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 mature insects, and the ocelli or simple 
eyes, which may or may not be present. When present 
there are usually three, which, if joined by lines, will 
descrbe 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. 5, fff ) 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 
Fic. 13. 
Fic. 12. 
Facets of Compound Eyes, Section of Compound Eye. 
F Facets. e Cornea. C Cells. 
ff Hairs. # Rods. 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. Larve, 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)—retinule—which are enclosed by pigment mem- 
