AQUATIC LARV^.. 277 



the mature state ; excepting that they are without 

 wings, these organs being added in the progress of 

 their growth, and constituting, when acquired, their 

 perfect or imago condition. 



§ 4. Aquatic Larvce. 



Many insects, which, when fully developed, are the 

 most perfectly constructed for flying, are, when in 

 the state of larvae, altogether aquatic animals. 

 Some of them are destitute of feet, or other external 

 instruments of motion, swimming only by means of 

 the alternate inflections of the body from side to 

 side, in the same manner as the Nais, and the 

 Leech. Sometimes these actions are 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 the larva of the LibeUula, or dragon- 

 fly, a singular artifice has been resorted to for 

 giving an impulse to the body, without the help of 

 external members. It is that of the alternate ab- 

 sorption of water into a cavity in the hinder part 

 of the body, and its sudden ejection from that 

 cavity, so that the animal is impelled in a contrary 

 direction, on 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 production of an effect exactly 

 similar to that of which Nature here presents us 

 with so perfect an example, for the purpose of pro- 

 pelling ships, instead of the ordinary mode of steam 

 navigation. 



