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 Ichneumonidae, 

 according to Mr. Walsh, the males are almost universally 

 lighter-colored than the females. On the other hand, in the 

 Tenthredinidse the males are generally darker than the 

 females. In the Siricidae the sexes frequently differ ; thus 

 the male of Sirex juvewus is banded with orange, whilst the 



