\ 97 



.tremely limited; for MH( c I he foi in of t In- Im 



distance between the l.-n- an. I tin- retina, th> of accom- 



modation, and inx| external ob fC out ., make an 



imam 1 , then, tin- ohjccl nm>t he at one definite distance from th 

 and as the lens is usually strongly convex, th- 

 in other \vord>. insects, like spiders, ore very D 



ocelli art- concerned; furthermore, the >mall number of retinal rods 

 implies an imai^e of only the coarsest kind. 



If the compound ryes of a ijra>shoppiT are covered with an OJM 

 varnish and the insect is placed in a box with only a single opening, it 

 readily finds its way out by means of its ocelli; if the three ocelli also 

 are covered, however, it no longer does so, c .T>-^ ^-^_ 

 except by accident, though it can make its 

 escape when only one of the ocelli is left 

 uncovered. The ocelli, then, can distinguish 

 light from darkness and they are probably 

 more serviceable to the insect in this way 

 than in forming images. 



Compound Eyes. As regards delicacy and 

 intricacy of structure, the compound eye of 

 an insect is scarcely if at all inferior to the eye 

 of a vertebrate. In radial section (Fig. 143), 

 a compound eye appears as an aggregation 

 of] similar elongate elements, or ommatidia, 

 each of which ends externally ina facet. The FlG - I43 '~, Po fl rtion f f . c 



pound eye of ny, Calltpnora 



following Structures Compose, Or are Concerned vomitoria, radial section, c, cor- 

 .r v j.*j* / \ / \ nea: i, iris pigment; n, nerve 



with, each ommatidmm: (i) cornea, (2) crys- fibe ; s; nCt nerve cells; r retinal 

 tallinc lens, or cone, (3) rhabdom and retinula, pigment; t, trachea. After 

 (4) pigment (iris and retinal), (5) fenestrate 

 membrane, (6) fibers of the optic nerve, (7) trachea. 



The cornea (Fig. 144) is a biconvex transparent portion of the exter- 

 nal chitinous cuticula. Immediately beneath it are the cone cells, which 

 may contain a clear fluid or else, as in most insects, solid transparent 

 . The rhabdom is a transparent chitinous rod or a group of rods 

 (rhabdo meres) situated in the long axis of the ommatidium and sur- 

 rounded by greatly elongated cells, which constitute the retinula. 

 Two zones of pigment are present: an outer zone, of iris pigment, in 

 which the pigment in the form of fine black granules is contained 

 chiefly in short cells that surround the retinula distally; and an inner 

 f retinal pigment, in which the pigment cells are long and slender, 



