no 



ENTOMOLOGY 



FIG. 140. 



somewhat 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 distance between the lens and 

 the retina, there is no power of accommo- 

 dation, 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 concerned ; 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 \vay 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 dis- 

 tinguish light from darkness and they 

 are probably more serviceable to the in- 

 sect in this way than in forming images. 

 Compound Eyes. As regards deli- 

 cacy and intricacy of structure, the com- 

 pound eye of an insect is scarcely if at all 

 inferior to the eye of a vertebrate. In 

 radial section (Fig. 141), a compound eye 

 appears as an aggregation of similar 

 elongate elements, or ommatidia, each of which ends exter- 

 nally in a facet. * The following structures compose, or are 

 concerned with, each ommatidium : (i) cornea, (2) crystal- 

 line lens, or cone, (3) rHabdom and retinula, (4) pigment (iris 



An ocellar retinula of 

 the honey bee, composed 

 of two retinal cells. A, 

 longitudinal section; B, 

 transverse section; n, n, 

 nerves; p, pigment; r, 

 rhabdom. After REDI- 

 KORZEW. 



