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 humming 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, torn, vi., 1837. 



