NOISES OF INSECTS. 381 
flight—the action of the air perhaps upon these organs 
giving it some modifications. Whether, in the beetles 
that fly with noise, the elytra contribute more or less to 
produce it, seems not to have been clearly ascertained : 
yet, since they fly with force as well as velocity, the 
action of the air may cause some motion in them, enough 
to occasion friction. With respect to Diptera, Latreille 
contends that the noise of flies on the wing cannot be 
the result of friction, because their wings are then ex- 
panded; but though to us flies seem to sail through the 
air without moving these organs, yet they are doubtless 
all the while in motion, though too rapid for the eye to 
perceive it. When the aphidivorous flies are hovering, 
the vertical play of their wings, though very rapid, is 
easily seen; but when they fly off it is no longer visible. 
Repeated experiments have been tried to ascertain the 
cause of sound in this tribe, but it should seem with 
different results. De Geer, whose observations were 
made upon one of the flies just mentioned, appears to 
have proved that, in the insect he examined, the sounds 
were produced by the friction of the root or base of the 
wings against the sides of the cavity in which they are 
inserted. To be convinced of this, he affirms, the ob- 
server has nothing to do but to hold each wing with the 
finger and thumb, and stretching them out, taking care 
not to hurt the animal, in opposite directions, thus to 
prevent their motion,—and immediately all sound will 
cease. For further satisfaction he made the following 
experiment. He first cut off the wings of one of these 
flies very near the base; but finding that it still continued 
to buz as before, he thought that the winglets and 
poisers, which he remarked were in a constant vibration, 
