— 34 — 



(fig. 3, C) the facets seem to be plane, or plano-convex. The 

 form and size of the facets vary in different parts of the 

 snrface of the eye; the largest facets being those of the 

 anterior side of the eye, and not of the dorsal as is the case 

 in most insects (figs. 4 and 5). The form of the facets de 

 pends on their size. Figures 6 and 7 represent facets taken 

 from the same transverse section of the eye, but belonging* 

 to two different parts of the latter. The facets of the 

 anterior surface (fig. 6) are large, plane and concavo-convex, 

 with the interior side slightly convex. The facets of the 

 posterior surface (fig. 7) having the same thickness as the 

 abovementioned, are twice smaller with a comparatively 

 very convex external surface. However, between these two 

 forms there is a whole series of gradations, which do not allow 

 to distinguish an external differentiation of the eye into two 

 different parts, as is the case in the so-called double eyes. 

 Towards the posterior edge of the eye the facets undergo 

 a still greater change, becoming with a small size biconvex. 



The Pseudocone. 



Pseudocone is the name given to a formation especially 

 peculiar to the eyes of Diptera. It consists of a more or 

 less fluid substance lying under the facets (fig. 3, 9 and U, 

 psc). Where it borders with the facets, the pseudocone still 

 remains hexagonal (fig. 11, on the left), but deeper it gradu- 

 ally rounds, becomes thinner, and assumes a conical shape 

 in this form the substance of the pseudocone is maintained 

 by the two chief pigment cells surrounding it (P^). Proximally^ 

 i. e. deeper inside, in front of the apex of the pseudocone 

 there are four crystalline cells the nuclei of which (Semper's 

 nuclei) are always clearly visible in my preparations (n. s.). 

 On the contrary, the pseudocone itself is badly preserved in 

 my preparations— it shrinks very much and, probably, consists 

 of a very viscous, fluid mass. On the anterior surface of the 

 eye the facets are larger, and the pseudocone is of a consi- 

 derable size (fig. 4 and 5). 



The origin of the pseudocone is still interpreted by diffe- 

 rent authors in a different manner. Grenacher considers 

 it to be the product of secretion of the four crystalline cells, 

 i. e. as homologous to the crystalline cone of other insects. 



