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Show 370 SEXUAL SELECTION. Par::r IT. A most remarkable distinction between the sexes. of many beetles is presented by the great horns which rise from the head, thorax, or clypens of the males ; and in some few cases from the under surface of th? body. These horns, in the great family of tbe Lamelh-corns, resem ble tllose Of Various quadrupeds, snch as stags, rhinoceroses, &c., and are wonderful both !r?m their size and diversified shapes. Instead of descnbmg them, I have given figures of the males ~nd fe~ales of some of the more remarkable forms. (Figs. lu to 1 9.) The females o·enerally exhibit rudiments of the horns in the form ~f small knobs or ridges ; but some arc destitute of even a rudiment. On the other hand, tho horns are nearly as well developed in the fem~le as in the male of Phanrous lancifer; and only a httl~ less well developed in the females. of some other spemes of the same genus and of Copr1s. In th? several sub~ divisions of the family, the differences m. structure of the horns do not run parallel, as I am mformed_ b.y 1\fr. Bates, with their more important and cha~actenstic differences· thus within the' same natural section of the genus Ontbophagus, there are s~e~ies which have either a single cephalic horn, or two d1stmct horns. In almost all cases, the horns are remarkable fr~m their excessive variability; so that a graduated ser1es can be formed from the most highly developed males to others so d;o-enerate that they can barely be distino- uished from the females. Mr Walsh 59 found that in Phanrous carnifex the horns were thrice as long in some males as in others. 1\Ir. Bates, after examining above a hunch·ed males of Onthophag~~s rangifer (fig. 19), thouo·ht that he had at last discoYered a species in 0 sn 'Proc. Entomolog. Soc. of Philadelphia,' 18G4, p. 228. 'CI!AP. X. COLEOPTERA. 371 which tho 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 h~ve been formed for some important purpose; but thell' 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 show marks of friction, as if used for any ordinary work. Some authors suppose r.o that as the males wander much more than the females, they require horns as a defence against their enemies; but in many cases the horns {;10 not seem well adapted for defence, as they are not sharp. The most obvious conjecture is that they are used by the males for fighting together; but they have never been observed to fight; nor could 1\fr. Bates, ~fter a c~reful e~amination of numerous species, find any suffiment evidence in their mutilated or broken condition of their having been thus used. If the males had been h~bitual fighters, their size would probably have been mcreased through sexual selection, so as to have e~ceeded that of the female; but Mr. Bates, after comparm.g the two sexes in above a hundred species of the Copndre, does not find in well-developed individuals any marked difference in this respect. There is, moreover, one beetle, belonging to the same great division of the Lamellicorns, namely Lethrus, the males of which are known to fight, but they are not provided with horns, though their mandibles are much laro·er than those of the female. ;:, The conclusion, which best agrees with the fact of the horns having been so immensely yet not fixedly .developed,-as shewn by their extreme variability in 60 Kirby and Spence, 'Introduct. Entomolog.' vol. iii. p. 300. 2 B 2 |
