Sexual Modifications 327 



male smoky black. In both these instances the male 

 is clearly the divergent sex, and it is of great interest 

 to find that a primitive race of either species still 

 exists ; in the Shetland Isles the males of Hepialus 

 hiimuli are yellow-brown, while in Ireland the males 

 of Diaphora tm'tidica are, almost without exception, 

 white. These insular races doubtless represent the 

 ancestral forms of the two species ; it is hard to 

 suggest what can have led to the striking modifica- 

 tions which their males have undergone over the 

 greater part of their range. 



Among the Lyc^nida? already referred to, with their 

 numerous species with blue males and brown females, 

 it is probable that the male is the more highly modified 

 sex, such a butterfly as Polyommatus astrarche, in which 

 both sexes have brown wings, representing the ancestral 

 type of the group. There is much reason for believing 

 that dulness preceded brilliance in the evolution of 

 insect colour. The partial blue suffusion over the 

 wings of a female whose male's wings are entirely 

 blue indicates that he has advanced farther in the 

 development of colour than she. But whether the 

 action of natural selection keeps down the tendency 

 to adornment among the females, or the natural 

 vivacity of the male necessarily expresses itself in 

 brilliant colour, must for the present remain undecided. 

 The fact that in Ireland and the Scottish Highlands 

 the female of P. icarus is often almost as blue as the 

 male shows that the causes of these divergences cannot 

 lie entirely within the insects themselves. 



The specially developed sense-organs of male in- 

 sects are undoubtedly of use in helping them quickly 

 to find mates and pair. The highly complex feelers 

 of many male moths (fig. 7) give them a wonderful 

 power of discovering the position of the females. A 

 single captive female in a box will cause the "as- 



