The Nervous System and the Senses 



167 



--CL 



CC 



OrftC. 

 C.-p.C. 



hb. 



ret. 



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 crystaUine cone (CC) 

 and (3) the rhabdome (rhb) surrounded by 

 eight or nine sense cells or retinula {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 



-BM 



Nv 



Fig. 87. — Section 

 of entire omma- 

 tidium. 



