110 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



peripheral parts become the pigmented iris. The cuticula covering the 

 rudiment gradually thickens in a chitinous lens (I). The deeper cup-like 

 layer forms the retina {ret) from the cuticular margins of which the optic 

 rods are derived. A slit (si) traversing the retina is bordered by the 

 horizontally placed rods of the large retinal cells (x) which lie next to it. 

 Not until later larval stages do the stemmata become fully developed. 

 Whether the ocelli of insects originate by an invagination or by delamina- 

 tion of the epidermis has been the subject of some controversy. Both 

 of the examples given above are of the first type. It is true also for 

 Hydrophilus (Fig. 52) (Patten, 1887) as well as for Peri-planeta, but in 



mid 



Fig. 52. — Hydrophilus. Ocellus of newly hatched larva, (ep) Epidermis. (/) Lens. 

 {mid) Middle layer of optic rudiment, {ret) Retinal layer, {vit) Vitreous body (lentigen 

 layer). {From Patten.) 



the latter case development of the ocelli is wholly postembryonic. 

 The eye in Julus likewise develops postembryonically. 



Imaginal Disks. — Although not usually thought of in connection 

 with the embryonic development, it should not be overlooked that some 

 imaginal histoblasts become obvious before the holometabolous insect 

 hatches from the egg. Pratt (1900) has shown that three imaginal disks 

 are already present in the head of Melophagus ovinus in the embryo in 

 the form of simple ectodermic thickenings. One of these lies in front of 

 the stomodaeum, and a pair behind it. In the embryo, likewise, three 

 pairs of dorsal and three pairs of ventral imaginal disks appear. The 

 latter give rise to the imaginal legs ; of the former the first two pairs remain 

 rudimentary; the last pair forms the imaginal halteres. Likewise also 

 during embryonic life, but only shortly before the rupture of the egg 

 envelopes, two pairs of disks appear in front of the anus for the forma- 

 tion of external genitalia. Imaginal disks for the formation of the 

 abdominal hypodermis and of the internal imaginal organs first become 

 recognizable as such during postembiyonic life. 



