THE SENSES OF INSECTS. 



23 



the crystalline cone, the retina, and the optic nerve. How 

 an insect sees is not well understood; but 

 the corneal lens acts like an ordinary 

 glass lens to condense the light, or "form 

 an image of a moving body, either of A 

 which, as the case may be, falls upon the 

 cone behind the lens. This cone has 

 been found by Mr. Patten to be rich in 

 extremely fine nerve-threads, the ends of 

 one of the fibres which unite to form the 

 optic nerve. It is thus highly sensitive. 

 The crystalline cone then, as Patten ob- 

 serves, is that part which is sensitive to B 

 the light or perceives objects. The retina, 

 a mass of black pigment-cells enveloping 

 the ends of the cones and their stalks or 

 rods, and further comprising, as Hickson 

 thinks, all that part of the eye lying be- 

 tween the crystalline cones and true optic c 

 nerve, is of use, especially in the com- 

 pound eye, in elaborating and combining 

 the sensations formed in the cones. Now 

 the compound eye is simply, so to speak, 

 a compound simple eye: not, as used to be 

 thought, a collection of simple eyes joined D 

 together. The compound eye grows out 

 of, or is "differentiated" from, a simple 

 eye; it is, as Patten says, "a modified 

 ocellus;" and this observer concludes that 

 the majority of compound eyes are adapted " for the per- 

 ception of inverted images formed by the corneal facets 

 upon the crystalline cones." Of course, as with us, the 

 effect upon the insect's mind is that of seeing a single object. 

 Experiments by Plateau on the simple eyes of centipedes 

 show quite decidedly that these creatures can do little more 

 than distinguish light from darkness; they do not make out 

 the form of objects, though some can perceive the more 



FIG. 19. 

 forms o 

 eyes. A, a" bug 

 (Pyrrhocoris): B. 

 worker bee; C, drone; 

 ),male Bibio. From 

 Judeich and Nitsclie. 



