THB COMPOUND EYES. 87 



concerned in its formation. At present nothing certain can 

 be said on the subject, although I incline entirely to the 

 former view. 



The cones and rods form a layer rather more than 1-1 00th of 

 an inch in thickness beneath the cornea, and are so arranged 

 that all the rods stand at right angles to their respective 

 facets. The layer of nerve cells behind the rods, is connected 

 with the cephalic nerve centre by a number of very clear nerve 

 filaments, these are about l-2500th of an inch in diameter. 

 I shall call them collectively the optic nerves (Plate VII., figs. 

 1 and 5). 



A thick chitinous ridge, probably the anterior margin of the 

 dorsal plate of the fifth segment, surrounds the anterior edge 

 of the compound cornea within, and bears an oval ring of 

 chitine connected with it below by a narrow pedicle. This 

 ring is very similar to the valves of the spiracles, and is 

 jointed like them, both above and below. A thin loose mem- 

 brane unites its outer edge with the edge of the cornea all 

 round. A narrow tendon is connected with its posterior half 

 about midway between its upper and lower joints ; by means of 

 which a small round muscle, which arises from the margin of 

 the occipital foramen, is inserted into it ; I shall call it the 

 epineuron* (Plate VIII., fig. 2). 



A large tracheal sac, as has been already stated, spreads out 

 behind the cephalic nerve centre, and almost surrounds the 

 optic nerve. This sac sends a vast number of small tracheal vessels 

 between the nerves and the epineuron, which dip into the spaces 

 between the rods, forming a dense plexus around their inner ex- 

 tremities. A long slightly tortuous branch is given off from this 



* In giving this name I have been guided by its similarity to the 

 epitrema, to which I believe it is homologous. 



