392 PHYSIOLOGY OF THE INVERTEBRATA. 



ground, the other tripod is raised, and so on. Dixon's 

 observations show, however, that while no leg of one tripod 

 ever moves simultaneously with any leg of the other, yet 

 tliere is a succession in the movements of the legs of each 

 tripod. The hind leg on one side is first moved, then the 

 middle on the other, and when the hind leg has been moved 

 forward and almost reached the ground, the front leg of that 

 side is raised. The middle leg and the front leg of the 

 opposite sides come to the ground almost simultaneously. It 

 is usually just when the hind leg is reaching the ground, and 

 the front leg is being raised, that the tripod on which the 

 fly is resting thrusts the body forward. After the movement 

 of each tripod there appears to be a short pause, during which 

 all six legs are on the ground together. 



Dixon has also observed the tripodic walk in earwigs, water 

 scorpions, aphides, and some beetles. In the case of some 

 slowly moving beetles and aphides, which can be observed 

 without photographic means, quite irregular movements have 

 been observed. By cooling aphides, they can be made to 

 move very slowly. In this condition one was observed to 

 move its legs in slow succession in the following order: — 

 (a) Right hind, (&) right middle, (t) right front, (f?) left hind, 

 (c) left middle, (/) left front. This walk was continued for 

 some time, occasionally interrupted by the following order, or 

 some other quite irregular walk : — {({) Eight hind, {h) right 

 middle, (c) left hind, {d) left middle, (c) left front, (/) right 

 front. 



In caterpillars the legs forming a pair seem to move simul- 

 taneously ; the motion begins at the posterior end of the 

 body, and proceeds regularly forward till the most anterior 

 pair of legs are moved.* 



According to Darwin,t Papilioferonia, of Brazil (one of the 



* Mr. Dixon kindly sent the author several photograi^hs illustrating the 

 above movements, but unfortunately they cannot be reproduced as wood- 

 cuts. 



t Journal of Jiesearches (chap, ii.). 



