332 



INSECT A. 



must be considered as the rudiment of the retina (r). From its 

 cuticular margin are derived the optic rods. Certain peculiarities 

 characteristic of the eye of Acilius now develop. The chief of 

 these is a slit traversing the retina perpendicularly (sp), which is 

 bordered by the horizontally-placed rods of the large retinal cells 

 (x) which lie next to it. In the further course of development 

 (Fig. 163) a flattening of the cup-like cavity occurs, owing to the 

 growth of the cells forming the base of the optic cup, and the rods 



belonging to these cells 

 P s ffk l consequently assume a 



more vertical position. The 

 retinal cells at the edge of 

 the cup, on the contrary, 

 curve inward and form an 

 inverted marginal layer (m) 

 with its rods directed 

 towards the base of the 

 retinal cup ; these cells may 

 be regarded as the rudiment 

 of a third layer intercalated 

 between the two principal 

 layers of the eye. 



The above would justify us in deriving the trilaminar Insectan eye from a 

 three-layered eye by the atrophy or incomplete development of the middle 

 layer. The original presence of three layers in the ocellus is, according to 

 Patten (No. 66), still more distinctly recognisable in the eye of the youngest 

 larvae of Hydropliilus (Fig. 164), in which the optic invagination presses into 

 the optic rudiment not from the middle, but from the edge and from the dorsal 

 side (Fig. 164 A). Even in later stages a vestige of the middle cell-layer (Fig. 

 164 B, m) is retained. According to Grenacher's observations (No. 151), thc- 

 ocelli of certain Insects appear to remain throughout life in a much more 

 primitive condition than would be expected from Patten's statements, the 

 optic vesicle in them never closing completely, and the layers of retinal cells 

 and of cells forming the lens remaining in direct continuity with the hypo- 

 dermis (e.g., Fig. 165). 



The statements of Patten do not agree with those of Carriere (No. 14). 

 If we rightly understand the latter author, in the development of the ocelli of 

 pupae of Chrysididae and Ichncumonidac, the separation of the retinal layer 

 from the lentigen layer takes place by delamination, while the optic invagination 

 which forms later develops according to the type of the cup-eye, and at the 

 same time stands in a certain relation to the development of the corneal lens. 



The larvae of the holometabolic Insecta are, as a rule, devoid 

 of compound lateral eyes (facet-eyes). These develop only in the 

 gradual transition to the imaginal stage. The larvae, on the contrary, 



Fig. 165.— Section through the eye of a Coleopteran 

 larva (Dytiscus) (after Grenacher, from Hatschek's 

 Text-book), c, chitinous cuticle ; I, corneal lens ; 

 h, hypodermis; pz, pigment - cells ; gk, vitreous 

 body ; r, retina ; b, basal membrane. 



