MOTIONS OF INSECTS. 359 
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. 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 connexion 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 after- 
wards 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 Sezoptera vibrans,- before mentioned. 
This renders Shelver’s conjecture—that they are con- 
nected with respiration—not improbable. Perhaps by 
their action some effect may be preduced 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 
4 Phys. Theol. 13th Ed. 366, note (7.) 
b Wiedemann’s Archiv. i. 210—. 
