260 SEXUAL DIMORPHISM 



on the principle of sexual selection. It may perhaps be 

 suggested that, the emission of light serving as a guide to the 

 male, the female does not need to fly, and has therefore lost 

 her wings, through the cessation of selection with regard to 

 the wings. But unless the wings were positively disadvan- 

 tageous the cessation of selection alone could never cause 

 their disappearance in this case, where they have disappeared 

 only in one sex. It would be difficult to prove that the 

 wings became a positive disadvantage to the female. On 

 the view that disuse of the wings in the female only has led 

 directly to their degeneration, the evolution becomes intelli- 

 gible, and it is noteworthy that apterous females both in 

 Lepidoptera and Coleoptera occur chiefly in nocturnal species, 

 because these are naturally apt to develop to the extreme 

 their retiring and inactive habits. The glow-worms in both 

 sexes, and in both larval and perfect stages, feed on small 

 snails and slugs, which are to be found in damp grass, and 

 the wings are therefore only required by the males in seeking 

 the females. Here again, if the development of organs is as 

 a general principle associated with the sex or the time of life 

 in which functional stimulation of them occurs, we can under- 

 stand the sexual dimorphism, but on the selection principle 

 there is no reason why a variation occurring and surviving in 

 one sex should not be equally transmitted to the other. 



I do not profess to be able to explain the origin of 

 the property of phosphorescence in Zampyris, but this, al- 

 though more developed in the female, has been observed in 

 all the stages, and in both sexes of the insect. The greater 

 size of the eyes in the male indicates that the light of the 

 female is the indication by which he finds her. But apterous 

 females occur also in species which are not phosphorescent, 

 for instance in Drilus Jlavescens, a beetle of the same family. 



Hymenoptera. In some species of Bees, according to 

 Darwin, the mandibles of the males are larger than those of 



