82 THE BIOLOGY OF INSECTS 



tension of the membrane, the inner surface of which, formed 

 by a modified air-tube, is in touch with a ganglion and 

 tympanic thread surrounded by a sheet of tissue derived 

 from the air-tube system and containing scolopales character- 

 istic of chordotonal organs (Fig. 27). In a noctuid or other 

 moth in which these organs are thoracic, they are present on 

 the metathorax, or hindmost segment of that region of the 

 body. Eltringham summarises their structure by stating 

 that each organ '* consists essentially of a modified tracheal 

 vesicle carrying two drums. One of these is the true 

 tympanum with its chordotonal thread and the other would 

 seem to be a kind of resonator." The tympanum lies, 

 according to Eggers, in a lateral and ventral position with 

 regard to the " resonator." 



The nature of these organs with their drums and nerve- 

 cords leaves little doubt that their function is auditory, and 

 with regard to the grasshoppers and locusts, this conclusion 

 is supported by the well-known fact that the males of such 

 insects produce a shrill chirping noise by rubbing certain 

 wing-nervures over blunt pegs on their hind thighs. Noises 

 produced by moths, though less familiar, have been detected 

 by reliable observers ; it is remarkable that the Death's-head 

 moth (Acherontia) whose mouse-like squeak has often been 

 described, belongs to a family (Sphingidae) in no member of 

 which have tympanal organs been detected. 



In long-horned grasshoppers and crickets it is well 

 known that there are ears situated in the front shins, paired 

 inpushings of cuticle below the knee-joint being in contact 

 with an expanded air- tube, so that the vibrations can affect 

 a set of nerve-endings arranged along an extended ridge. 

 The organs have been well described by N. von Adelung 

 (1892) and others, and J. Regen (19 12) has shown that 

 females of a cricket Liogryllus campestris, normally attracted 

 by the chirping of the males, were no longer influenced after 

 their auditory organs had been removed. It is possible that 

 in many insects such organs as these may be responsive 

 to vibrations too rapid for appreciation by means of the 

 human ear. 



