628 THE SENSES AND SENSORY ORGANS. 



intermittent expulsion of air from the tympanic spiracle, and 

 the vibration of the air in the tympanic bullae and scutellum, 

 as the whole thorax vibrates distinctly when the insect is held 

 between the linger and thumb. The vibration of the wings in 

 Eristalis during the production of sound is not a vibration due 

 to a movement of the wing on its articulations, but a wave 

 which proceeds from the root to the apex of the wing, as if it 

 were produced by a current of air impinging upon the wing 

 from below. When I observed this phenomenon, the wings 

 were at rest over the back, and the current apparently came 

 from the tympanic spiracles. 



The mere coincidence of the number of wing vibrations 

 with the ground tone of the sound emitted is not conclusive, 

 as is sometimes apparently held, that the note is due to these 

 vibrations ; both the vibrations of the wing and the rhythmic 

 impulses which produce the sound are probably directly 

 dependent on the number of muscular contractions per second 

 which are normal to the species, and it is probable, therefore, 

 that in both cases the number of vibrations per second would 

 be the same ; the mere fact that silent flight is possibly as 

 rapid, or even more rapid than the buzzing flight supposed to 

 be constant in these insects, is in itself conclusive against the 

 usually received view. And as Landois asserts, the note emitted 

 is frequently an octave higher than the ground tone, or of an 

 even higher pitch. 



The close relation of the tympanic apparatus of the Blow-fly 

 with a supratympanic organ, and the presence of a Mullerian 

 organ, apparently indicate that this structure is concerned in 

 the perception of sounds. If the halteres are organs of audition, 

 it may appear at first sight highly improbable that two such 

 complex organs are both concerned in the reception of sounds. 

 When, however, the complex character of the internal ear in 

 Vertebrates is borne in mind, the improbability becomes less 

 manifest. That insects must hear the sounds they produce is 

 evident, and that they distinguish between these sounds and 

 others is scarcely doubtful. That the function of the halteres 

 differs from that of the tympanic apparatus is indubitable, but 



