THE COMMON COCKROACH 133 



furnished with stout spines on its lower edge ; this is 

 the femur or thigh. It is movable upon the coxa with an 

 up-and-down movement, i.e., in a vertical plane. When 

 the insect is viewed from above, the coxae are not seen, as 

 they are then concealed by the body, and the legs thus 

 appear to commence with the femora. The femur is 

 succeeded by a longer and narrower straight piece, set 

 with spines on both edges, which is called the tibia or 

 shank. It is movable upon the femur in a vertical plane, 

 but in the opposite direction to that of the femur upon 

 the coxa. Finally, the leg terminates in a series of five 

 joints, called collectively the tarsus, or foot, of which the 

 one attached to the tibia is by far the longest, and the 

 next three regularly diminish in length ; but the last one 

 is longer again, and carries at its extremity a pair of 

 hooked claws. The joints of the tarsus are movable 

 upon one another in the same plane as the tibia and 

 femur, and possess at the same time the power of motion 

 to a slight extent from side to side. These are all the 

 parts of the leg that are obvious at a first glance, but 

 there is still another small joint which a careful exami- 

 nation will detect at the outer extremity of the coxa ; it is 

 a tiny triangular piece called the trochanter, and has very 

 little freedom of motion. The other two pairs of legs are 

 similarly constituted to the hind pair, the chief differences 

 being in the proportional length of the parts, a matter 

 in which great uniformity is preserved in all specimens. 

 But though the structure of all six legs is similar, the 

 functions of the three pairs are somewhat different. In 

 running, the first and third leg of one side are moved 

 forward simultaneously with the middle one of the 

 opposite side ; then the other three follow for the next 

 step, and so on. There are, therefore, never less than 

 three legs in contact with the ground at the same time, 



