AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 377 



of evolution the eyes of the males were no larger, the gradual 

 acquirement of the character as we now find it must have been 

 of great advantage. 



We see, in the related Scatopse, that the eyes of both the 

 sexes have proportionately enormously increased ; therefore this 

 character would not be of the particular advantage in these flies 

 that it is in Bibio and Dilophus, and consequently has not estab- 

 lished itself in the genus or in the other families of the order. 

 The influence of the character of the genitalia in these genera 

 will be referred to later. 



The second problem before us is, What part do the larger facets 

 play ? Are they simply caused by an enlargement of space — i.e. 

 a spreading out of the lenses to take up the larger surface of 

 the holoptic eye ? It is certainly the fact that these larger 

 facets cover the areas which are bare of facets in the female ; 

 but as we find the whole head completely covered with facets in 

 Oncodes, and there is no difference, either sexual or in the sizes 

 of the lenses, these facts negative the idea. 



Or are they modified to give the male a long and a short sight, 

 that is to say, two foci of vision ? Do the smaller afford a more 

 powerful magnification, possibly a means of discrimination, so 

 that the true species is mated with ? 



The females in Bibio are in many species dark red, or, at 

 any rate, differ in colour from the males, which are a deep 

 black ; and some connection might be thought to exist between 

 the colour and the highly modified sight, but this is at once put 

 out of consideration by the fact that the females of Dilophus are 

 as black as the males. It has occurred to me that the double eye 

 might have arisen as a compensation for the failure of another 

 sense, such as the olfactory pits on the antennae or the sense- 

 organs that are sometimes found on the male genitalia ; for 

 instance, those on the forcipes inferiores of Tipula oleracea, L. 



The number of joints in the antennae appear variable, 

 especially in Dilophus, and though the olfactory organs are 

 very well marked in the males, they are even more marked in 



