MOTIONS OF INSECTS. 359 



The animal moves these organs with great vivacity, 

 often when at rest, and probably when flying. Their 

 winglets (AIuIce) are different from those of Dt/Uscus 

 marginalise and the moth before noticed. Like them, 

 they are of rigid membrane, and fringed ; but they con- 

 sist generally of two concavo-convex pieces (some- 

 times surrounded 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 nei- 

 ther 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^ 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 ex- 

 periment, 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 after- 

 terwards cut off both its poisers, when it could neither 

 fly nor walk. Hence he conjectures that the poisers 



" Phys. TheoL 13th Ed. 366, note (j.) 



