274 



INVERTEBEATA 



CHAP. 



visual rods, much smaller than the basal rods but of the same 

 character. Above these horizontal visual cells come cells bearing 



pigment, which form the upper part 

 of the wall of the pit. At the 

 edges of the pit are cells which 

 secrete a "glass body," that is a 

 mass of gelatinous refracting sub- 

 stance, whilst outside these again are 

 the cells which secrete the thicken- 

 ing of the ordinary cuticle termed 

 the lens (Fig. 219). 



When the larva leaves the water 

 and seeks a spot on land to bury 

 itself and undergo the pupal moult, 

 a fine pigmented line can be observed 

 surrounding each group of ocelli 

 like a horse-shoe. As development 

 proceeds, this horseshoe-shaped line 

 thickens, and the lenses of the ocelli 

 are torn away from the deeper 

 pigmented portions, since the lenses 

 belong to the larval cuticle which 

 is now being stripped off. The 

 horse-shoe pigmented area has now 

 increased in breadth till it has 

 become a broad crescent, and the 

 pigmented portions of the ocelli are 

 close to its concave border. 



Soon, before the final moult to 

 form the imago is completed, the 



Fig. 219. — Longitudinal section through 

 simple eye of larva of Bytiscits mar- 

 ginalis parallel to its shortest 

 diameter. (After Gtinther. ) 



eci, ordinary ectoderm : -n.f, nei've fibres ; 

 l.v.c, large visual cells ; l.v.r, large visual 

 rods; s,v.c, small visual cells; s.v.r, small 

 visual rods ; vit, vitelligenous substance. 



Fig. 220. — Lateral views of the head of the larva of Dytiscus margiTmlis, in three stages 

 of development, to show the gradual growth of the rudiment of the compound eye. 

 (After Giinther.) 



Letters as in Fig. 218. set, line of setae, marking posterior limit of eye area. 



remnants of the ocelli recede from the surface altogether, and the 

 pigmented area now becomes also circular in outline, and spreads over 



