MOTIONS OF INSECTS. 



289 



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 (Tipularice). The bodies of 

 these are light, their wings narrow, and their legs long, and 

 they have no winglets. Next are those whose bodies, though 

 slender, are more weighty — the Asilidce, Conopsidoe, &c. ; 

 these have larger wings, shorter legs, and very minute and 

 sometimes even obsolete winglets. Lastly come the flies, 

 the MuscidcB, &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 1 ; 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 carves : — all 

 these lines they describe with a kind of skipping motion. 

 Sometimes they would seem to flit in every possible way — 

 upwards, downwards, athwart, obliquely, and sometimes 

 almost in circles. The common gnat ( Culex pipiens) seems 

 to sail along also in various directions. The motion of its 



i To those that frequent meadows and pastures (Tipula oleracea L. &c.) they 

 are also useful as I have hefore observed, as stilts, to euable them to walk over 

 the grass. Reaum. v. Pref. i. t. iii. f. 10. 



VOL. II. TJ 



