478 MOTIONS OF INSECTS. 
unclouded his beam, the more insects are there upon the wing, and every 
diurnal species seems fitted for longer and more frequent excursions. 
Having given you all the information that I,can collect with respect to 
the motions of perfect insects in the air, 1 must next say something con- 
cerning 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 (Wotonecta); or by having their 
terminal joints very much dilated—as in the whirlgig (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. 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 yelocity 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.’ _, 
These, however, are neither fringed nor dilated, but very long and slen- 
der, 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 in the ease with Z/ophorus, and. many other aquatic insects —is 
clothed with a thick coat of grey 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 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 asthey doon theland. Many smaller species of water-beetles, 
belonging to the genera Hydrophilus, Elophorus, Hydrena, Parnus, Limnius, 
&c., thus win their way in the waves. —Thus also the. water-scorpion 
(Nepa) pursues its prey ; and the little water-mites, (7Zydrachna) 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 sur- 
face 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 
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 sur- 
face of the water. 
2 De Geer, iii, 814. 
