MOTIONS OF INSECTS. 513 



or by having their terminal joints very much dilated — as in the whirl gig 

 (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 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 extended 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, separated from ISotonecta 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 move- 

 ment. They dive down, however, with great ease and velocity when 

 alarmed. The common water-bug (Gerris lacusiris), though it never 

 goes under water, will sometimes swim upon the surface, which it does by 

 strokes of the intermediate and posterior legs.^ 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 conceals 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 naturally aquatic, if they fall into the water will swim very 

 well. I once saw a kind of grasshopper (^Acrydium), which by the pow- 

 erful 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 species of water- 

 beetles, belonging to the genera Hydrophilus, Elophorus, Hydrana, Par- 

 nus, Limnius, he, thus win their way in the waves. — Thus also the 

 water-scorpion (^Nepa) pursues its prey ; and the little water-mites (Hy- 

 drachna) may be seen in every pool thus working their little legs with 

 great rapidity, and moving about in all directions. — Some spiders also will 

 not only traverse the surface of the waters, but as you have heard with 

 respect to one, descend into their bosom. There are other insects moving 

 in this way that are not divers. Of this kind are the aquatic bugs (Gerris 

 lacusiris, Hydrometra stagnorum, Velia rivulonim, &;c. Latr.) The first 

 can walk, run, and even leap, which it does upon its prey, as well as 

 swim upon the surface. The second, remarkable for its extreme slender- 

 ness, and for its prominent hemispherical eyes — which, though they are 

 really in the head, appear to be in the middle of the body — rambles about 

 in chase of other insects, in considerable numbers, in most stagnant waters. 

 The Velia is to be met with chiefly in running streams and rivers, coursing 

 very rapidly over their waves. ^ The two last species neither jump nor 

 swim. The species of one genus of this group {Halolates Eschscholtz) 



' Mr. Briggs observes that this insect appears to move all its legs at once, with wonderful 

 rapidil)', by which motion it produces a radiating vibration on the surface of the water. 

 * De Geer, iii. 314. 3 Curtis, Brit. Ent. t. ii. 



