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The Descent of Man and Selection in Relation to Sex

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.