THE COMPOUND EYE 



159 



retinular cells, the axons of these cells do not conduct impulses even although 

 they form the majority of the fibres of the optic nerve, while this function is 

 taken over by other structures analogous to the bipolar cells of the vertebrate 

 retina, the electronic potentials induced by the primary i-eceptor process 

 giving rise to propagated impulses in closely contiguous conducting neurones. 

 It is obvious that many fascinating problems still remain to be elucidated. 



As in other evolutionary processes it cannot be said that a ciit- 

 and-dried differentiation exists between the simple and the compound 

 eye. Intermediate forms between the two may 

 be seen in some worms. On the one hand, as we 

 have already seen,i some sedentary poly chaste 

 worms and lamellibranch molluscs are provided 

 M ith structures superficially resembling a com- 

 pound eye, but since each element contains a 

 single visual cell they are more correctly termed 

 AGGREGATE EYES. On the other hand, the 

 stemmata of the larvse of most holometabolous 

 insects and the lateral ocelli of many adult 

 types such as butterflies and moths (Lepidop- 

 tera) and all the ocelli of springtails (Collem- 

 bola) have structures somewhat resembling the 

 single ommatidium of a compoimd eye, consist- 

 ing of a cornea, a crystalline lens and seven 

 retinular cells arranged around a central 

 rhabdome (Dethier, 1942-43 ; and others) ; 

 such an arrangement may be called a simple 

 OMMATiDiAL EYE (Fig. 138). The ventral eye 

 of Copepods forms a similar intermediate step 

 between an ocellus and an ommatidium. 

 Thus the female Ponfellojjsis regalis, for ex- 

 ample, has an eye composed of a single retinule of 6 cells arranged 

 in two groups of 3 (Vaissiere, 1954), while Copilia and its relatives have 

 a single group of 3 cells arranged around a rhabdome (Grenacher, 

 1879-80 ; Exner, 1891). 



These tiny crustaceans have unique eyes (Fig. 139) ; each is almost half 

 as long as the body and is pulled about in all directions with great rapidity 

 by muscles, a device presumably designed to increase its visual field. Moreover, 

 the optic nerve issues, not from the proximal end of the ommatidium, but from 

 its side. A similar point of exit for the optic nerve from the middle of the lateral 

 wall of the visual cells is seen in the anterior median ocelli of the common house 

 spider, Tegenaria domestica (Biitschli, 1921) (Fig. 107). 



True compound eyes, however, are seen only among the Arthro- 

 pods. They occur in several fossil forms (Trilobites, Eurypterids, 



Fig. 138.— The Simple 

 Ommatidial Eye of 

 THE Larva of the 

 Moth, Gaxtropacha 



RUBI. 



A lens and retinule are 

 arranged after the man- 

 ner of a single omma- 

 tidium (after Demoll). 



C, corneal lens ; Ep, 

 epithelial cell ; L, lens ; 

 M, mantle cell ; R, R, 

 visual cells ; Rh, rhab- 

 dome ; V, vitrellfe. 



Copilia 



Tegenaria 



p. 151. 



