HEXAPODA : LEGS AND WINGS. 289 



Hexapod Insects and these are attached to the under side 

 of the thorax, one pair to each of the three rings of which 

 the thorax is composed. The leg consists of the hip- 

 joint^ by which it is fastened to the body, the thigh, the 

 shank, and the foot, the last consisting generally of five 

 pieces placed end to end and called tarsi, and generally 

 armed at the end with one or two claws. The fore legs 

 are directed forward, and the two hind pair backward. 

 The wings are normally four, but in some, as in Flies, etc., 

 there are only two, and in others, as In Fleas, etc., these 

 organs are wholly wanting. 



The wings of insects are at first little soft sac-like bodies contain- 

 ing tracheae. They grow from the side of the thorax of the pupa at 

 points above the insertion of the legs. During the pupa stage they 

 are pad-like, but when the pupa-skin is shed, they rapidly expand 

 with air and become broad and delicate wings. The wings of insects 

 are thus simply expansions of the general covering of the body spread- 

 over a net-work of horn-like veins or tubes. These tubes are double, 

 consisting of a central air-tube enclosed within a larger tube filled 

 with blood ; and hence the aeration of the blood is also carried on in 

 the wings, and thus these organs serve the purpose both of respira- 

 tory organs and of flight. And it may be further remarked here that 



FIG. 359. 



Fore wing of a Saw-fly showing Venatiou. 



c, costal vein; sc, sub-costal; m, median; sm, sub-median; i, internal; aud the 

 spaces inclosed by the veins are going from the front edge backward the costal, 

 sub-costal, median, sub-median, and internal cells. The opaque space is called the 

 pterostigma. 



the number and position of these veins or tubes are of very great 

 importance in classifying the genera and species of insects. The 

 typical number of primary veins is five. They diverge from the base 

 of the wing, and divide into veinlets, from which cross veins arise, 



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