74 THE ANIMALS AND MAN 



delicate vibratile hairs. Auditory organs of this general 

 character are known among the polyps, the worms, the 

 crustaceans, and the molluscs. Recent studies seem to 

 show that the otoliths have a special use as organs which 

 help the animal to keep its equilibrium. In the common 

 crayfish the "ears" are situated in the basal segment of the 

 inner antennae or feelers. They consist each of a small 

 sac filled with liquid, in which are suspended several grains 

 of sand or other hard bodies. The inner surface of the 

 sac is lined with fine auditory hairs. ,The sound-waves 

 coming through the air or water outside strike against this 

 sac, which lies in a hollow on the upper or outer side of the 

 antennas. The sound-waves are taken up by the contents of 

 the sac and stimulate the fine hairs, which in turn give this 

 stimulus to the nerves which run from them to the principal 

 auditory nerve and thus to the brain of the crayfish. Among 

 the insects other kinds of auditory organs exist. The com- 

 mon locust or grasshopper has on the upper surface of the 

 first abdominal segment a pair of tympana or ear-drums (fig. 

 31), composed simply of the thinned, tightly-stretched 

 chitinous cuticle of the body. On the inner surface of this 

 ear-drum there are a tiny auditory sac, a fine nerve leading 

 from it to a small auditory ganglion lying near the tympanum, 

 and a large nerve leading from this ganglion to one of the 

 larger ganglia situated on the floor of the thorax. In the 

 crickets and katydids, insects related to the locusts, the 

 auditory organs or ears are situated in the fore legs. 



Certain other insects, as th& mosquitoes and other midges 

 or gnats, undoubtedly hear by means of numerous delicate 

 hairs borne on the antennje. The male mosquitoes have 

 many hundreds of these long, fine antennal hairs, and on 

 the sounding of a tuning-fork they 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 



