476 MOTIOXS OF IXSECTS. 



poisers, which, when the insect reposes, fold over each other like the valves 

 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 win^let 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 oft' 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, and the same result followed. He 

 found, upon removing one of these organs, that they were not properly 

 compared to balancers. Observing that a common cvanc-Hy (Tipiu'ci 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, ami are air-holders.'^ 

 I have often seen flies move their poisers very briskly when at rest, 

 particularly Scioptera 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 {Tipidar'uE). 

 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 AsilidcE, Conopsidce, &c. ; these have larger wings, shorter 

 legs, and very minute and sometimes even obsolete winglets. Lastly come 

 the flies, the Muscidce, &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 tlie 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 zigzng 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, obliquely, and sometimes almost in circles. 

 The common gnat (jCidex- pipiens) seems to sail along also in various de- 



1 Phys. Theol. 13th ed. 366. note {i). 



2 Wiedemann's Archiv. ii. 210. 



5 To tliose that frequent meadows and pastures ( Tipula oleracea L. &c.) they are 

 also useful, as I have before observed, as stilts to enable them to walk over the grass. 

 Iveaum. v. Pref. i. t. iii. f. 10. 



