88 



ENTOMOLOGY 



the same plan as the human eye, its capacity for forming images must be 

 extremely limited; for since the form of the lens is fixed and also the dis- 

 tance between the lens and the retina, there is no power of accommodation, 

 and most external objects are out of focus; to make an image, then, the 

 object must be at one definite distance from the lens, and as the lens is 

 usually strongly convex, this distance must be small; in other words, 

 insects, like spiders, are very near-sighted, so far as the ocelli are con- 

 cerned; furthermore, the small number of retinal rods implies an image 

 of only the coarsest kind. 



If the compound eyes of a grasshopper are covered with an opaque 

 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 all three ocelli are also covered, 

 however, it no longer does so, 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 sec- 

 tion (Fig. 141), a compound eye appears 

 as an aggregation of similar elongate ele- 

 ments, or ommatidia, each of which ends 

 externally in a facet. The following struc- 

 tures compose, or are concerned with, each 



ommatidium: (i) cornea, (2) crystalline lens, or cone, (3) rhabdom and 

 retinula, (4) pigment (iris and retinal), ($) fenestrate membrane, (6) fibers 

 of the optic nerve, (7) trachea. 



The cornea (Fig. 142) 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 

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

 (rhabdomeres) situated in the long axis of the ommatidium and surrounded 

 by greatly elongated cells, which constitute the retinula. Two zones of 

 pigment are present: an outer zone, of iris pigment, in which the pig- 

 ment in the form of fine black granules is contained chiefly in short cells 



'i>nc 



FIG. 141. Portion of compound 

 eye of fly, Calliphora vomitoria, 

 radial section, c, cornea; i, iris 

 pigment; , nerve fibers; nc, nerve 

 cells; r, retinal pigment; /, trachea. 

 After HICKSON. 



