THE METATHORACIC MUSCULATURE OF 



CRYMODES DEVASTATOR (BRACE) 



(NOCTUIDAE) WITH SPECIAL 



REFERENCE TO THE 



TYMPANIC ORGAN 



By ASHER E. treat 



The City College of Nczv York. Special Research Fellow of the National Institute 

 of Allergy and Infections Diseases 



(With i6 Plates) 



The most distinctive feature of the noctuid metathorax is the 

 tympanum. Unique among auditory organs in having only two cellu- 

 lar elements specifically concerned with the acoustic response, this 

 structure in its evolution has entailed drastic modifications in the 

 skeletal parts both of its own segment and of the adjacent abdominal 

 region. The integumentary features associated with the tympanum 

 have been described in considerable detail and have entered promi- 

 nently into current schemes of classification. The histology of the 

 tympanic sensilla has been investigated by Eggers (1919, 1928). 

 Neurophysiological studies (Haskell and Belton, 1956; Roeder and 

 Treat, 1957) have confirmed the impressions of earlier writers that 

 the tympanum is a true acoustic receptor having maximum sensitivity 

 at frequencies somewhat higher than those perceptible to the human 

 ear. It is not yet known, however, whether hearing is the sole or even 

 the chief function of the tympanum, and the work of Roeder and 

 Treat has revealed an active neuronal component of the organ to 

 which no function can as yet be assigned. This element has been 

 found to include a large cell body closely associated with the tympanic 

 sensilla but attached to the surface of the skeletal structure referred 

 to by Eggers and by others as the Biigel. From this neurone is re- 

 corded a continuous and apparently spontaneous impulse discharge 

 having no obvious relation to the acoustic response and surviving the 

 destruction of the acoustic sensilla. The discharge frequency may 

 be increased experimentally by stretching the sheath of tracheal epi- 

 thelium which covers both the Biigel and the tympanic nerve. Could 

 the Bugel cell represent some kind of proprioceptive device, and if 

 so, what might be its relation to the movements of flight or of other 

 behavior? In seeking answers to these questions it appeared neces- 



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