AQUATIC LARViE. 221 



Somollmcs, these actions arc performed by abrupt strokes, 

 giving rise to an irregular zig-zag course: this is the case 

 with the larva of the gnat, and with many others which have 

 no feet. In the structure of tlie larva of the Libellula, or 

 dragon-fly, a singular artifice has been resorted to for giving 

 an impulse to the body, without the help of external mem- 

 bers. It is that of the alternate absorption of water into a 

 cavity in the hinder part of the body, and its sudden ejec- 

 tion from that cavity, so that the animal is impelled in a con- 

 trary direction, upon the same principle that a rocket rises 

 in the air by the reaction of that fluid. It has, at various 

 times, been proposed to apply the power of steam to the ])ro- 

 duction of an effect exactly similar to that of which Nature 

 here presents us with so pcriect an example, for the purpose 

 of propelling ships, instead of the ordinary mode of steam 

 navigation. 



Some larvae, such as that of the Stratioinys, collect 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 effica- 

 cy 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 evolu- 

 tions of the hydra.* 



The impulse given by the lateral inflections of the body 

 are in many cases assisted by short legs; but the larvae of 

 the Ephcmer a, i\\ou^\ furnished with legs, do not use them 

 for this purpose, and swim simply by the action of the tail. 

 Those of the Dyiiscus 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 larvaj 

 of the Hy drop hi I us are also admirably formed for swim- 

 ming; and they not only dart forwards with surprising velo- 

 city, but also turn in all directions with the utmost facility. 



• Page 133. 



