THE OCELLI AND THE COMPOUND EYES. 



333 



possess a number of laterally placed ocelli (very often six). The 

 question now arises as to the relation of the compound lateral eyes 

 of the imago to the ocelli of the larva. It is certain that the latter 

 degenerate and are not taken over into the imago. In the pupa of 

 the Lepidoptera, the degenerating ocelli can be seen detached from 

 the hypodermis, and drawn back into the interior of the larva on the 

 optic nerve as on a stalk (Carriere, 

 No. 147). Since, at this time, the 

 rudiment of the compound eye can 

 be seen as a hypodermal thickening, 

 it might be thought that the latter 

 was altogether a new acquisition. 

 But, according to Patten's observa- 

 tions on Acilius, there seems to be 

 a certain relation between the larval 

 and imaginal eyes. In Acilhis, the 

 highly developed and complicated 

 larval eye (the first) has a peculiar 

 dorsal appendage, which perhaps 

 represents the vestige of an ocellus. 

 The hypodermal thickening, which 

 leads to the development of the 

 imaginal lateral eye, develops first in 

 the neighbourhood of this appendage. 

 In later stages, this rudiment forms 

 a thickened band which almost com- 

 pletely surrounds the six ocelli. This 



position perhaps favours a view which regards the complex of the 

 six larval eyes and the compound eye that develops later merely as 

 differently developed parts of one and the same optical area. We 

 should here recall Grenacher's view, according to which the omma- 

 tidia of the compound eye on the one hand, and the ocelli on the 

 other, represent merely different ontogenetic forms and grades of 

 development of one and the same type of eye (chap, xxviii.). 



The frontal ocelli of the imagines of many Insecta have nothing to do with 

 the larval ocelli. Against Patten's view that they may perhaps stand in nearer 

 relation to the compound eyes we might adduce the independent condition of 

 their innervation. These ocelli are often three in number. Patten (No. 66) 

 observed in Vespa that the median unpaired ocellus is derived by fusion from 

 a paired rudiment. 



The details of the development of the compound lateral eyes 

 (fan-shaped or facet-eyes) are so far chiefly known in connection 



Fig. 1C6.— Section through the rudi- 

 ment of the compound eye oUVcspa 

 (after Carriere, from Hatschek's 

 Text-bool). 1, cylindrical cells (later 

 accessory pigment-cells); 2, crystal- 

 line cone-cells ; 3, principal pigment- 

 cells ; U, retinulae ; 5, nerve, which 

 gives off branches to the different 

 ommatidia. 



