<h2><SPAN name="link2HCH0010" id="link2HCH0010"></SPAN> CHAPTER X.<br/> SECONDARY SEXUAL CHARACTERS OF INSECTS.</h2>
<p class="letter">
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.</p>
<p>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.</p>
<p>[Fig. 9. Crabro cribrarius. Upper figure, male; lower figure, female.]</p>
<p>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.)</p>
<p>[Fig. 10. Taphroderes distortus (much enlarged). Upper figure, male; lower
figure, female.]</p>
<p>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.</p>
<p>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.</p>
<h3>DIFFERENCE IN SIZE BETWEEN THE SEXES.</h3>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<h3>ORDER, THYSANURA.</h3>
<p>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.”</p>
<h3>ORDER, DIPTERA (FLIES).</h3>
<p>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.”)</p>
<h3>ORDER, HEMIPTERA (FIELD-BUGS).</h3>
<p>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.</p>
<p>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.</p>
<h3>ORDER: HOMOPTERA.</h3>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<h3>ORDER, ORTHOPTERA (CRICKETS AND GRASSHOPPERS).</h3>
<p>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.)</p>
<p>[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.</p>
<p>Fig.12. Teeth of Nervure of Gryllus domesticus (from Landois).]</p>
<p>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.</p>
<p>[Fig.13. Chlorocoelus Tanana (from Bates). a,b. Lobes of opposite wing-covers.]</p>
<p>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.)</p>
<p>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.</p>
<p>[Fig.14. Hind-leg of Stenobothrus pratorum: r, the stridulating ridge; lower
figure, the teeth forming the ridge, much magnified (from Landois).</p>
<p>Fig.15. Pneumora (from specimens in the British Museum). Upper figure, male;
lower figure, female.]</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<h3>ORDER, NEUROPTERA.</h3>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.)</p>
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