ECTODERMAL DERIVATIVES 



109 



the end of the first instar the cuticular lens first formed is shed with the 

 body cuticula, the new one secreted by the vitreous body taking its place. 

 Entad of the vitreous body, between it and the retina, a large irregularly 

 shaped crystalline body is formed, possibly by the cells of the vitreous 

 body. The definitive ocellus thus consists of four parts : the lens secreted 

 by the vitreous body, the vitreous body, the crystalline body possibly also 

 secreted by the vitreous body, and the retina. The retinal cells are at 

 first nucleated, but the nuclei probably pass to the nerve fibers each one 

 of which is connected to a retinal cell. Ocelli developed in the embryo 

 remain unchanged throughout the insect's life. 



Precursors of the compound eyes of the holometabolous insects are 

 the simple eyes (stemmata, adaptive ocelli) of the larvae. Patten's 



•' .vit 



Fig. 51. — Adaptive ocellus of Acilius larva. 

 (n) Nerve, {ret) Retina, {rod) Cuticxilar rods. 

 {x) Retinal cells bordering slit. {From Patten.) 



{ep) Epidermis. {I) Rudiment of lens. 

 {si) Slit in retina, {vit) Vitreous body. 



account of the development of these structures in the embryo of Acilius, 

 one of the water beetles, has shown that the later developmental stages 

 occur after hatching. In the larva of Acilius there are three pairs of 

 stemmata on each side which differ more or less from one another in 

 structure. The ventral stemma of the third pair, being somewhat typical, 

 will be described here. The rudiment, which at an early stage reminds 

 one of the cup-shaped eyes of some mollusks, consists of a simple pit-like 

 depression in a thickened part of the epidermis (Fig. 51A). The deep 

 cells which form the wall of the pit and which are continuous with the 

 thinner adjacent body wall (ep) are arranged in a single layer. The distal 

 end of the elongate cells have a cuticular margin (rod) and at their inner 

 ends give off nerve fibers which unite to form the optic nerve (n) . Before 

 this stage is reached the larva has emerged from the egg. Subsequently, 

 the eye pit closes toward the exterior (Fig. 51B), the epidermal marginal 

 parts pushing toward the center of the pit until they meet. Thus an 

 eye cup is formed which is two-layered, the central part of the outer 

 layer (vit) becoming the lenticular layer or vitreous body, while the 



