Glass 3. Insecta. 



235 



Tlie thorax is composed of three segments : prothorax, meso- 

 thorax, and metathorax. Usually the last two are immovably 

 united, the pro-thorax freely articulated. Each bears a pair of legs, 

 which are divided into the following parts: coxa, trochanter, 

 femur, tibia, and tarsus; each of the first four consists of a 

 single joint only, whilst the tarsus is generally multiarticulate. The 

 coxa and trochanter are usually short, the femur and tibia long, the 

 former thicker than the latter ; articulating with the lower end of 

 the tibia there is generally a pair of movable spines (spurs). The 

 tarsus in many Insects consists of five joints (sometimes of fewer), 

 and usually bears at its tip two movable hooks, the claws. The 

 legs are true loco- 

 motor organs ; in 

 walking, the animal 

 rests on the lower 

 side of the tarsus, 

 which is often hairy ; 

 the distal end of the 

 femur is turned out- 

 wards, that of the 

 tibia downwards, the 

 tip of the tarsus out- 

 wards ; in the first 

 pair of legs the foot 

 is forwardly directed, 

 in the last pair back- 

 wardly. In many 



forms the legs, or some of them, have another function besides that 

 already mentioned. The front pair in the Cockchafer serve not 

 only for ambulatory purposes but also for digging; in others the 

 legs are so modified in connection with the secondary office, that 

 their primitive function is lost. The first pair in the Mole- 

 •cricket, for instance, is only used for digging; the same pair in 

 Water-scorpions for organs of prehension ; the last pair in the Locust 

 forms a springing apparatus, whilst in Dytiscus it is pre-eminently 

 natatory in function. 



The thorax usually bears two pairs of wings, which arise dorso- 

 laterally from the meso- and meta-thorax. Each wing is a large, 

 laminate, tegumentary outgrowth, which primitively possesses the 

 same layers as the rest of the skin, i.e., is covered on each side 

 with a chitinous layer (cuticle), within which, on each side, is an 

 epidermal layer, whilst between the two epidermal layers run 

 tracheae, nerves, etc. When the wing is fully developed, however, 

 the soft parts disappear, so that it then consists of little else than 

 two closely apposed chitinous plates. The wings articulate with 



Pig. 197. Transverse section through the thorax of 

 a Beetle (diagrammatic), a' elytra, a? wings, h body- wall; 

 c coxa, t (upper) trochanter, t' (lower) tibia, ta tarsns, u 

 claws. — Orig. 



