THE SIMPLE AND COMPOUND EYES OP INSECTS. 
579 
suspect that the function of the ocelli is the perception of the intensity and the 
direction of light rather than vision in the ordinary acceptation of the term. 
II. On the Structure of the Compound Eye in Tipula. (Figs. 2 to 5.) 
The eye in Tipula oleracea is intermediate in structure between a true compound 
eye and a collection of ocelli. 
The curvature of the common cornea is nearly hemispherical. It is divided into a 
number of strongly convex hexagonal facets, each of which is ToVolh of an inch in 
diameter, and -^Wth of an inch in thickness in its thinnest part. The outer surface 
is more strongly curved than the inner. The axes of the adjacent lenses make an 
angle of from four to five degrees with each other. Each lens is surrounded by a 
deeply pigmented portion of the cornea, which forms a black hexagonal framework 
between the lenses. 
Beneath each lens there are sixteen roddike cells (a"), which are easily distinguished 
in the immature imago. 
In the mature imago these cells are so strongly pigmented with deep black pigment, 
that even in the thinnest sections I have been unable to detect the divisions between 
them ; neither do they exhibit any transparent openings in transverse sections. I 
have found this to be the case both in specimens hardened in chromic and osmic 
acids. 
Between each of these opaque cells and the facet of the cornea is a minute higlily- 
refractive globule (a'), of a bright purple colour. These cells bring to mind the highly 
pigmented retinal cells of the Pigeon.'"' 
Beneath the rod-cell layer is an elongated chamber containing a very remarkable 
structure, the “retinula” of Dr. Grenacher, which I shall name the facellus (/). 
The facellus consists of seven fusiform cells, the outer extremities of which terminate 
in fine hair-like points, which appear to pass into the rod-like cells of the more super- 
ficial layer. The points of the cells of the facellus are the extremities of fine highly- 
refractive threads, which pass through the fusiform cells of which it is composed, and 
are prolonged through the long cylindrical organ which connects the facellus with the 
ganglionic retina. These axial threads are easily distinguished in transverse sections 
through the facellus (figs. 4 and 4a). 
The cells of the facellus appear to become chitinous in the fully-formed imago, and 
are yellow in specimens hardened in chromic acid ; like all the highly-refractive struc- 
tures of the eye, including the cornea, they resist the action of solutions of caustic 
alkali for a considerable time. In the immature imago they are slightly granular, 
especially near their surface, but they contain no pigment. 
A strong chitinous membrane (m) separates the parts already described from the 
* Max Schultze, Archiv., bd. iii. 
