90 ON THE ANATOMY OF THE FLY. 



demonstrated over the ocelli, but from the manner in which 

 they are developed, their structure is probably similar to that 

 of the compound corneas (Plate VII., fig 9). 



I think it quite probable that, in both the compound and 

 simple eyes of insects, the cornea consists of the protoderm and 

 mesoderm ; and the choroid or pigment layer of the modified 

 endoderm. If so, they are essentially tegumentary organs, 

 with the nervous structures piercing the endoderm, and ex- 

 panding between it and the mesodermic layer. 



Immediately behind the cornea are several layers of nerve 

 cells ; at least, I have not been able to distinguish any inter- 

 mediate structure, although a fluid probably intervenes during 

 life. 



The nerve cells form a cone, about l-600th of an inch in 

 depth, from the back of the cornea to the entrance of the optic 

 nerve. This is surrounded by a layer of hexagonal pigment cells, 

 filled with bright rose-coloured pigment, and containing well 

 marked nuclei. This choroid is surrounded by a plexus of 

 tracheal vessels (Plate VII., fig. 8). 



The ocelli first appear about the fourth day of the pupa 

 state, and the pigment on their posterior surface is seen in 

 beautiful hexagonal nucleated cells about two days later. 

 During the whole pupa stage they are nearly twice as large as 

 they are in the adult insect. In the same way, the legs and 

 several other organs are larger when first formed, for condensa- 

 tion and contraction frequently accompany the development of 

 organs in the pupa ; but as the ocelli are flat at this period, 

 something must be allowed to the subsequent convexity of the 

 cornea. 



