— 40 — 



A little more proximally from the cells of the crystalline 

 cone there are several accessory pigment cells. The number 

 of these cells is very variable in the different families of 

 Diptera; in Diopsidae to each ommatidinm correspond 

 at least four accessory cells. All of them Avedge in between 

 the ommatidia on the border between the pseudocone and 

 retinula (fig. 9. n. P) at the point, where the ommatidia are 

 most constricted. 



The pigment of the retina (figs. 9, 12 and 13, P. ret.) covers 

 the exterior surface of the retinulae in the form of a very 

 thin layer. It is composed of fine granules seen only at high 

 magnification. The retinulae of the anterior area of the eye 

 are provided with more developed retinal pigment which 

 passes into the tracheal spaces, and in some places slightly 

 connects the separate retinulae. 



Optic ganglia. 



The facetted eye of D i o p s i d a e may be compared with a 

 hollow hemispherical formation limited from the exterior by 

 the cornea, and from the interior by the basilar membrane. 

 It appears that the volume of the interior cavity of the eye 

 is comparatively small (cp. fig. 3). In other Diptera also pro- 

 vided with very convex eyes, e. g. Microchrysa, Chrysopihis, 

 Chrysogaster and many others, the interior cavity is so strongly 

 developed that it not only contains the exterior and interior 

 optic ganglia, but inside the outgrowth of the basilar mem- 

 brane there are large tracheal cavities. 



In Diopsidae the cavity inside the eye is occupied only 

 by the exterior optic ganglion (fig. 3, G. o. e.), and large tracheal 

 cavities are altogether absent. As concerns the interior optic 

 ganglion, it is situated in the widened distal end of the 

 eye-stalk. 



If we compare the ommatidia of the eye of Diopsidae 

 as a whole, with those of other Diptera, we shall notice tlie 

 following peculiarities. Firstly, the interior cavity of the eye 

 of Diopsidae is small not only with regard to the volume 

 of the optic ganglia, but also in relation to the thickness of 

 the eye measured by the length of the ommatidia. If we take 

 as a scale of the cavity of the eye the average half diameter 

 of the curve of the basilar membrane, this half diameter will 



