The first of these, motions of insects reposing, will not detain us long. The most remarkable is that of the long-legged gnats or crane-flies (Tipulæ).—When at rest upon any wall or ceiling, sometimes standing upon four legs, and sometimes upon five, you may observe them elevate and depress their body alternately. This oscillating movement is produced by the weight of their body and the elasticity of their legs, and is constant and uninterrupted during their repose. Unless it be connected with the respiration of the animal, it is not easy to say what is the object of it. Moths, when feeling the stimulus of desire, or under alarm, set their whole body into a tremor[468]. A living specimen of the hawk-moth of the willow being once brought me, upon placing it upon my hand, after ejecting a milky fluid from its anus, it put its wings and body into a most rapid vibration, which continued more than a minute, when it flew away. A butterfly, called by Aurelians "The large skipper," (Hesperia Sylvanus,) when it alights, which it does very often, for they are never long on the wing, always turns half-way round; so that, if it settles with its head from you, it turns it towards you.
Others of the motions in question are merely those of parts. Butterflies, when standing still in the sun, as you have doubtless often observed,
"Their golden pinions ope and close;"
thus, it should seem, unless this motion be connected with their respiration, alternately warming and cooling their bodies. You have probably noticed a very common little fly, of a shining black, with a black spot at the end of its wings (Seioptera vibrans[469]). It has received its trivial name (vibrans) from the constant vibration which, when reposing, it imparts to its wings. This motion also, I have reason to think, assists its respiration.—Some insects when awake are very active with their antennæ, though their bodies are at rest. I remember one evening attending for some time to the proceedings of one of those caseworm-flies (Leptocerus), that are remarkable, like certain moths, for their long antennæ. It was perched upon a blade of grass, and kept moving these organs, which were twice as long as itself, in all directions, as if by means of them it was exploring every thing that occurred in its vicinity.—Many Tipulæ, and likewise some mites (Acarus vibrans and Gamasus motatorius), distinguished by long anterior legs, from this circumstance denominated pedes motatorii by Linné, holding them up in the air impart to them a vibratory motion, resembling that of the antennæ of some insects[470].—I scarcely need mention, what must often have attracted your attention, the actions of flies when they clean themselves; how busily they rub and wipe their head and thorax with their fore legs, and their wings and abdomen with their hind ones.—Perhaps you are not equally aware of the use to which the rove-beetles (Staphylinus, L.) put their long abdomen. They turn it over their back not only to put themselves in a threatening attitude, as I lately related[471], but also to fold up their wings with it, and pack them under their short elytra.
With respect to the motions of insects in action, they may be subdivided, as was just observed, into motions whose object is change of place—and sportive motions.
The locomotions of these animals are walking, running, jumping, climbing, flying, swimming, and burrowing. I begin with the walkers.
The mode of their walking depends upon the number and kind of their legs. With regard to these, insects may be divided into four natural classes; viz. Hexapods, or those that have only six legs: such are those of every order except the Aptera of Linné, of which only three or four genera belong to this class.—Octopods, or those that have eight legs, including the tribes of mites (Acarina); spiders (Araneidæ); long-legged spiders (Phalangidæ); and scorpions (Scorpionidæ):—Polypods, or those that have fourteen legs, consisting of the woodlouse tribe (Oniscidæ);—and Myriapods, or those that have more than fourteen legs—often more than a hundred—composed of the two tribes of centipedes (Scolopendridæ) and millepedes (Julidæ). The first of these classes may be denominated proper, and the rest improper insects. The legs of all seem to consist of the same general parts; the hip, trochanter, thigh, shank, and foot; the four first being usually without joints (though in the Araneidæ, &c. the shank has two), and the foot having from one to above forty[472].
In walking and running, the hexapods, like the larvæ that have perfect legs, move the anterior and posterior leg of one side and the intermediate of the other alternately, as I have often witnessed. De Geer, however, affirms that they advance each pair of legs at the same time[473]; but this is contrary to fact, and indeed would make their ordinary motions, instead of walking and running, a kind of canter and gallop. Whether those that have more than six feet move in this way—which is not improbable—from the difficulty of attending at the same time to the movements of so many members, is not easily ascertained.
The dog-tick (Ixodes Ricinus), if when young and active it moves in the same way that it does when swoln to an enormous size with blood, seems to afford an exception to the mode of walking just described. It first uses, says Ray, its two anterior legs as antennæ to feel out its way, and then fixing them, brings the next pair beyond them, which being also fixed, it takes a second step with the anterior, and so drags its bloated carcase along[474]. Redi observes, that when scorpions walk they use those remarkable comb-like processes at the base of their posterior legs to assist them in their motions, extending them and setting them out from the body, as if they were wings: and his observation is confirmed by Amoreux, who calls them ventral swimmers[475]. I have often noticed a millepede (Julus terrestris), frequently found under the bark of trees, and where there is not a free circulation of air, the motions of which are worthy of attention. Observed at a little distance, it seems to glide over the surface, like a serpent, without legs; but a nearer inspection shows how its movement is accomplished. Alternate portions of its numerous legs are extended beyond the line of the body, so as to form an obtuse angle with it, while those in the intervals preserve a vertical direction. So that, as long as it keeps moving, little bunches of the legs are alternately in and out from one end to the other of its long body; and an amusing sight it is to see the undulating line of motion successively beginning at the head and passing off at the tail.—The motion of centipedes (Scolopendra), as well as that of this insect and its congeners, is retrogressive as well as progressive. Put your finger to the common one (Lithobius forficatus), and it will immediately retrograde, and with the same facility as if it was going forwards. This difference, however, is then observable—it uses its four hind legs, which, when it moves in the usual way, are dragged after it. Almost all the other apterous insects, as well as many of those in the other orders, can move in all directions; backwards, and towards both sides, as well as forwards. Bonnet mentions a spider (not a spinner) that always walked backwards when it attacked a large insect of its own tribe; but when it had succeeded in driving it from a captive fly, which however it did not eat, it walked forwards in the ordinary way[476].