘Modern
Classification of Insects,’ vol. ii. p. 473.) to be effected by the
movement of the neck within the pro-thoracic cavity. According to
Westring, Reduvius personatus also stridulates. But I have no reason to
suppose that this is a sexual character, excepting that with non-social
insects there seems to be no use for sound-producing organs, unless it
be as a sexual call.
ORDER: HOMOPTERA.
Every one who has wandered in a tropical forest must have been
astonished at the din made by the male Cicadae. The females are mute;
as the Grecian poet Xenarchus says, “Happy the Cicadas live, since they
all have voiceless wives.” The noise thus made could be plainly heard
on board the “Beagle,” when anchored at a quarter of a mile from the
shore of Brazil; and Captain Hancock says it can be heard at the
distance of a mile. The Greeks formerly kept, and the Chinese now keep
these insects in cages for the sake of their song, so that it must be
pleasing to the ears of some men. (23. These particulars are taken from
Westwood’s ‘Modern Classification of Insects,’ vol. ii. 1840, p. 422.
See, also, on the Fulgoridae, Kirby and Spence, ‘Introduct.’ vol. ii.
p. 401.) The Cicadidae usually sing during the day, whilst the
Fulgoridae appear to be night-songsters. The sound, according to
Landois (24. ‘Zeitschrift für wissenschaft. Zoolog.’ B. xvii. 1867, ss.
152-158.), is produced by the vibration of the lips of the spiracles,
which are set into motion by a current of air emitted from the
tracheae; but this view has lately been disputed. Dr. Powell appears to
have proved (25. ‘Transactions of the New Zealand Institute,’ vol. v.
1873, p. 286.) that it is produced by the vibration of a membrane, set
into action by a special muscle. In the living insect, whilst
stridulating, this membrane can be seen to vibrate; and in the dead
insect the proper sound is heard, if the muscle, when a little dried
and hardened, is pulled with the point of a pin. In the female the
whole complex musical apparatus is present, but is much less developed
than in the male, and is never used for producing sound.
With respect to the object of the music, Dr. Hartman, in speaking of
the Cicada septemdecim of the United States, says (26. I am indebted to
Mr. Walsh for having sent me this extract from ‘A Journal of the Doings
of Cicada septemdecim,’ by Dr. Hartman.), “the drums are now (June 6th
and 7th, 1851) heard in all directions. This I believe to be the
marital summons from the males. Standing in thick chestnut sprouts
about as high as my head, where hundreds were around me, I observed the
females coming around the drumming males.” He adds, “this season (Aug.
1868) a dwarf 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 Müller 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, ‘Über die Heuschrecken in Südrussland,’ 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 für 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 für
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 für 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 in 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 für 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 Facultés 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. Müller (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 (Müller, 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. Müller
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’après 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. Müller (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 für 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.