20 STRUCTURE OF THE LEGS. 



the three segments are denominated, according to 

 their position in the thorax, the prothorax, or first 

 segment, the mesothorax, middle or second segment, 

 and the metathorax, hinder or third segment; that 

 the centre of the lower surface of the thorax is called 

 the sternum, divided like the thorax itself inio pro-, 

 mesa-, and metasternum and that the terms pro-, 

 meso-, and metanotum are applied to the nf per sur- 

 faces or backs of the same segments. 



But the thorax bears certain appendages which 

 require notice. These are the organs of motion, con- 

 sisting of six legs, and in the majority of insects, four 

 wings. Of the legs one pair is inserted on the lower 

 surface of each segment of the thorax, the sockets for 

 their reception being placed on each side of the ster- 

 num. Each leg consists of five parts, the first of 

 which, called the coxa, or hip, serves for the attach- 

 ment of the whole organ to the thorax. It is some- 

 times immoveably fixed to the latter, and sometimes 

 articulated with it, so as to form a ball and socket 

 joint. The joint which follows the coxa is called the 

 trochanter. It is either a ring-like or triangular 

 piece, the base or head of which is completely enclosed 

 within the coxa, with which it forms a strong hinge- 

 joint ; its opposite extremity is firmly united with the 

 base of the thigh. The latter, which is also called 

 the femur, is usually the largest and strongest piece 

 of the whole apparatus ; it is often greatly thickened 

 in the middle, or towards the tip, and in leaping 

 insects the hinder thighs are generally remarkably 

 strong. The fourth piece is the tibia, or shank, which 

 is attached by a firm hinge-joint to the extremity of the 

 thigh ; it is usually about equal to the thigh in length, 

 but instead of being rounded or flattened in its general 



