408 



ZOOLOGY. 



of the head, constituting organs of mastication or suction. 

 We shall return to these when speaking of digestion ( 421 

 and 522). 



514. The thorax of insects occupies the middle portion 

 of the body, and carries limbs and wings. It is always com- 

 posed of three circles or rings, named protkorax, meso- 

 thorax, and metathorax (a I c, Pig. 416) ; and it is to the 

 ventral arch of each of these rings that one of the pairs of 

 limbs are fixed. The wings, on the contrary, spring from 

 the dorsal arch of the thoracic rings ; but the prothorax (a) 

 never carries any, and there never exists more than a pair of 

 these appendages on each of the two following rings, so that 

 there are never more than two pairs. 



Fig. 420. Notonectes. 

 Boat Fly. 



Fig. 421. Cricket, 



515. In the limbs of insects we distinguish a hnunch 

 composed of two joints, a thigh, and leg, and a sort of foot 

 called tarsus, divided into several joints, the numher of which 

 varies from two to five. They terminate in nails. It m<y 

 be readily imagined that their forms vary in unison 

 with the habits of the animals. Thus, insects 

 which have the hinder limbs long (Fig. 421), leap 

 rather than walk. In insects which swim, such as 

 the dystica3, the notonectes^Pig. 420), and the 

 gyrini, commonly called whirlygigs (Pig. 422), the 

 tarsi are generally flattened, ciliated, and disposed 

 like oars ; and in those which can walk on smooth 

 Gyrinus,or surfaces with the body suspended, we find on the 

 Whirlygig. terminal joint a sucker, by which they adhere 

 to the body they touch. Sometimes, also, the 

 anterior limbs are enlarged, as in moles, to enable them 

 to dig into the soil. The mole-cricket (Pig. 423), which does 



Fig. 422 



