476 MOTIONS OF INSECTS. 
poisers, which, when the insect reposes, fold over each other like the valyes 
of a bivalve shell; but when it flies they are extended. The use of neither 
of these organs seems to have been satisfactorily ascertained. Dr. Derham 
thinks they are for keeping the body steady in flight ; and asserts that if 
either a poiser or winglet be cut off, the insect will fly as if one side over. 
balanced the other, till it falls to the ground; and that if both be cut off, 
they will fly awkwardly and unsteadily, as if they had lost some very 
necessary part.’ Shelver cut off the winglets of a fly, leaving both wings 
and poisers, but it could no longer fly. He next cut off the poisers of 
another, leaving the wings and winglets, andthe same result followed. He 
found, upon removing one of these organs, that they were not properly 
compared to balancers. Observing that a common crane-fly (Tipula crocata) 
moved the knee of the hinder tibia in connection with the wing and poiser, 
he cut it off, and it could no longer fly: this last experiment, however, 
seems contradicted by the fact, which has been often observed, that the 
insects of this genus will fly when half their legs are gone. He afterwards 
cut off both its poisers, when it could neither fly nor walk. Hence he con- 
jectures that the poisers are connected with the feet, and are air-holders.? 
I have often seen flies move their poisers very briskly when at rest, 
particularly Seioptera vibrans, before mentioned. This renders Shelver’s ' 
conjecture—that they are connected with respiration—not improbable, 
Perhaps by their action some effect may be produced upon the spiracle in 
their vicinity, either as to the opening or closing of it. 
There are three classes of fliers in this order, the form of whose bodies, 
as well as the shape and circumstances of their wings, is different. First 
are the slender flies—the gnats, gnat-like flies, and crane-flies (Tipulavia). 
The bodies of these are light, their wings narrow, and their legs long, and 
they have no winglets. Next to those whose bodies, though slender, are 
more weighty—the Asilide, Conopside, &c.; these have larger wings, shorter 
legs, and very minute and sometimes even obsolete winglets. Lastly come 
the flies, the Muscide, &c., and their affinities, whose bodies being short, thick, 
and often very heavy, are furnished not only with proportionate wings and 
shorter legs, but also with conspicuous winglets. From these comparative 
differences and distinctions, we may conjecture in the first place — since 
the lightest bodies are furnished with the longest legs, and the heaviest 
with the shortest—that the legs act as poisers and rudders, that keep them 
steady while they fly, and assist them in directing their course; and in the 
next— since the winglets are largest in the heaviest bodies, and altogether 
wanting in the lightest—that one of their principal uses is to assist the 
wings when the insect is flying. 
The flight of the Tipularian genera is very various. Sometimes, as I 
have observed, they fly up and down with a zigzag course; at others in 
vertical curves of small diameter, like some birds ; at others, again, in 
horizontal curves; —all these lines they describe with a kind of skipping 
motion. Sometimes they would seem to flit in every possible way —up- 
wards, downwards, athwart, obli uely, and sometimes almost in circles. 
The common gnat (Culex iient seems to sail along also in various de- 
1 Phys. Theol. 18th ed. 866, note (i). 
2 Wiedemann’s Archiv. ii. 210. 
5 To those that frequent meadows and pastures (T'pula oleracea L. &c.) they are 
also useful, as I have before observed, as stilts to enable them to walk over the grass. 
Reaum, y. Pref. i, t. iii, f. 10, 
