158 PHYSIOLOGY OF INSECTS. 



times solid, sometimes consisting of a single joint, some- 

 times composed of many joints : perfoliate or i^erf orates, in 

 which a portion of each joint is dilated and flattened, 

 and the remaining portion being cylindrical and slender, ap- 

 pears like a thread on which the dilated portions are strung : 

 and setigerous or setiger(je, (fig. 18), in which the basal 

 joints are stout and short, the tliird being much the largest, 

 of various shapes, and having the remaining joints, which 

 are formed into a kind of bristle, attached on one side of 

 it, often at a right angle. 



Antennce are of various kinds besides those here de- 

 scribed, but all the common forms may be found in the 

 foregoing list : when other forms occur, their peculiarities 

 are described ; no general name is applied to them. The 

 length as well as the structure of antenna is very various ; 

 sometimes they are so short as to be concealed from the 

 sight in the little cavities in which they stand ; sometimes 

 they are five times the length of the body. 



Eyes. — The eyes of an insect commonly form the lateral 

 portions of its head ; they are two in number, and are com- 

 posed 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. Every one 

 of these lenses receives the image of an object, and ap- 

 pears very nearly to correspond in properties with the 

 crystalline lens of the human eye, so that a butterfly may, 

 without exaggeration, be said to possess forty thousand 

 eyes. Many of our commonest insects have each of the 

 eyes divided horizontally by a line, wliich is sometimes 

 obviously elevated, thus giving them the appearance of 

 being double ; in such instances the insect appears to have 

 foiu" eyes : the lenses in the upper division are larger than 

 those in the lower; the object of this structure is to enable 



