138 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 154. — A diagram illustrating the structure of 

 a primary ocellus; c, cornea; c. hy, corneal 

 hypodermis; ret, retina; n, ocellar nerve; p, 

 accessory pigment cell; r, rhabdom. 



the two kinds forming a single layer of cells beneath the 

 cornea; but in the ocelli of insects, the sense-cells form a distinct 



layer beneath the hypo- 

 dermal cells. In this 

 type of ocellus the fol- 

 lowing parts can be dis- 

 tinguished : 



The cornea. — T h e 

 cornea (Fig. 154, c) is a 

 transparent portion of 

 the cuticula of the body- 

 wall ; this may be lenti- 

 cular in form or not. 



The corneal hypoder- 

 mis. — The hypodermis 

 of the body- wall is con- 

 tinued beneath the 

 cornea (Fig. 154, c. hy.); 

 this part of the hypo- 

 dermis is termed by 

 many writers the vitreous 

 layer of the ocellus; but the term corneal hypodermis, being a self- 

 explanatory term, is preferable. Other terms have been applied to it, 

 as the lentigen layer and the corneagen, both referring to the fact that 

 this part of the hypodermis produces the cornea. 



The retina. — Beneath the corneal hypodermis is a second cellular 

 layer, which is termed the retina, being composed chiefly or entirely of 

 visual cells (Fig. 154, ret). 



The visual cells of the retina are grouped, as described above (Fig. 

 153), so that the rhabdomeres of several of them, two, three or four, 

 unite to form a rhabdom; such a group of retinal cells is termed a 

 retinula. 



The visual cells are nerve-end-cells, each constituting the termina- 

 tion of a fiber of the ocellar nerve, and are thus connected with the 

 central nervous system. 



Accessory pigment cells. — In some ocelli there are densely pig- 

 mented cells between the retinulas, which serve to isolate them in a 

 similar way to that in which the retinula of an ommatidium of a com- 

 pound eye is isolated (Fig. 154, p). Even in cases where accessory 

 pigment cells are wanting a degree of isolation of the rhabdoms of thd 

 retinulae of an ocellus is secured by pigment within the visual cells 

 (Fig. 153, p). 



