OP MUSCULAR MOTION. 451 



chief motion, and they are then therefore altogether larger, and also 

 some of their joints are more fully developed. This development chiefly 

 affects the thigh ; it is not only longer than in the anterior legs but also 

 much thicker, particularly at its lower end when it is very long, or in 

 the middle when it but little exceeds the rest in length. In the first 

 case it is obclavate, and in the last it is either ovate or conical. When, 

 therefore, a leap is to be made, the posterior leg bends at its knee joint 

 as much as possible; usually the femur and tibia then touch each other. 

 The tarsus is also so much bent back that its superior surface touches 

 the tibia, but the entire femur is so depressed that its axis is parallel 

 to the surface upon which the insect rests. In this position, with its 

 anterior legs somewhat withdrawn, the insect stops for some seconds, 

 as it were to collect itself, when it distends all and chiefly the posterior 

 legs with considerable force and rapidity, and by means of which it 

 throws itself from the surface. We therefore perceive that it is chiefly 

 the extensors which produce the leap ; they are consequently through- 

 out the whole posterior leg, and particularly the thigh, the strongest 

 and largest, whence it is also that the greater convexity of the thigh 

 is always above, and not beneath. 



The line described by the insect in its leap, if, for instance, when 

 winged it should not expand its wings, and by their action supporting 

 the leap continue it by flight, is that of all projectiles, namely, para- 

 bolic, which is explained by the gradually increasing gravity of the body, 

 and in consequence its decreasing power of flight. But the extent of 

 the leap depends partly upon the force with which it is made and 

 partly upon the size of the body, but particularly upon the latter, so 

 that we may consider as a law that the larger the body of the leaping 

 insect the less is the extent of its leap, and the flatter is the parabolic 

 line. Both, however, depend upon the flexure and correlative position 

 of the femur and tibia, so that the smaller the bend the shorter and 

 flatter is the leap. Thus every insect regulates both the direction and 

 height of its leap by the position of its feet and the force of their 

 extension. 



The power of leaping in insects, from the lightness of their bodies, 

 and the relative strength and size of their muscles in general, is consi- 

 derable, and doubtlessly greater than in any other animals. No 

 mammal can leap proportionately so high and far as the flea, which of 

 all insects possesses this power most strongly developed, for in one leap 

 it will spring a height exceeding two hundred times the length of its 



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