390 PHYSIOLOGY OF THE INVERTEBRATA. 



notwithstanding their immense number of limbs, are always 

 very slow, and they generally try to escape danger by rolling 

 themselTes up into a ball. 



The Insecta. . 



In this class the thorax always bears the organs of loco- 

 motion, which, in -most insects, consist of six ambulatory 

 limbs and four wiugs. The form of these organs is very 

 various, but their general anatomy is always similar. The 

 centre of the ventral surface of the thorax is occupied by a 

 narrow piece termed the sternum, which frequently projects 

 as a ridge externally, and generally gives off an internal 

 process for the insertion of muscles. On each side of this 

 are the sockets for the legs, of which each segment of the 

 thorax bears a pair. The first joint or coxa of the legs is 

 sometimes immovably attached to the thorax, sometimes 

 articulated with it by a sort of ball-and-socket joint. The 

 next four joints are termed respectively — the trochanter, 

 femur, tibia, and tarsus. The tarsus or foot sometimes con- 

 sists of one, but generally of from three to six joints. The 

 terminal or sixth tarsal joint is furnished with two curved 

 and pointed claws or ungues, often toothed, and in many 

 cases accompanied by a pair of soft membranous organs or 

 pulvilli, which are very distinct in Musca (house fly). These 

 adhere, like suckers, to any object ag&inst which they may be 

 applied, and thus enable their possessors to walk securely 

 even in a reversed position. 



The ambulatory limbs and their various joints undergo very 

 many modifications in the different orders and groups of the 

 Insecta ; always, however, in exact coincidence with the habits 

 of the individuals — in leaping or jumping insects, such as 

 Eiopteryx,* Locusia, and Gryllus, the posterior limbs are much 

 lengthened and the femora " very thick, forming powerful 

 jumping organs. In Mantis, the anterior limbs are so much 



* See Dr. Griffiths' The Diseases of Crops, p. 51 (Bell & Sons). 



