288 



MOTIONS OF INSECTS. 



winged flies ; but, to replace the under wings of the tetrap- 

 terous insects, they are furnished with poisers, and numbers 

 of them also with winglets. The poisers (lialteres) are little 

 membranaceous threads placed one under the origin of each 

 wing, near a spiracle, and terminated by an oval, round, 

 or triangular button, which seems capable of dilatation and 

 contraction. The animal moves these organs with great 

 vivacity, often when at rest, and probably when flying. 

 Their winglets (alulcR) are different from those of Dytiscus 

 marginalis, and the moth before noticed. Like them, they 

 are of rigid membrane, and fringed; but they consist 

 generally of two concavo-convex pieces (sometimes sur- 

 rounded by a nervure), situated between the wing and the 

 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 winglet be cut off, the insect will fly as 

 if one side overbalanced 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. 1 

 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, 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 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 

 conjectures that the poisers are connected with the feet, and 

 are air-holders. 2 I have often seen flies move their poisers 



1 Phys. Theol. 13th ed. 366. note (*.) 



2 Wiedemann's ArcMv. ii. 210. 



