22 ILLINOIS BIOLOGICAL MONOGRAPHS [192 



The region of the vertex ventrad and mesad of each compound 

 eye is a gena. The size of the genae (ge) is dependent upon the location 

 of the compound eyes and the ventral extension of the head-capsule. 

 The figures show considerable variation in these respects. 



Compound Eyes and Ocelli. — The compound eyes (c. e) of a hypo- 

 thetical head are large oval structures located on the cephalo-lateral 

 aspects of the head-capsule. They cover from one-half to two-thirds 

 of the entire cephalic aspect and their caudal margins are adjacent to 

 the lateral margins of the head. The compound eyes of a majority of 

 the Diptera resemble in general the hypothetical type. The shape and 

 size vary considerably with the different species. Variations are most 

 prevalent in the families of the Orthorrhapha. This variability agrees 

 with the decided variability of other parts. In such genera as Tipula 

 (Pig. 95), Psorophora (Fig. 96), and Limnobia (Fig. 93) the compound 

 eyes are exceptional in that they extend onto the caudal aspect of the 

 liead. The variations in shape are well illustrated by the numerous 

 figures. 



The compound eyes show secondary characters in a greater number 

 of species than any other fixed or movable part. This sexual variation 

 is most prevalent among the Nematoeera and the Brachycera, and was 

 not observed in the Acalyptratae. Among the Calyptratae, slight 

 differences occur in Musca (Fig. 71 and 72) and Hydrotaea (Fig. 69 

 and 70). When sexual variation occurs, the eyes of the male are larger 

 than those of the female, and they are usually adjacent along a portion 

 of their mesal margins. Such species are said to be holoptic ; while all 

 the females, and some of the males, having the eyes distinctly separated, 

 are dichoptic. The extent of the holoptic condition depends upon the 

 size of the eyes and the location of the antennal fossae, as in Simulium 

 (Fig. 2 and 3) and Bibio (Fig. 13 and 14). In the male of Bibio the 

 compound eyes are adjacent along their mesal margin and the antennal 

 fossae (a. f ) are located ventrad of the eyes. The extent and nature 

 of the sexual variation is shown in the figures. Except in the case of 

 Empis the heads of the male and female have both been drawn when 

 decided differences are present. 



The facets or ommatidia of the compound eyes vary in number, 

 form, and size thruout the order. In the Nematoeera they are usually 

 large and not as closely compacted as in the Cyclorrhapha. An inter- 

 esting variation occurs in the male of Simulium, the facets (fa) of the 

 ventral half of the eye being smaller than those of the dorsal half. This 

 difference is also foi;nd in the female of Bibiocephala (Fig. 5). In the 

 male of Bibio (Fig. 154) the facets (fa) in the ventro-caudal portions 

 of the eyes are smaller than the others. The compound eyes of Bibio- 

 cephala and Blepharocera are divided into a dorsal and a ventral por- 



