360 MOTIONS OF INSECTS. 



are connected with the feet, and are air-holders^. I 

 have often seen flies move their poisers very briskly 

 when at rest, particularly Seioptera xihrans, 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 open- 

 ing or closing of it. 



There are three classes of fliers in this order, the 

 form of whose bodies, as well as the shape and circum- 

 stances of their wings, is different. First are the slen- 

 der flies — the gnats, gnat-like flies, and crane-flies 

 ( Tipulid(£). The bodies of these are light, their wings 

 narrow, and their legs long, and they have no wing- 

 lets. Next are those whose bodies, though slender, 

 are more weighty — i\\Q Asilidce^ Conopsidce, &c.; these 

 have larger wings, shorter legs, and very minute and 

 sometimes even obsolete winglets. Lastly come the 

 flies, the Muscidcp, and their aflSnities, 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 com- 

 parative differences and distinctions, we may conjec- 

 ture 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 



" Wiedemann's -<d^rcAiu. ii. 210— . 



'' To those that frequent meadows and pastures {Tipula oleracea, L- &c. ) 

 thej aieaJso useful, as I have before observed, as stilts, to enable them 

 to walk over the grass. Kcaum. v. Pref. i. t. iii. /. 10. 



