70 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. jy 



from the normal position and from contact with the central part 

 of the cornea. Under the corneagenous cells, or directly beneath the 

 cornea when the corneagenous cells are displaced, are four cells, 

 sometimes called the cells of Semper, that usually unite to form a 

 conical crystalline body (Cm) having its apex directed inward. The 

 four component cells are, therefore, usually known as the cone cells. 

 Proximal to the cone or the cone cells are the retina cells (Ret) of 



IP^Cl 



if \trCorCI 



Fig. 32 — Diagrammatic structure of anommatidium of the compound 

 eye. 



A, type of eye in which the corneal cells (C'orC/) lie immediately be- 

 neath the cornea, exemplified by the adult eye of Machilis and of many 

 crustaceans and by the immature eye of some insects. 



B, type of eye usual in adult insects in which the corneal cells 

 (CorCl) have become pigment cells surrounding sides of cone. 



C, cross-section of ommatidium through cone, showing the two 

 corneal pigment cells (CorCl) surrounding sides of cone. 



D, cross-section of ommatidium through retinula, showing rudimen- 

 tary eighth retinula cell (a) at surface of retinula and taking no part 

 in formation of rhabdom (Rhb). 



the ommatidium, which typically are slender and elongate, and con- 

 stitute a retinula with an axial rhabdom {Rhb) directly beneath the 

 apex of the cone. The primitive number of retinula cells in each 

 ommatidium is probably eight, but generally one is aborted or crowded 

 away from the axis (D, a), leaving seven as the typical number tak- 

 ing part in the formation of the rhabdom. The lower ends of the 

 retinulse rest upon the basement membrane (BM), and the fibers 

 of the optic nerves (Nv) entering the optic lobes of the brain pene- 

 trate the membrane to end in the ganglia of the lobes. 



