MECHANICS OF WALKING 



107 



"The case is the same in the middle and hind legs, which must make a 

 double course also, yet in such a way that the straight line is drawn, not during 

 the retraction, but during the extension ; during which, however, quite as in the 

 fore leg, the members mentioned (,R 3 ) gradually approach the body. 



"When the legs have reached the maximum of their retraction, or of their 

 extension, as the case may be, and therefore the end of their active course for 

 that time, then begins the opposite or backward movement ; that is, the fore legs 

 are again extended, while their levers draw the remaining legs together again. 



FIG. 118. A Carabus beetle in the act of walking or running : three legs (Z,, 7? 2 , Z 3 ) are directed 

 forward, while the others (_/? t , Z 2 , ff a ), which are directed backward toward the tail, have ended 

 their activity ; nb, cd, and ef are curves described by the end of the tibia-, and passing back to the 

 end of the body ; lih, di, and fg are curves described by the same legs during their passive change 

 of position. After Graber. 



"At the same time, as we may see by the uniting leg, the limb is either a 

 little raised, that there may be no unnecessary friction, or it remains during the 

 passive step also, with its means of locomotion in slight contact with the ground. 



" The curve of two steps, as inscribed by the end of the tibia of the left fore 

 leg of a stag-beetle, affords an instructive summary of the conditions of which 

 we have been speaking (Fig. 121, B). We see two curves. The thick one (aft), 

 directed toward the axis of the body, corresponds to the effective act of a single 



