378 BURMEISTER ON THE SOUND PRODUCED 



me, therefore, not improper to call the attention of natural philosophers 

 to this subject bj- a special memoir, and in particular to give a sketch 

 of the mechanism which forms the sound. I must next remark, that 

 the sound which tlie insect emits is capable of cousiderable variations. 

 It may be tliat it maintains an cqualitj' of pitch and strength during a 

 uniform motion of the vings, for so in fact it appears; but every 

 change in the velocity of the flight, every disturbance of the ordinary 

 motion, generally causes also an alteration in the tone. An idea of the 

 origin of the tone is however only to be obtained when the insect is held 

 by the legs, and excited by pressure or other means to go through all its 

 motions of the wing, and thus to produce a sound. I found in this 

 manner that the tone of the common gad-fly ( Tabanus bovinus) varied 



?./• !- — as the effort to extncate it- 



»)' ♦ 



from y / • P —— to 



self from the hands of the troublesome observer was shown with greater 

 or less energy. Such a difference might be explained, it is true, upon 

 the supposition that the agitation of the wing produces the tone by the 

 varying rapidity with which the vibrations are made; but this expla- 

 nation is untenable, as the same phcenomenon continues when the wings 

 are entirely cut away; an operation which produces only a variation of 

 the tone, but does not render its formation impossible. 



Before I proceed to assign the true cause of the sound, I think it ne- 

 cessary to give a short description of that part of the insect by which 

 alone the sound is produced. This part is the breast or lliorax. This 

 consists, in two-winged insects (^Diptera, Linn.) of a simple cavity, co- 

 vered by a thin elastic parchment-like envelope, which exhibits on its 

 surface various sj'mmctrically arranged elevations and depressions (fig. 7. 

 Plate v.), but is notwithstanding perfectly continuous. These elevations, 

 the relative maguitude and form of which differ very much in different 

 diptera, originate either in the muscles attaching themselves to the in- 

 ternal surface of the cavity, or in air-bladders forming contmuations of 

 the tracheas, M'hich stretch in these parts the coriaceous skin and make it 

 vesicular. The largest of these elevations is the vaulted partition which 

 forms the limit between the thorax and the abdomen (Kirby's meta- 

 phrag)iia. Fig. 7. B), and to which the great dorsal muscle, of which a 

 horizontal section is represented in fig.S. A Plate V., is attached in the di- 

 rection A B. On the middle of the back the further point of connection of 

 this muscle foinns a broad longitudinal strijie. Near to this, on each side, 

 appear two elevations, a front one lesser (fig. 7. C), and a hinder one 

 greater (fig. 7. E, where it appears partly covered by the wing) : both 

 originate in the lateral muscles, which are extended in the direction C D 

 and E F through the cavity of the thorax. In fig. 8. a section of them 



