106 TEXT-BOOK OF ENTOMOLOGY 



and the mode of -walking of these insects in general. This subject 

 had been but slightly investigated until Graber made a series of 

 observations and experiments, of which we can give only the most 

 important results. 



The locomotion of insects is an extremely complicated subject. 



Let us consider, Graber says, first, a running or carabid beetle, when walk- 

 ing merely with the fore and hind legs. The former will be bent forward and 

 the latter backward. 



"Let us begin with the left fore leg (Fig. 118, LI). Let the same be ex- 

 tended and fixed on the ground by means of its sharp claws and its pointed heel. 

 Now what happens when the tibial flexors draw together ? As the foot, and 

 therefore the tibia also, have a firm position, then the contraction of the muscles 

 named must cause the femur to approach the tibia, whereby the whole body is 

 drawn along with it. This individual act of motion may be well studied in 

 grasshoppers when they are climbing on a twig by stretching out their long fore 

 leg directly forward, and then drawing up the body through the shortening of 

 the tibial flexors until the middle leg also reaches the branch. 



"But while the fore legs advance the body by drawing the free lever to the 

 fixed leg-segment, the hind legs do this in exactly the opposite way. The hind 

 leg, namely, seeks to stretch out the tibia, and thus to increase the angle of the 

 knee (83), thereby giving a push on the ground, by means of which the body 

 is shoved forward a bit. 



" Though it might be supposed that the feet would remain stationary during 

 the extension or retraction of the limbs, this never occurs in actual walking. 

 Not merely the upper, but also the lower, thigh is either drawn in or stretched 

 out, as the case may be. The latter then describes a straight line with its point 

 during this scraping or scratching motion (Fig. 115, no), which is obviously the 

 chord to that quadrant which would be drawn by the tibia or foot in a yielding 

 medium, as water, for instance. But even this motion results extremely rarely, 

 and never in actual walking. If we fix our eye anew upon the fore leg at the 

 very moment when it is again retracted, after the resultant 'fixing,' we shall 

 then observe that the hip also is simultaneously turned backward in a definite 

 angle. The tibia would describe the arc nq (Fig. 115) by means of the lat- 

 ter alone. 



"This plane, in conjunction with the rectilinear 'movement 1 (no) obtained 

 by the retraction of the tibia, produces a path (nr), and this is what is actually 

 described by a painted foot upon a properly prepared surface, as a sheet of 

 paper ; 1 supposing, however, that the body in the meantime is not moved for- 

 ward by other forces. In the last case, and this indeed always takes place in 

 running, the trunk is moved a bit forward, together with the leg which is just 

 describing its curve with a rapidity corresponding to the momentum obtained ; 

 the result of this is that the curve of the foot from its beginning (w) to its end 

 (a) bends round close to itself, just as a man who, when on board a ship in 

 motion, walks across it diagonally, and yet on the whole moves forward, be- 

 cause his line of march, uniting with that of the ship, results in a change of 

 position in space. 



1 In his account of his studies on the locomotion of insects, De Moor states that he 

 obtained the track of each of the feet in different colors by coating them with differ- 

 ent pigments ; the insect, as it moved, left its track on a strip of paper. (Archives de 

 Biologic, Liege, 1890.) 



