NOISES OF INSECTS. 377 



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, 



