MOTIONS OF INSECTS. 511 



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 conjectures 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 ren- 

 ders 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-flres {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, Conopsidce, &c. ; these have larger wings, 

 shorter legs, and very minute and sometimes even obsolete winglets. 

 Lastly come the flies, the Muscida, he, and their affinities, whose bodies 

 being short, thick, and often very heavy, are furnished not only with pro- 

 portionate 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 — 

 upwards, downwards, athwart, obliquely, and sometimes almost in circles. 

 The common gnat (^Culex pipiens) seems to sail along also in various direc- 

 tions. The motion of its wings, if it does not fly like a hawk, is so rapid 

 as not to be perceptible. When the crane-fly {Tipula oleracea) is upon 

 the wing, its fore-legs are placed horizontally, pointing forwards, and the 

 four hind ones stretched out in an opposite direction, the one forming the 

 prow and the other the stern of the vessel, in its voyage through the ocean 

 of air. The legs of another insect of this tribe {Hirtcea Marci) all point 

 towards the anus in flight, the long anterior pair forming an acute angle 

 with the body: — thus, perhaps, it can better cut the air. 



I have often been amused in my walks with the motions of the hornet- 

 fly (^Asilus crahroniformis) , belonging to the second division just mentioned. 

 This insect is carnivorous, living upon small flies. When yau are taking 

 your rambles, you may often observe it alight just before you ; as soon as 



' Wiedemann's Archio. ii. 210. 



• To those that frequent meadows and pastures fTt/jM/a oleracea L. &c.) they are also 

 asefiil as I have before observed, as stilts, to enable thein to walk over the grass. Reaum. 

 ». Pref. i. t. ill. f. 10. 



