The Nervous System and the Senses 



167 



CL 



CC 



rhb. 



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 retinulae (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 



a-p.c. 



-BM 



Nv 



^ Section 



tidium. 



