THE SPECIAL SENSES 235 
or gnats, undoubtedly hear by means of numerous delicate 
hairs borne on the antenne. The male mosquitoes (Fig. 
149) have many hundreds of these long, fine antennal hairs, 
and on the sounding of a tuning-fork these hairs have been 
observed to vibrate strongly. In the base of each antenna 
there is a most elaborate organ, 
composed of fine chitinous 
rods, and accompanying nerves 
and nerve cells whose function 
it is to take up and transmit 
through the auditory nerve to 
the brain the stimuli received 
from the external auditory 
hairs. 
123. Sound - making, — The 
sense of hearing enables ani- 
mals not only to hear the 
warning natural sounds of 
storms and falling trees and 
plunging avalanches, but the 
sounds made by each other. 
Sound-making among animals 
serves to aid in frightening 
away enemies or in warning "SS paite (ty cn the ae 
companions of their approach, _ tenne. 
for recognition among mates 
and members of a band or species, for the attracting and 
wooing of mates, and for the interchange of information. 
With the cries and roars of mammals, the songs of birds, 
and the shrilling and calling of insects all of us are familiar. 
These are all sounds that can be heard by the human ear. 
But that there are many sounds made by animals that 
we can not hear—that is, that are of too high a pitch for 
our hearing organs to be stimulated by—is believed by nat- 
uralists. Especially is this almost certainly true in the case 
of the insects, The peculiar sound-producing organs of 
