278 THE MECHAiNICAL M^ NOTIONS. 



Some laivfp, such as that of the Stratiomys, col- 

 lect a bubble of air, which they retain within a 

 tuft of hair at the extremity of the tail, evidently 

 with a view of diminishing the specific gravity of 

 the body, and thus giving greater efficacy to the 

 muscular actions which they employ in their pro- 

 gression through the water. Another use is also 

 made of these tufts of hair ; for by repelling the 

 water, they allow of the insect's suspending itself 

 from the surface of the fluid in the manner already 

 noticed in giving the history of the evolutions of 

 the hydra.* 



The impulse given by the lateral inflections of 

 the body are in many cases assisted by short legs ; 

 but the larvas of the Ephemera, though furnished 

 with legs, do not use them for this purpose, and 

 swim simply by the action of the tail. Those of 

 the Dytiscus are furnished with a pair of very long 

 members, projecting to a considerable distance from 

 the sides, and flattened at the ends, to serve as 

 oars. The larvae of the Hydrophilus are also admi- 

 rably formed for swimming ; and they not only 

 dart forwards with surprising velocity, but also turn 

 in all directions with the utmost facility. 



§ o. Terrestrial Larvce. 



The movements of larvae that are not aquatic are 

 perfectly analogous to those of the Annelida, which 

 thev much resemble in their outward form and 

 mechanical structure. The muscles by which the 



* Page 160. 



