LEGS OF INSECTS. 87 



from the inferior part of the head, and constitute the organs of 

 mastication or of suction ; we shall return to these, when speak- 

 ing of the Digestive apparatus. 



663. The thorax of Insects occupies the middle part of their 

 body, and bears the legs and the wings. It is always composed 

 of three rings, named prothorax, mesothorax, and metathorax 

 (Fig. 393, , b, c) ; and to the ventral arch of each of these seg- 

 ments is fixed one of the pairs of legs. The wings arise, on the 

 contrary, from the dorsal arch of the thoracic rings ; but the 

 prothorax never bears them, and there never exist more than 

 one pair of these appendages on each of the succeeding rings ; 

 so that their number can never exceed two pairs. 



664. We may distinguish, in the limbs of Insects, a haunch 

 composed of two joints, a thigh, a shank, and a kind of finger, 

 named tarsus, which is divided into several joints whose num- 

 ber reaches to five, and is terminated by claws. Their con- 

 formation differs ; but, as may be readily believed, it is always 

 in relation with the habits of the animals. Thus the Insects 

 whose posterior legs present a great length (Fig. 397), usually 

 jump rather than walk ; amongst the swimming insects, such 

 as the Dytiscus, the Notonecta (Fig. 398), and the Gytinus, vul- 

 garly called Whirligigs (Fig. 399), the tarsi are usually flat- 

 tened, fringed with hairs, and arranged like oars ; and amongst 



FIG. 31,7. LOCUST. FK.. wx NO.OM'CTA. FIG. 399. GYRINITS. 



those which can walk suspended from smooth surfaces, we find, 

 under the last joint of these organs, a kind of cushion or cup, 

 fitted to make them adhere to the body which they touch. 

 Sometimes, also, the anterior, legs ar^ widened like those of the 

 Mole, in order to enable them to dig in the ground ; the Mole- 

 Cricket, which cften occasions considerable injury in our fields, 



