628 THE SENSES AND SENSORY ORGANS. 
intermittent expulsion of air from the tympanic spiracle, and 
the vibration of the air in the tympanic bull and scutellum, 
as the whole thorax vibrates distinctly when the insect is held 
between the finger 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 Miillerian 
organ, apparently indicate that this structure is concerned in 
the perception of sounds. Ifthe 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 
