THE TYMPANIC ORGANS OF THE BLOW-FLY. 627 
organs, it is not supposed that they are the primary organs 
concerned, and their function is regarded as that of resonators ; 
that is, it is supposed that their vibrations ‘are sympathetic, 
and serve only to intensify the sounds otherwise produced. A 
membrane set into such vibrations is essentially similar to the 
tympanum of a vertebrate ear, and evidence is wanting that 
such vibrations produce any sensible effect on the volume of 
sound produced ; even the larger tympanum of a phonograph 
produces sounds of small intensity when set into vibration, and 
except when it is directly connected with the ear by a tube, or 
its vibrations are intensified by a trumpet, sounding-board or 
other resonator, they are scarcely audible ; on the other hand, 
it is undoubtedly a most efficient receptive surface, and the 
tympanic mirror of a Cicada supplied with a receptive nerve 
mechanism would be a most perfect auditory apparatus. The 
fact that the mirrors are equally developed in the mute females 
and in the strident males is a fact which points to a sound- 
receiving rather than a sound-producing function. And M. 
Solier* affirmed that the perforation or tearing of these parts 
produces but little effect on the volume of sound emitted by the 
insects. 
The humming produced by the Diptera and Hymenoptera is 
usually attributed to the rapid vibrations of the wings. If this 
movement produces the sound emitted, it is difficult to under- 
stand why even the most rapid flight is usually unattended by 
humming. In the Blow-fly at least, the emission of sound 
is entirely under the control of the insect. 
Landois [281], although he attributes some part of the 
sound to the movements of the wings, regards the humming 
of the Diptera as due to an expiratory effort, and, further, attri- 
butes it to the rapid vibration of the valves of the thoracic 
spiracles. I am convinced that it is not due to the expulsion 
of air through those spiracles ; when an insect is hamming the 
spiracles are closed. Moreover, I have already shown that the 
ordinary spiracles are not expiratory. 
It appears probable that the sound emitted is due to an 
* Ann. Entom. Soc. de France, tom. vi., 1837. 
