PHYSIOLOGY OP INSECTS. 1 1 1 



lateral portions of its head ; they are two in 

 number, and are composed of numerous highly 

 convex lenses ; these lenses are hexagonal ; and 

 yet not fitting closely to each other, but each 

 situated in a square. The number of lenses 

 varies from fifty to twenty thousand in a single 

 eye. 



386. Every one of these lenses receives the 

 image of an object, and appears very nearly to 

 correspond in properties with the crystalline lens 

 of the human eye ; so that a butterfly may, with- 

 out exaggeration, be said to possess forty thousand 

 eyes. 



387. Many insects have each of the eyes di- 

 vided horizontally by an elevated line ; thus 

 giving them the appearance of being double : in 

 such instances the insect appears to have four 

 eyes. 



388. Petiolated eyes are those which are placed 

 at the end of distant stalks, which are situated one 

 on each side of the head : this is a very rare and 

 remarkable character in the eyes of insects : it 

 occurs in a peculiar kind of flies. 



389. The form of eyes is very various ; the 

 variations are expressed by the usual descriptive 

 terms of shape, as round, heart-shaped, oblong, 

 kidney-shaped, &c. 



390. The mouth of an insect is situated below 

 the head, and generally projects slightly forwards ; 

 this being by far the most convenient position for 



