io8 FIRST LESSONS IN ZOOLOGY 



which one or more tiny spherical hard bodies called 

 otoliths are held. This auditory sac is formed of, or 

 lined internally by, auditory cells, specialized nerve-cells, 

 which often bear delicate vibratilc hairs. Auditory or- 

 gans of this general character are known among tjie 

 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 

 \\-ith 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 

 antenna;. The sound-waves are taken up b_\- the con- 

 tents of the sac anil stimulate the fine hairs, which in 

 turn gi\'e 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 common locust or grasshopper has 

 on the upper surface of the first abdominal segment a 

 pair of tympana or ear-drums (fig. 74), 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 crick- 

 ets anti katydids, insects related to the locusts, the audi- 

 tory organs or cars arc situated in the fore legs. 



Certain other insects, as the mosquitoes and other 

 midges or gnats, undoubtedly hear by means of numerous 

 delicate hairs borne on the antcnn.e. The male mosqui- 



