CHAPTER X. 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 colours - 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 organs for locomotion, and often in their sense-organs, as in the pectinated and beautifully plumose antennæ of the males of many species. In one of the Ephemeræ, namely Chloëon, the male has great pillared eyes, of which the female is entirely destitute.1 The ocelli are absent in the females of certain other insects, as in the Mutillida, which are likewise destitute of wings. 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. 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 6 The 1 Sir J. Lubbock, Transact. Linnean Soc.' vol. xxv. 1866, p. 484. With respect to the Mutillidæ see Westwood, Modern Class. of Insects,' vol. ii. p. 213. 66 66 organs," it is astonishing," as Mr. B. D. Walsh 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, &c.) has immense curved jaws, many times longer than those of the female; and they are smooth instead of being toothed, by which means he is enabled to seize her without injury. 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 exceed"ingly ardent, seizing their partners round the neck "with their sickle-shaped jaws; "5 whilst the females use 2 These organs in the male often differ in closely-allied species, and afford excellent specific characters. But their importance, under 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 Phryganida, which present strongly-pronounced differences of this kind, were confined together by Dr. Aug. Meyer, they coupled, and one pair produced fertile ova. 88. 3The Practical Entomologist,' Philadelphia, vol. ii. May, 1867, p. 4 Mr. Walsh, ibid. p. 107. 5 Modern Classification of Insects,' vol. ii. 1840, p. 206, 205. Mr. Walsh, who called my attention to this double use of the jaws, says that he has repeatedly observed this fact. these organs for burrowing in sand-banks and making their nests. 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 waterbeetles (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 object. In the male of Crabro cribrarius (fig. 8.), 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." In the male of Penthe (a male; lower figure, female. genus of beetles) a few of Fig. 8. Crabro cribrarius. Upper figure, the middle joints of the antennæ are dilated and furnished on the inferior surface 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 sulcated or punctured and quite smooth elytra have been observed. See Dr. H. Schaum, as quoted in the 'Zoologist,' vol. v.-vi. 1847-48, p. 1896. Also Kirby and Spence, Introduction to Entomology, vol. iii. 1826, p. 305. . 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. ii. p. 88. with cushions of hair, exactly like those on the tarsi of the Carabidæ, "and obviously for the same end." In male dragon-flys, "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 In all the orders, the sexes of many species present differences, of which the meaning is not understood. One curious case is that of a beetle (fig. 9), the male of which has the left mandible much enlarged; so that the mouth is greatly distorted. In another Carabidous beetle, the Eurygnathus, we have the unique case, 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 f Fig. 9. Taphroderes distortus (much enlarged). Upper figure, male; lower figure, female. 8 Kirby and Spence, Introduct.' &c., vol. iii. p. 332-336. less atrophied, with the tibia and tarsi reduced to mere rudimentary knobs. The wings, also, in the two sexes often differ in neuration,1o and sometimes considerably in outline, as in the Aricoris epitus, which was shown 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." In several British butterflies, the males alone, as shewn by Mr. Wonfor, are in parts clothed with peculiar scales. The purpose of the luminosity in the female glowworm is likewise not understood; for it is very doubtful whether the primary use of the light is to guide the male to the female. It is no serious objection to this latter belief that the males emit a feeble light; for secondary sexual characters proper to one sex are often developed in a slight degree in the other sex. It is a more valid objection that the larvæ shine, and in some species brilliantly: Fritz Müller informs me that the most luminous insect which he ever beheld in Brazil, was the larva of some beetle. Both sexes of certain luminous species of Elater emit light. Kirby and Spence suspect that the phosphorescence serves to frighten and drive away enemies. Difference in Size between the Sexes.-With insects of all kinds the males are commonly smaller than the females; 12 and this difference can often be detected even in the larval state. So considerable is the difference 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 Hymenop.' 1837, p. 39-43) differ in neuration according to sex. 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. 12 Kirby and Spence, Introduction to Entomology,' vol. iii. p. 299. |