522 



NOISES OF INSECTS. 



respect to Diptcra, Latreille contends that the noi^ 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 

 Ofo-ans, 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 ver- 

 tical 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 con- 

 vinced of this, he affirms, the observer has nothing to do but to hold each 

 wing with the finger and thumb, and stretching them out, taking care not 

 to hurt the tinimal, 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 buzz as before, he 

 thought that the winglets and poisers, which he remarked were in a con- 

 stant vibration, might occasion the sound. Upon this, cutting both off, 

 he examined the mutilated fly with a niicroscope, and found that the 

 remaining fragments of the wings were in constant motion all the time 

 that the buzzing continued ; but that upon pulling them up by the roots 

 all sound ceased.^ Shelver's experiments, noticed in my last letter, go 

 to prove, with respect to the insects that he examined, that the winglets 

 are more particularly concerned with the buzzing. Upon cutting oft' the 

 wings of a fly — but he does not state that he pulled them up by the roots 

 — he found the sound continued. He next cut off the poisers — the buz- 

 zing went on. This experiment was repeated eighteen times with the 

 same result. Lastly, when he took off the winglets, either wholly or 

 partially, the buzzing ceased. This, however, if correct, can only be a 

 cause of this noise in the insects that have winglets. Numbers have them 

 not. He next, therefore, cut off the poisers of a crane-fly (Tipula cro- 

 cata), and found that it buzzed when it moved the wing. He cut off 

 half the latter, yet still the sound continued ; but when he had cut off 

 the whole of these organs the sound entirely ceased.^ 



Dr. Burmeister, however, was led by his experiments to a different 

 conclusion. Finding that the buzz of a large fly (^Eristalis ienax) still 

 continued after the winglets, the poisers, and even the wings had been 

 quite cut off except their very stumps (only in this last case the sound 

 was somewhat weaker and higher), he conceived that the spiracles lying 

 between the meso-and meta-thorax must be the instruments of the sound, 

 which accordingly he found to cease entirely when they were stopped 

 with gum, though while the wings were in vibration. Pursuing his 

 researches, he extracted one of these spiracles, and opening it carefully, 

 found its posterior and inner lip, which is directed towards the commence- 

 ment of the trachea, to be expanded into a small flat crescent-shaped 

 plate, upon which arc nine parallel very delicate horny laminae, the central 

 one being the largest, while those on each side became gradually smaller 



> De Geer, vi. 13. * Wiedemann's Archiv. ii. 210. 217. 



