THE HALTERES OF THE BLOW-FLY. 619 
Since the density of the atmosphere varies with the phase of 
a sound-wave, and as such variations of density are an efficient 
cause of retardation, and acceleration in a body vibrating in 
air, it may be assumed that the rate of the vibration of the 
halteres is affected by them. It can be shown that the more 
rapidly a body moves, the greater the resistance of the atmo- 
sphere becomes ; and this resistance increases with the square 
of the velocity of the moving body (see p. 387). Sound vibra- 
tions must therefore affect the moving halter and produce 
disturbances of its movement. The more rapid the movement 
of the halter itself the greater these disturbances become; and 
as they are produced by the varying density of the air through 
which it moves, or by the waves of sound, we may regard 
these disturbances as vibrations superimposed upon the proper 
vibrations of the halter due to the action of its own muscles. 
Since every form of ear possesses the power of analysing 
sound-waves or separating superimposed vibrations, the vibra- 
ting halter endowed with such a power would become a most 
efficient organ of hearing. 
Airy [297, p. 142] states that: ‘ The only experiment which 
is sufficiently delicate to give a measure of the pressure of a 
sound-wave is the observation of its influence on the move- 
ment of a pendulum whose bob is a sphere’; and he shows 
that the effect is directly proportional to the square of the 
number of vibrations per second or to what comes to the 
same thing, the square of the mean velocity of the moving 
pendulum bob. As we know that the movements of the tym- 
panic membrane of the ear are entirely due to changes of the 
density of the air in its vicinity, it is clear that these changes 
would have a marked effect on a minute vibrating halter. So 
the velocity of the movement of the insect in flight would give 
rise to variations of a similar character, differing, however, 
from those of sound in not being rhythmic, but more or less 
constant pressures modifying the envelope of the curves in 
which the halter vibrates. 
We may further assume that the fluid in the capitellum, stalk 
and scale of the halter moves as a rigid body, like the fluid in 
