DIVIDED EYES OF CERTAIN INSECTS 469 



Zimmer, 1897). The large facetted dorsal eyes have been 

 called turban eyes and the smaller deeply pigmented eyes, the 

 lateral eyes. The females have only the small lateral pigmented 

 eyes. Zimmer, 1897, has given the histological structure of the 

 eyes of 7 genera of mayflies according to Pictet's classification 

 and he discussed also the physiological significance of the turban 

 eyes of these insects. 



The structure of the eyes of Callibatis hageni differs in only 

 a few points from that given by Zimmer for Cloe fuscata Pict. 

 It will be well, however, to describe briefly the structure of the 

 eye in the adult male of Callibcetis hageni before taking up the 

 development of the turban eye in that species. Microphoto- 

 graph 24 (a cross-section through the head) shows the relative 

 size, position, pigmentation and the general structure of the 

 right turban and lateral eyes. The large and small eye ele- 

 ments are entirely separated here by a deep, rather wide, groove. 

 A single partly divided optic ganglion lies beneath the right 

 turban and lateral eyes and a similar ganglion beneath the left 

 eye o-pg in Figs. 23, 25 and 26. Drawings in Fig. 16 show 

 more clearly the structure of 2 entire elements of the turban eye. 

 The light-gathering or dioptric apparatus consists of a corneal 

 lens, 16 Ac, a cone, Aco, and a hypodermal space between the 

 lens and the cone, 16 Ahs. The cornea is made up of rather 

 distinct convex lenses, Ac, which are continuous with each other. 

 The outer third of each of these lenses appears to be denser 

 than the inner two thirds. The cone is composed of 4 crystaline 

 bodies so closely associated along their inner faces that they 

 appear in all except cross-sections as one solid cone body with 

 its slightly convex base facing the cornea. This is the eucone 

 type of Grenacher, 1879. The outer faces of each crystaline 

 body are surrounded by the less dense protoplasm of the mother 

 cone cell and in this protoplasm just distal to the base of the 

 cone are the cone cell nuclei (Fig. 16, A, en). The cross-sec- 

 tion made just distal to the cone base B, shows the 4 cone cells 

 and their nuclei. The hypodermal space contains no nuclei, 

 and it is filled by transparent fluid only. Zimmer demonstrated 

 2 nuclei in this space for Cloe. He did not figure the nuclei in 

 this space for the eye of Bcetis cerea Pict., or for that of Chiro- 



