228 THE SCIENCE OF LIFE. 



a head, a threefold thorax, and a ringed abdomen. 

 (Fig. 128.) 



The head contains the organs of sense, and sup- 

 ports two antennae, which are supposed to be organs of 

 touch and of hearing, as well as of communication be- 

 tween one insect and another. There are many forms 

 of antennse, but all are tubular and jointed. The eyes 

 are usually compound, although there are also some- 

 times a cluster of simple eyes, or ocelli. The compound 



eyes have a trans- 

 parent surface, or 

 cornea, divided 

 into many facets, 

 each of the nerve- 

 rods having its 

 own pigment mass 

 A n G ',, I29 'T H r d and f yes f * e B n e r "' "' A f" na; - and itsown cornea. 



b. Ocelli. A. Facets enlarged. B. 1 he same with hairs 



growing between them. (Fig. 129.) The 



common house-fly has two thousand such facets in each 

 eye, and in the dragon-fly there are twenty-eight thou- 

 sand. 



The thorax consists of three pieces, the prothorax, 

 mesothorax, and metathorax, each having a pair of legs ; 

 the wings, when present, arise from the last two seg- 

 ments. 



The abdomen contains the viscera and the organs of 

 reproduction. Legs are never attached to it. 



The " brain," as it is called, is a mass of ganglia lying 

 across the upper side of the throat behind the mouth, 

 and the principal nerve cord, with a ganglion for each 

 segment, runs along the ventral side of the body. 



