64 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J'] 



Concerning the function of the chordotonal organs nothing defi- 

 nite can be said. In the text books the chordotonal organs are 

 presented as " organs of hearing." It is certain, however, that the 

 perception of sound has not been proved to be connected with any 

 of them, and those organs situated within the legs, the wing bases, 

 and various regions of the body where they are alifixed to solid parts 

 of the body wall, even though they may be associated with enlarged 

 trachae, seem poorly adapted for acoustic purposes. On the other 

 hand, the internal position of the organs suggests that they must 

 have some function connected with the workings of internal parts 

 of the body. Following this idea, the discussion of Eggers (1923) 

 on the possible uses of the chordotonal organs leads to conclusions 

 more convincing than any other yet presented bearing on the func- 

 tion of these enigmatical structures peculiar to insects. 



Eggers points out that most of the movements made by insects 

 result in rhythms. Especially is this true of the action of the wing 

 mechanism, which sets the whole body into rapid vibration ; but 

 also the motions of the antennas and the legs tend to become rhyth- 

 mical, while the movements of respiration, the pulsations of the heart, 

 the bodily motions of locomotion in certain aquatic larvae are all 

 of a rhythmic nature. Since rhythm, then, is such a characteristic 

 feature of muscular activity in insects, it would seem that there 

 should be special organs for registering it and for regulating the 

 action of the muscles that produce it. The chordotonal organs sug- 

 gest themselves at once as organs adapted for this purpose and as 

 the only organs that could serve in such a capacity. According to 

 this idea, therefore, the chordotonal organs are to be regarded as 

 rhythmometers. 



Finally, it is conceivable, as suggested by Eggers, that if a chordo- 

 tonal organ is connected with a thin membrane of the body wall, or 

 is sufficiently delicate in its construction, it might be responsive to 

 motions of the surrounding medium ; i. e., to vibrations of air or 

 water, and hence might act as a receptor of sound waves. Thus, for 

 example, the highly developed organ of Johnston in the antenna of 

 the Culicidae (fig. 31 B) or the tympanal organs of the Orthoptera 

 may be organs of hearing. 



VIII. THE ORGAN OF JOHNSTON 



Located in the second segment of the antenna of most insects, the 

 segment commonly distinguished as the pedicel, is the sense organ that 

 has long been known as the organ of Johnston. Structurally it 

 scarcely deserves to be placed in a class by itself, since it appears 



