Darwin's Theory of Sexual Selection 181 
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 pro- 
nounced 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 ver- 
milion and metallic greens prevail—that we are tempted to 
attribute the result to sexual selection. In the Ichneumonide, 
according to Mr. Walsh, the males are almost universally 
lighter-colored than the females. On the other hand, in the 
Tenthredinide the males are generally darker than the 
females. In the Siricide the sexes frequently differ; thus 
the male of Sivex_ juvencus is banded with orange, whilst the 
