Eyes of Insects. 



59 



one, for nothing answering in the least to ears, structurally, 

 has yet been discovered. 



The eyes are of two kinds, the compound, which are 

 always present in m'ature insects, ahd the ocelli or simple 

 eyes, which may or may not be present. When present 

 there are usually three, which, if joined by lines, will 

 describe a triangle, in the vertices of whose angles are the 

 ocelli. Rarely there are but two ocelli, and very rarely 

 but one. 



The simple eyes (Fig. ^i ffj ) are circular, and possess 

 a cornea, lens, and retina, which receives the nerve of sight. ' 



From the experiments of Reaumur and Swammerdam, 

 which consisted in covering the eyes with varnish, they 



^ Fig. i^. 



Fig. 13. ^ 



Facets of C-ompoimd Eyes, 



F Facets. 

 H Hairs. 



Section oj Compound Fye. 



c Cornea. 

 R Rods. 



C Cells. 

 O Nerve. 



concluded that vision with these simple eyes is very indis- 

 tinct, though by them the insect can distinguish light. 

 Some have thought that these simple eyes were for vision 

 at slight distances. Larvae, like spiders and myriapods, 

 have only simple eyes. 



The compound eyes (Fig. 8), are simply a cluster of 

 simple eyes, so crowded that they are hexagonal (Fig. 12). 

 The cornea is transparent, modified, chitinous skin. Just 

 within, for each facet is the crystalline lens, back of which 

 extend the rods (Fig. 13), which consist of united chitinoUs 

 thread. Each rod is surrounded by eight rounded columns 

 (Fig. 14) — retinulae — which are enclosed by pigment mem- 



