THE CHORDOTONAL ORGANS AND PLEURAL DISCS 

 OF CERAMBYCID LARVAE.* 



Walter N. Hess. 

 INTRODUCTION. 



For years systematists who have attempted the classification 

 of certain coleopterous larvae, have been perplexed by the 

 pecuHar and varied structures on the abdominal pleural region. 

 These often take the form of a ray-hke disc in the larger species, 

 while in the smaller species, a small elliptical enlargement is 

 present in this region. Many larvae, even among the Coleop- 

 tera, do not possess the characters in question, but they seem to 

 be universally present among the Cerambycids. Moreover, 

 they are often much modified in the different genera. 



During the summer of 1914, Dr. W. A. Riley, of this depart- 

 ment, found these pecuHar pleural structures on various larvae. 

 About the same time Mr. F. C. Craighead, of the Department 

 of Entomology at Washington, wrote Dr. Riley asking about 

 their functions. It was at this time that the writer undertook a 

 study of their structure. 



The writer is sincerely indebted to Mr. Craighead for 

 mounts of these structures which he had turned over to Dr. 

 Riley, also to Dr. Riley himself for much valuable advice and 

 assistance. 



In some larvae, especially the larger species of Prionids, 

 these structures are very pronounced, and instances have been 

 known of students mistaking them for spiracles. 



References in the literature are practically limited to a 

 brief mention by Ferris (1877). This writer spoke of them in 

 the Cerambycids as "accessory locomotor organs." However, 

 this interpretation was not accepted by Craighead (1915), who 

 refers to them by the non-committal name of "pleural discs." 



It will be shown in this paper that these external characters 

 are not constant in ah genera of the Cerambycids, that in spite 

 of an external variation, the internal condition is usually con- 

 stant, and finally, that the pleural discs are the points of attach- 

 ment of abdominal chordotonal organs. The detailed structure 

 of the latter will be described. 



*Contriluition from the Entomological Laboratory of Cornell University. 



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