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, I 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 (Tty&cttf), or the water-boatmen (Notonecta); or by having their 

 terminal joints very much dilated as in the whirlgig (Gyrinus) so as to 

 resemble the paddle of an oar. 1 When the Dy tisci 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 velocity when alarmed. The common water-bug (Gerris 

 lacustrii), 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 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 case with Elophorus, 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 as they do on the land. Many smaller species of water-beetles, 

 belonging to the genera Hydroplnlus, Elophorus, Hydrcena, Parnus, Limnius, 

 &c., thus win their way in the waves. Thus also the water-scorpion 

 (Nepa) pursues its prey -, and the little water-mites (Hydrachnd) 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. 314. 



