380 BURMEISTER ON THE SOUND PRODUCED 
pends, is produced by the contraction of the lateral muscles ; for, the co- 
vering of the back being drawn down presses on the base of the wing, and 
thereby the short base end of the wing, projecting freely into the cavity of 
the thorax, is forced downwards beneath the wing about the more out- 
ward point of support; in consequence of which the opposite end rises. 
This raising of the wing is then accelerated by its proper muscles of ele- 
vation E E and F F, but ceases as soon as these and the simultaneously 
contracted lateral muscles are relaxed. The wing now falls again, which 
falling is accelerated by the contraction of the dorsal muscle and the 
concomitant curvature of the back. Such then is the mechanism which 
produces the action of the wing. But at the same time this mechanism 
also causes rhythmical contractions and expansions of the whole thorax 
and of thenumerous air-canalsof its interior. Now the contraction drives 
out a part of the air; the expansion, on the contrary, causes an equal 
quantity of fresh air to stream through the air-pipes above described ; 
and there is, therefore, connected with the motion of the wings, a con- 
stant, proportionally rapid, and intensive breathing ; and this breathing 
is the cause of the sound. It produces the sound by the alternate efflux 
and influx of the air, just as the current of air sounds the zolian harp 
when forced at short intervals through the small holes of the sound- 
board, or (to take a more familiar example) by a mechanism similar to 
that of the mouth in whistling. The sound of the zolian harp bears 
a wonderful resemblance to that of many insects; and the observation 
that the sound is no longer heard when the air-holes of the thorax are 
closed, without however injuring the insect in any other manner, is de- 
cisive. It is true that the insect dies of suffocation soon after this ex- 
periment, but not directly, because the respiration through the air-holes 
of the abdomen continues for some time; but these emit no sound during 
the flight of the insect, for they are then inactive, as other observations 
have proved ; the insect breathes through the air-holes of the abdominal 
part when it sits and crawls, but through the air-holes of the thorax 
when on the wing. According to this the hum of insects is in reality a 
whistle. The variation which the mutilation of the wings causes in the 
tone is easily to be accounted for, if we consider that by this action the 
moveable part becomes lighter, and also that the motion of the same, by 
the continued equal exertion, is quicker; but this causes a quicker cur- 
rent of air, which must produce a higher tone. On the contrary, a mu- 
tilation of the moveable apparatus produces a slower motion, a slower 
current of air, a deeper tone. I believe that the foregoing facts and 
observations offer sufficient evidence, but the reader can satisfy him- 
self as to the truth of the phenomenon by performing the same simple 
experiments. 
Ihave now only to add, that I have, in my Manual of Entomology, 
made mention of small moveable plates which are found behind the aper- 
