MOTIONS OF INSECl'S. 359 



tions, enters into the muscles and nervures of their wings, 

 maintaining their tension, and by the greater or less ra- 

 pidity of its pulsations accelerating or diminishing their 

 action. 



Having given you all the information that I can col- 

 lect with respect to the motions of perfect insects in the 

 aiy\ I must next say something concerning their modes 

 of locomotion in or upon the 'water. These are of two 

 kinds, sivimnmig and "doalking. Observe — I call that 

 movement swimming, in which the animal pushes itself 

 along by strokes — while in walking, the motion of the 

 legs is not different from what it would be if they were 

 on land. Most insects that sxoim have their posterior 

 legs peculiarly fitted for it, either by a dense fringe of 

 hairs on the shank and foot, as in the water-beetles 

 [Di/tiscus) % or the water-boatmen (Notonecta) ; or by 

 having their terminal joints very much dilated — as in 

 the whirlwig [Gijrinus) — so as to resemble the paddle of 

 an oar ''. When the Dytisci rise to the surface to take 

 in fresh air — a silver bubble of which may often be seen 

 suspended at their anus — they ascend, as it should seem, 

 merely in consequence of their being specifically lighter 

 than the water ; but when they descend or move hori- 

 zontally, which they do with considerable rapidity, it 

 is by regular and successive strokes of their swimming 

 legs. While tiiey remain suspended at the surface, 

 these legs are extended so as to form a right annle with 

 their body. The water-boatmen swim upon their back, 



=" Plate XIV. Fig. 6. 



•' Mr. Briggs observes that this insect appears to move all its legs 

 at once, with wonderful rapidity, by which motion it produces a 

 radiating vibration on the surface of the water. 



