292 



MOTIONS OF INSECTS. 



joints very much dilated — as in the whirlgig (Gyrinus) — 

 so as to resemble the paddle of an oar. 1 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 as- 

 cend, as it should seem, merely in consequence of their being 

 specifically lighter than the water; but when they descend 

 or move horizontally, 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 legs are ex- 

 tended so as to form a right angle with their body. The 

 water-boatmen swim upon their back, which enables them to 

 see readily and seize the insects that fall upon the water, 

 which are their prey. Sigara, however, a cognate genus, se- 

 parated from Notonecta by Fabricius, swims in the ordinary 

 way. As the Gyrini are usually in motion at the surface, 

 whirling round and round in circles, it is probable that their 

 legs are best adapted to this movement. They dive down, 

 however, with great ease and velocity when alarmed. The 

 common water-bug (Gerris lacustris), though it never goes 

 under water, will sometimes swim upon the surface, which it 

 does by strokes of the intermediate and posterior legs. 2 

 These, however, are neither fringed nor dilated, but very long, 

 and slender, with claws, not easily detected, situated under 

 the apex of the last joint of the foot, which covers and con- 

 ceals them. The under side of their body — as is the case 

 with Elophorus, and many other aquatic insects — is clothed 

 with a thick coat of gray hairs like satin, which in certain 

 lights have no small degree of lustre, and protect its body 

 from the effects of the water. Some insects, that are not na- 

 turally aquatic, if they fall into the water will swim very 

 well. I once saw a kind of grasshopper (Acrydium), which 

 by the powerful strokes of its hind legs pushed itself across a 

 stream with great rapidity. 



Other insects walk, as it were, in the water, moving their legs 

 in much the same way as they do on the land. Many smaller 



1 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. 



2 De Geer, hi. 314. 



