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



In the Cyclorrapha we find the large family of the Syrphidae 

 with the lenses still well defined, and separated by chitinous 

 bands ; but this is markedly less in the Conopodae, and in the 

 Muscidae the opaque structure has completely disappeared, so 

 that in prepared specimens it is, in the majority of cases, 

 difficult to see the lenses clearly. 



Summary. 



A number of observations have been recorded (made on about 



forty-two families and a number of genera) with regard to 



the eye-structure. From these it is assumed that the highly 



chitinised plate, pierced with circular apertures for the lenses, 



is the most primitive form, which is quite in agreement with 



observations on insects of very ancient type in other orders — 



insects which possess only simple eyes — that as we ascend in 



the scale of the flies, the facets begin to be hexagonal and there 



is much less opaque chitin, until finally, in specialised insects 



like the Blow-fly (Calliphora), the opaque structure has quite 



disappeared, permitting all the rays of light to enter the eye. 



One very remarkable eye shows these stages in transition ; this 



is the eye of a male, the female retaining the more archaic type 



Most of the large families include species which have attained 



conditions similar to those which we find in the Muscidae, the 



family Empidae furnishing examples, in three different genera, 



of the transitional stages. 



The Sexual Characters. 

 1. The Holojrtic and Dichoptic Eyes. 



Little need be said on this subject except that the holoptic eye 

 is rare in the Nemocera, my only examples being in a Cecido- 

 myid, Bibio, Dilophns, and Simulium ornatum, Mg. 



In the Brachycera the holoptic eye is more often met with 

 than the dichoptic, and in the Pipunculidae and the Cyrtidae the 

 character has even gone beyond that condition, as the whole head 

 in both sexes is occupied by the eyes, a slightly wider separation 



