BALANCE AND HEARING 



37 



t.b. 



of the higher Crustaceans are able to emit sounds, it is quite 

 possible that their otocysts are beginning to acquire a new 

 use, i.e. that of serving as auditory organs. The Rock- Lobster 

 {Palinurus}, for example, makes a creaking noise by moving 

 the basal joints of the large feelers, which then rub against 

 their sockets. An unpleasant sound of similar nature can be 

 produced by twisting a glass stopper in the neck of its bottle. 

 A more specialized case is that of the Musical Strand- Crab 

 (Ocypoda macrocera], which has been described by Alcock (in 

 A Naturalist in Indian 

 Seas). In this animal the 

 inner side of the large 

 nippers is provided with 

 a ridge or scraper placed 

 near the base of the limb, 

 and a rasp-like ridge or 

 key-board on the fixed joint 

 of the claw. By drawing 

 the scraper over the key- 

 board a sort of chirping 

 sound is produced, not 

 unlike the one with which 

 our native grasshoppers 

 have made us familiar. 

 The same zoologist speaks 



glion (gn.}, nerve, and ligament \hg.) still further enlarged. 



of the Squeaker Crab 



(Psopheticus stridulans) of the Andaman Sea as making a dismal 

 noise by rubbing a spine which projects from the base of its 

 nippers against a rough knob near the eye-socket. 



ORGANS OF BALANCE AND HEARING IN INSECTS (INSECTA). 

 A variety of organs situated in different parts of the body are 

 probably connected with balance or hearing, or both. Among 

 those which are most likely concerned with equilibrium and move- 

 ment are certain peculiar structures (chordotonal organs) that 

 are especially characteristic of aquatic larvae, though not limited 

 to these. Gnat larvae, for example, possess such organs, one of 

 which is represented in fig. 1047. ^ consists essentially oi a 

 group of rod -shaped cells contained in a tube that opens to 

 the exterior. 



Many insects make sounds which are doubtless heard by 



km. 



Fig. 1047. Chordotonal Organs. On the right is shown part of 

 an abdominal segment of the larva of a Gnat (Corethra plumicor- 

 nis], seen as a transparent object, enlarged. In the centre is the 

 nerve-cord (darkly shaded) with the ganglion (gn.} of the segment; 

 l.m., longitudinal muscles; ch.n., ch.g., ck.l., and ch., chordotonal 

 nerve, ganglion, ligament, and organ; t.b., branched tactile bristles. 

 On the left is seen part of the chordotonal organ with its rods, gan- 



