72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. // 



times with one cell at the base of the ommatidium and the other 

 seven distal, sometimes with three in a basal layer and four in a 

 distal layer. As already noted, the primitive number of retinula 

 cells appears to be eight ; but one is commonly reduced in size and 

 separated from the rhabdom, leaving only seven taking part in the 

 formation of the latter (fig. 32 D), and in such cases, if one of 

 these becomes basal, only six appear in cross-sections through the- 

 distal part of the retinula. 



The^ommatidia are usually separated from one another by cells con- 

 taining a dark pigment. When this pigment extends through the en- 

 tire depth of the eye, it serves to isolate optically the individual omma- 

 tidia, and to make each a separate receptive element of the eye. The 

 efifect of light entering an eye thus divided into isolated tubes must 

 be to give a mosaic image of the exterior, and this is regarded as 

 the usual form of vision with diurnal insects. In the eyes of some 

 nocturnal species, however, it is said that on the decrease of light 

 the pigment condenses between the outer parts of the ommatidia, 

 allowing light rays from any one point to spread over the re- 

 tinulse of several ommatidia, thus giving a more effective vision 

 in dim illuminations. Usually but one set of pigment cells is de- 

 scribed, besides the corneal pigment cells, but in some eyes there 

 are two sets, a distal one and a proximal one. The distal pigment 

 cells invest the cone and the corneal cells and are conveniently dis- 

 tinguished as the iris pigment cells (fig. 32 A, B, C, IPgCl). The 

 proximal pigment cells surround and separate the retinulx and are 

 usually known as the retinal pigtnent cells (A, B, D, RPgCl), though 

 they may also embrace the base of the cone. The number of pig- 

 ment cells of both sets varies much in the eyes of different species 

 of insects. There is some confusion in the names applied by writers 

 to the several sets of pigment cells in the compound eye, including the 

 pigmented corneal cells, due in part to the lack of uniformity in the 

 cells themselves, and in part to the fact that it has not always been 

 recognized by reviewers that the corneagenous cells are the pigment 

 cells immediately investing the cone, except when they retain their 

 primitive subcuticular position. 



Compound eyes divided into two parts are common in several 

 orders of insects, and frequently the two parts differ in the internal 

 structure of the ommatidia, probably in accommodation to different 

 ranges of vision or different intensities of light. 



THE OCELLI 



The various forms of simple eyes of insects have probably been 

 developed independently of one another, and none of them has been 



