366 



THE LOCUST 



type of sound production is the 

 " seventeen-year locust," which is 



ANTERIOR SIDE 



POSTERIOR SIDE 



Fig. 185. — Diagrammatic transverse 

 section through anterior thoracic ap- 

 pendage of katydid, Microce7itru7n rhom- 

 bifolium (cf. Fig. 191, p. 373), showing 

 structure of the organ of hearing. 



Outer chambers {OTC) communicate ex- 

 ternally by the openings CS. A tympanum or 

 drum {TYM) separates these two outer cavities 

 from the single inner tympanic cavity {ITC, 

 AC and PC) with which sensory cells (SC) arc 

 connected. Vibrations in the air are transferred 

 to the sensory cells shown by the arrows. 

 AC, anterior portion of inner tympanic cham- 

 ber; BL, blood vessel; DP, tracheal tube; CS, 

 opening of outer tympanic chamber; GC, sec- 

 ondary ganglion ceUs which relay impulse to 

 central nervous system; ITC, inner tympanic 

 chamber; MU, cross-sections of muscle; 

 N, nerve; NU, nucleus of sensory cell; OTC, 

 outer tympanic chamber; PC, posterior portion 

 of inner tympanic chamber; SC, sensory cell; 

 SH, shield that protects tympanum; STR, 

 prop-like support of sensory cell; STY, sensory 

 process of cell; TYM, tympanum; V, mem- 

 brane between inner cavities. (After Dahl- 

 gren. Natural History, Vol. XXV, No. -i, 

 courtesy of American Museum of Natural 

 History.) 



with nerves at their inner ends. 



" shrilling " of the cicada, or 

 produced by the vibration of 

 membranes on either side of 

 the most anterior segment 

 of the abdomen and operated 

 by special muscles. 



The sense of hearing may 

 be regarded as developed in 

 correlation with the foregoing 

 power of sound production. 

 Indeed, the very presence of 

 special sound-producing or- 

 gans would be presumptive 

 evidence that insects could 

 hear even if one could not 

 locate the special auditory 

 organs {cf. Fig. 185). In con- 

 • trast with vertebrate animals, 

 in which the ears are homo- 

 logous structures and always 

 located in the same region, 

 the "ears" of insects are of 

 several types and located 

 in different places. They 

 may occur, according to the 

 species, upon the abdomen, 

 the legs, the antenna?, and 

 other portions of the body. 

 In the grasshopper the tym- 

 panic membranes and asso- 

 ciated internal parts are 

 assumed to be auditory 

 organs, more on account of 

 their structure than from any 

 direct experimental evidence, 

 since they consist of a 

 membrane against the inner 

 surface of which are proc- 

 esses that are connected 

 The mechanism resembles the 



