Eyes 



91 



attachment are met with where there can be no question 

 of spiracles, as in non-tracheate Arthropods, or in the 

 median thoracic region (cockroach and many other 

 insects). Chitinous tunnels like those of Chironomus 

 occur in a gnat, Anopheles (fig. 62) ^ 



The compound eyes are large in both sexes, but some- Eyes. 

 what larger in the male than in the female. AVe estimate 

 the number of facets in each eye as 225-250 in the male, 

 considerably fewer in the female. The corneal facets are 

 hemispherical on 

 their outer faces, 

 thick, and pro- 

 duced internally 

 into prominences 

 which look like 

 crystalline cones, 

 though they are 

 really crystalline 

 cells, four to- 

 gether. The outer 

 layer of the facets 

 is distinct and 



separable. No crystalline cones are formed. The pig- 

 mented retinal cells form retinulae of six or sometimes 

 seven cells each. 



There are no functional simple eyes, but between the J^*^|^^ ""^ 

 compound eyes and near the top of the head are a pair of ^3"^^ "^ 

 small stalks, which in the pupa are connected with the 

 brain by a single median nerve. Dufour^ has described, 

 in the crane-ly (Tipula oleracea), a minute ocellarj^ 

 nerve terminated by a pigmented retina, and also a small 



^ Mr. C. 0. Waterhouse (Labium and Submentum in Certain MandibuJafe 

 Insects, 1895) mentions that certain beetles show a pair of pits on the 

 submentum or gula. In Corydalis these are connected by a tube with 

 two openings in front of the antennae on the upper side of the head, so 

 that one can see daylight through the head, and a fine wire can be passed 

 freely through. ^ 1851, p. 178. 



Fig. 62. — Horizontal section throngli head of Ano- 

 2)JieIes mactdiipcnnis, showing chitinous tunnels (c). 



