NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 



57 



Scolopalse were discovered first in the tympanal organs of Or- 

 thoptera by von Siebold (1844) ; they were found next in organs of 

 the wing bases of Coleoptera and Diptera by Leydig (i860). The 

 first concise description, however, of the scolopalse and of the struc- 

 ture and distribution of the chordotonal organs in insects and in 

 different parts of the insect body was given by Graber (1882) in 

 an extensive paper on the chordotonal organs, which has served as 

 a basis for all subsequent studies of these organs. Graber, however, 

 embraced in his definition of " chordotonal " organs all sense or- 



FiG. 26. — Diagrammatic structure of a single sensillum of a chordo- 

 tonal organ. 



Presumably the sense cell (SCI) and the enveloping cell (ECl) ex- 

 tend to the cuticula, at least in a formative stage, but in the adult organ 

 they appear to end in the base of the cap {CCl). 



gans " in which there is present a nerve-end structure similar to the 

 well-known ' auditory rod ' of the Orthoptera," and he included 

 in his descriptions not only the organs which we now call " chordo- 

 tonal " but also the campaniform organs of the wing bases. 



It is clear that the chordotonal organs cannot be defined as " scolojjo- 

 phorous " organs, as is still done by some modern writers, since 

 it is now evident that a scolopala or sense rod of some sort is present 

 in the majority of insect sense organs. The term '" chordotonal," how- 

 ever, may be retained inasmuch as it has become well fixed by uni- 

 versal usage, though it carries an auditory implication which prob- 

 ably does not apply to chordotonal organs in general. 



