INSECT VARIETY . ' 231 



vibrations or sounds j and I have already drawn attention to certain 

 papers in the Transactions of the Linnean Society, on the struc- 

 tural suitability of some membraneous vesicles on their surface to 

 the acoustic faculty. But be this as it may, it is certain any 

 structure situated in the head or its appendages in Insecta, 

 considered as an organ of hearing, has hitherto proved exces- 

 sively minute ; while, on the other hand, it is now becoming 

 an axiom that organs of sense in Insecta — and I believe, too, 

 in Mollusca — have neither stable external position nor internal 

 connection, save that some communicate with the anterior 

 ganglia of the nervous chords, these nerve-knots themselves 

 here assuming a spontaneousness of action that in the Vertebrata 

 flows alone from the brain. 



ORGAN OF HEARING IN THE CICADID^ (PLATE III., 

 MG. 5, A, B ; PLATE VI., FIG. 5.) 



To commence with the cicadte, these challenge and reply 

 seated at a considerable distance, which, considering the low 

 pitch, the spherical propagation and refraction of their notes, 

 would accord an auditory adit of some dimensions ; the females 

 also "alight near'''''^ the musical males. Now, the corresponding 

 cavities at the base of the abdomen placed ventrally, and closed 

 inwardly by a posterior, tense, iridescent, and anterior soft mem- 

 brane (Plate III., Fig. 5a, m), answer the requirements of an 

 external ear structurally, as by intermittent exposure in the 

 males to atmospheric impressions, in the action accompanying 

 their music. These cavities, taking a considerably greater de- 

 velopment in the male, are similar in either sex, and seem to 

 correspond with the other auditory structures I shall notice. 

 Although considered by Reaumur f designed to augment the 

 sound of the tymbals, Mons. Sober, who put the conjectures 

 of that author to a practical test with living examples of 

 provincial cicad®, on the removal of their constituent parts, 

 hesitatingly assures us '' the sound became a little feebler and 

 modified; " while Colonel Goureau,J having not only removed the 



* Darwin, " Desc. of Man," Vol. I., chap. x. 



f Reaumur, "Mem. pour Ser.," T. V., Mem. iv. 



X "Annl. Ent. Soc. de France," T. VII., p. 403, 1838. 



