OF MUSCULAR MOTION. 453 



violently with the somewhat raised margin of the pronotum, and the base 

 of the elytra against the ground, which throws it upwards, yet, as this 

 blow does not proceed from the centre of the body, but its anterior 

 portion, this part receives the greater impetus, in consequence of which 

 the body turns over in the air, and it consequently falls with its ventral 

 surface to the ground. It retains itself in this position by the sudden 

 clinging of its legs, and so prevents the effects the concussion would 

 otherwise have of throwing it up again. The dagger-shaped process of 

 the prosternum, which fits into the funnel-shaped groove of the meso- 

 sternum, has no other purpose in this motion than to regulate and pre- 

 serve the direction of the prothorax during the contraction, without 

 participating in the least in the blow. 



In the spring- tails a furcate process originates at the ventral plate 

 of the penultimate and ante-penultimate abdominal segments, which in 

 repose lies extended towards the head along the belly, and reaches to 

 about the posterior legs. By the insect striking this process rapidly 

 and with force against the surface it is enabled frequently to make an 

 extensive leap. During this leap the fork is directed posteriorly, but 

 as soon as it again touches the ground it again bends forward. In 

 Smynthurus, Lat., De Geer found, besides, a conical process at the 

 sternum, whence the creature projected two long flexible filaments when 

 it wished to affix itself*. Probably these filaments also participate in 

 producing the leap, which is much greater in them than in Podura. 



265. 



In swimming, insects are assisted either by their legs or other organs, 

 which, in conjunction with other functions, exei'cise also that of fins. 

 Among these organs may be classed the branchial leaves of the larvae 

 which live in water, of which we have before noticed their incessant 

 motion backwards and forwards, whereby the larva moves and breathes. 

 This is however the case only in those larvae which have lateral 

 branchial leaves ; a portion of the rest, for instance the larvae of the 

 Agrions, move by the serpentine motion of their abdomen, and the 

 leaves at their caudal extremity, which act as a rudder. Thus also do 

 the larvae and pupae of the gnats move. Others, again, swim like the 

 leech, by a serpentining of the abdomen ; which motion sometimes 

 describes an undulating line, and sometimes, as in the red larva of 



* Memoircs, torn. vii. p. 20, PL III. f. 7, o. 



