376 W. WESCHE ON THE STRUCTURE OF THE EYE-SURFACE, 



There are examples in the Ephydrid Hydrellinae of pubescent 

 eyes, but these are common to both sexes ; I mention the fact as 

 this group retain some very archaic characters in the mouth 

 parts, and it bears on a later argument. 



On the Uses or Advantages of these Adaptations, and the 



Causes that have led to them. 



1. The Holoptic and Dichoptic Eyes. 



It seems agreed that a larger surface of eye-space is of advan- 

 tage to the possessor. It is also obvious, from that condition 

 prevailing in the Nemocera, that the dichoptic condition is the 

 primitive one, and that those families or genera exhibiting it simply 

 retain the ancient character. Space will not admit of an 

 inquiry into the causes that have prevented the acquirement of 

 the holoptic eyes, but I will mention that in the case of the 

 familiar crane-flies the males often possess longer legs than the 

 females ; so it may be that the struggle for the possession of 

 the female does not depend so much on sight as speed. That 

 there is a fierce struggle we know, as Westwood has recorded 

 several instances of the males of Tipula having been seen 

 fighting.* 



2 and 3. The Division of the Eyes and the Variations in 



the Sizes of the Lenses. 



An examination of the eyes of most insects will show that they 

 can only command a limited view of what is beneath the head. 

 In Bibio and Uilophus this limited view is further interfered with 

 by the palpi, which are long and four-jointed. The modification 

 of the eyes of the male in these genera gives him an increased 

 power of vision in this direction, and, as we have seen, he uses 

 the smaller or normal facets in the lower division of the eye. 



The eyes of the females, particularly in Dilophus, are but small 

 masses in proportion to the size of the body ; and as at one period 

 Modern Classification of Insects," vol. ii., 1840, p. 526. 



* <( 



