ARTEROPOJDA. 



405 



facet corresponds to a small chitinous leus (the number of which 

 varies, in different species, between a dozen and several thousand). 



1234. 5 



Fig. 408.— Section of compound eye of Forflcula. (After Carrltoe, from Hatscliek.) 

 i, cuticula. producing the cornea of manj^ lenses over the eye; '2, epidermis, 

 ^vliich in the eye forms the ommatidia; -3, basal membrane; i, reentrant chitin- 

 ous fold C sclerotic'); .5, rudimentary larval eye. 



and bounds the eye externally, whence this layer is called the 

 cornea (fig. 408). The part of the eye be- 

 neath tJie cornea consists of radially arranged 

 prismatic parts or ommatidia which corre- 

 spond in numlier and position to the facets, 

 tlieir broader ends being placed beneath the 

 facets, their narrower internal ends con- 

 necting with fibres of the optic nerve which 

 go to the brain. Each ommatidium (fig. 

 409) has essentially the structure of an 

 ocellus; (1) the lens (/) with its epi- 

 tlielium; {"2) tlie vitreous body {kz); (3) the 

 retiunla (z'i). The vitreous body is usually 

 comjiosed of four cells which iu the so-called 

 cnconous eyes surround a transparent body, 

 tlie crystalline cone {k), secreted by these cells. 

 Tile retinular cells are almost always seven in 

 number, each bearing on its inner surface a 

 rhabdome (/•), the seven rhabdomes frequently 

 fusing into a common mass. Each omma- 

 tidium is surrounded by a piigment sheath, 

 isolating it optically from its fellows. 



From this it api^ears that the compound 

 eve may Ije regarded as a complex of ocelli. 

 'J'his anattmiical conception must not, however, obscure the physio- 



FiG. 409.— A single om- 

 matidium (witli sec- 

 tions) of a compound 

 eye. fc, crystalline 

 cone; l-z, cone cells; /, 

 lens with hypodermis; 

 r. rhabdomes; rz, re- 

 tinular cell. 



