140 AN INTRODUCTION TO ENTOMOLOGY 



which constitute the corneal hypodermis of the ommatidium. These 

 cells are quite distinct in Machilis and their nuclei are prominent 

 (Fig. 155, hy); but in many insects they are greatly reduced, and 

 consequently are not represented in many of the published figures 

 of compound eyes. 



The crystalline-cone-cells. Next to the corneal hypodermis there 

 are four cells, which in one type of compound eyes, the eucone eyes, 

 form a body known as the crystalline -cone, for this reason these 

 cells are termed the crystalline-cone-cells (Fig. 155, cc). Two of 

 these cells are represented in the figure of a longitudinal section 

 and all four, in that of a transverse section. In each cell there is a 

 prominent nucleus at its distal end. 



The iris -pigment-cells. Surrounding the crystalline-cone-cells and 

 the corneal hypodermis, there is a curtain of densely pigmented cells, 

 which serves to exclude from the cone light entering other ommatidia ; 

 for this reason these cells are termed the iris -pigment (Fig. 155, i). 

 They are also known as the distal retinula cells; but as they are not a 

 part of the retina this term is misleading. 



There are six iris-pigment-cells surrounding each crystalline -cone; 

 but as each of these cells forms a part of the iris of three adjacent 

 ommatidia, there are only twice as many of these cells as there are 

 ommatidia. This is indicated in the diagram of a transverse section 



(Fig- i55. *') 



The retinula. At the base of each ommatidium, there is a group 

 of visual cells forming a retinula (Fig. 155, r} ; of these there are seven 

 in Machilis; but they vary in number in the eyes of different insects. 

 The visual cells are so grouped that their united rhabdomeres form a 

 rhabdom, which extends along the longitudinal axis of the ommati- 

 dium (Fig. 155, rh). The distal end of the rhabdom abuts against the 

 proximal end of the crystalline -cone; and the nerve -fibers of which the 

 visual cells are the endings pass through the basement membrane 

 (Fig. 155, 6) to the optic nerve. 



The visual cells are pigmented and thus aid in the isolation of the 

 ommatidium. 



The accessory pigment -cells. In addition to the two kinds of pig- 

 ment-cells described above there is a variable number of accessory 

 pigment -cells (Fig. 155, ap), which lie outside of and overlap them. 



From the above it will be seen that each ommatidium of a eucone 

 eye is composed of five kinds of cells, three of which, the corneal hypo- 

 dermis, the crystalline-cone-cells, and the retinular cells produce solid 

 structures; and three of them are pigmented. 



