THE EYES OF INSECTS 



121 



cells cohere to form the cone, the vesicles in them also coalescing to form 

 a single retractile body. The cells which will form the retinula retreat 

 from the surface, each group of these consisting of eight cells, one central 



Fig. 83. — Diagram showing the Formation of an Inverted Image on the 

 Retina of a Typical Lateral Eye of a Vertebrate. 



and seven peripheral. One of these peripheral cells is squeezed out from 

 between the rest, while the central cell and the remaining six peripheral 

 cells co-operate in forming one long visual rod or rhabdome. The lower 

 end of the rod has the form of a flask-shaped basal cell, while its upper 



Fig. 84. 



Diagram showing the course of rays of light from three points — x, y, z — through 

 nine visual rods (supposed to be empty tubes) A to /, of a compound eye ; a 

 to i : nerve-fibres connected with the visual rods. The arrows have been 

 inserted to indicate that the retinal image would not be inverted as in the 

 vertebrate eye (Fig. 83). 



(Slightly modified after T. H. Huxley.) 



end thins out into a tapering process before reaching the upper limit of 

 the retinula cells (Fig. 80). 



In both ocelli and in the adult compound eyes (Fig. 81), the nerve 

 fibres of the optic nerve originate as basal outgrowths from the retinal 

 cells. 



± (library 



