MOTIONS OF INSECTS. 3638 
quickly in them, invigorates all their motions, enters into 
the muscles and nervures of their wings, maintaining 
their tension, and by the greater or less rapidity of its pul- 
sations 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 
air, I must next say something concerning their modes 
of locomotion in or upon the water. These are of two 
kinds, swimming and walking. 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 swim 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 
(Dytiscus)*, or the water-boatmen (Notonecta); or by 
having their terminal joimts very much dilated—as in 
the whirlwig (Gyrinus)—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 horizon- 
tally, which they do with considerable rapidity, it is by 
regular and successive strokes of their swimming legs. 
While they remain suspended at the surface, these lees 
are extended so as to form a right angle with their body. 
The Notonccte swim upon their back, which enables 
@ PraTE XIV. Fic. 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, 
