In the Diptera, or flies, Wallace records one interesting case of sexual difference in the genus Elaphomyia of New Guinea, in which the males are furnished with horns, which the females lack. Darwin writes:—
“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 color, 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.”
Presumably, therefore, Darwin means these colored horns have been acquired by sexual selection.
In the Hemiptera, or bugs, both sexes of some species are “beautifully colored,” and as the members of the group are often unpalatable to other animals, the color in this case is supposed to act as a warning signal.
In other cases it is stated, however, that “a small pink and green species” could hardly be distinguished from the buds on the trunks of the lime trees which this insect frequents. In this case the color appears “to be directly protective.” Thus without any means of forming a correct judgment, the color of one animal is supposed to be the result of natural selection, since it resembles its surroundings, but of sexual selection if the color is present or more pronounced in one sex. Where neither view can easily be applied, the color is ascribed in a general way to the nature of the organism.
In respect to the group of Hymenoptera, or bees, Darwin records the following cases:—
“In this order slight differences in color, 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 colored—for instance in Chrysis, in which vermilion and metallic greens prevail—that we are tempted to attribute the result to sexual selection. In the Ichneumonidæ, according to Mr. Walsh, the males are almost universally lighter-colored than the females. On the other hand, in the Tenthredinidæ the males are generally darker than the females. In the Siricidæ 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.”