NOISES OF INSECTS. 



305 



the old bard's gnats into hornets. In Guiana these animals 

 are distinguished by a name still more tremendous, being 

 called the devil's trumpeters. 1 I have observed that early in 

 the spring, before their thirst for blood seizes them, gnats 

 when flying emit no sound. At this moment (Feb. 18.) two 

 females are flying about my windows in perfect silence. 



After this short account of insects that give notice when 

 they are upon the wing by the sounds that precede them, I 

 must inquire by what means these sounds are produced. 

 Ordinarily, except perhaps in the case of the gnat, they seem 

 perfectly independent of the will of the animal ; and in 

 almost every instance, the sole instruments that cause the 

 noise of flying insects are their wings, or some parts near to 

 them, which, by their friction against the trunk, occasion a 

 vibration — as the fingers upon the strings of a guitar — 

 yielding a sound more or less acute in proportion to the ra- 

 pidity of their 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 expanded ; 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. 



VOL. II. 



Stedman's Surinam, i. 24. 

 X 



