The Nervous System and the Senses 
167 
the outer surface, these units (ommatidia) are indicated 
by hexagonal facets in the chitinous covering of the eye. If 
a section is cut through the entire eye of 
a worker bee (including the optic lobes), 
the structure is that shown in Fig. 86. 
Numerous ommatidia are shown in full 
length and beneath these are the optic 
lobes, which need not be described here. 
An examination of a single ommatidium 
(Fig. 87) shows the following details of 
structure: (1) an outer corneal lens of 
chitin (CL) continuous with the chitin 
of the head, (2) the crystalline cone (CC) 
and (3) the rhabdome (rhb) surrounded by 
eight or nine sense cells or retinulee (ret). 
Surrounding the ommatidia are two types 
of pigment cells, (1) the corneal pigment 
cells (c.-p.c.), which in the pupal stage 
secreted the chitin of the corneal lens, 
and (2) the outer pigment cells ( o.-p.c .). 
So far as can be determined, the functions 
of these parts are as follows: rays of light 
pass through the lens and crystalline cone 
cells and enter the transparent rhabdome 
where the stimulus is received. Any rays 
of light which enter obliquely or which 
strike the edge of the crystalline cone are 
absorbed by the surrounding pigment cells 
so that it seems probable that only those 
rays which strike the surface of the eye 
at a right angle ever reach the sense cells. 
There is no apparatus for changing the 
focus of the lens. 
The type of image formed by the com¬ 
pound eye has been the subject of con¬ 
siderable speculation. The two theories 
on this subject are (1) that each facet forms a separate 
- CL 
- CC 
rhb. 
.ret.n. 
a-pc. 
_BM 
Fio. 87. — Section 
of entire omma¬ 
tidium. 
