240 INSECTA. 



to another ; but so loosely, that the edges of the individual plates 

 wrap over each other to some extent, and thus may be separated 

 by the slightest pressure from within. 



But in other regions there is an absolute necessity for a mode 

 of communication intermediate in character between the two kinds 

 mentioned above ; having neither the firmness of the one, nor 

 the mobility of the other. This is more especially the case in 

 the junction between the head and the anterior segment of the 

 thorax, and also between the last-named segment and the middle 

 piece of the thorax, in those cases where these two parts are not 

 joined by suture. The joint employed in this case is of very 

 beautiful construction, resembling in some respects that formed 

 by a ball and socket ; a conical prolongation of one segment is 

 admitted into a smooth cavity excavated in the corresponding 

 margin of the other, and secured in this position by muscles and 

 an external ligament. Such an articulation is of course capable of 

 being firmly fixed by muscular action, but at the same time admits 

 of sufficient freedom of motion to allow rotation in all directions. 



(282.) The legs of insects, as we have already stated, are in- 

 variably six in number, one pair being attached to each of the three 

 thoracic segments. Considered separately, every leg may be seen 

 to consist of several pieces, connected together by articula- 

 tions of different kinds, which require our notice. The first di- 

 vision of the leg, or that in immediate connection with the thorax, 

 to which it is united by a kind of ball-and-socket joint, enclosed 

 in a strong membranous capsule, and possessing very various de- 

 grees of motion in different insects, is called the hip (coxa)', and 

 upon this, as upon a centre, the movements of the limb are per- 

 formed. To the extremity of the coxa a small moveable piece is at- 

 tached, called the trochanter ; to which succeeds the thigh (femur), 

 which is the thickest and most robust of all the divisions of the 

 limb. The next piece, called the shank (tibia"), is occasionally of 

 considerable length, and is connected to the last by a hinge ; to its 

 extremity is appended the foot (tarsus), composed of a consecutive 

 series of small segments, varying in number from five to one, the 

 last of which is armed with claws, or other appendages, adapted 

 to different kinds of progression. These divisions of the leg the 

 reader will easily recognise ; they are for the most part united 

 together by articulations so constructed as to allow simply of flexion 

 and extension, which will be best understood by inspecting, in 

 some large insect, the junction between the femur and the tibia, 



