378 BURMEISTER ON THE SOUND PRODUCED 
me, therefore, not improper to call the attention of natural philosophers 
to this subject by 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 the insect emits is capable of considerable variations. 
It may be that it maintains an equality of pitch and strength during a 
uniform motion of the wings, 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 
: _be — 
from = to — as the effort to extricate it- 
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 phzenomenon 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 thorax. 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 symmetrically arranged elevations and depressions (fig.7. 
Plate V.), but is notwithstanding perfectly continuous. These elevations, 
the relative magnitude 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 continuations of 
the trachez, which 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 mefa- 
phragma. Fig.’7. B), and to which the great dorsal muscle, of which a 
horizontal section is represented in fig.8. A Plate V., is attached in the di- 
rection AB. On the middle of the back the further point of connection of 
this muscle forms a broad longitudinal stripe. 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 CD 
and EF through the cavity of the thorax. In fig. 8. a section of them 
