148 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



but it seems probable that in some way or other these afferent 

 impulses are registered by chordotonal organs. 



The significance of Johnston's organs is equally problematical. 

 The constancy of their occurrence in so manj^ different groups of 

 insects and their essential similarity of structure, which only varies 

 in its complexity and the number of sensillae involved, suggests 

 that these organs play an important role in the lives of the insects 

 possessing them. Eggers holds the view that they register 

 antennal movements, which exercise varying stresses upon the 

 intersegmented membrane to which the component sensillse are 

 attached. It has also been suggested that wind pressure, air 

 currents, mechanical vibrations, etc., might also be perceived by 

 these organs. In this connection, it may be pointed out, the 

 general structure of the cuticle investing the antennal segment 

 bearing Johnston's organ does not exhibit any special adaptation 

 for the reception of such stimuli. In the case of Chironomidae 

 and Culicidac, however, where these organs are greatly developed, 

 the distal extremities of the sensilla^ are all attached to a special 

 conjunctival plate. Mayer (1874) claimed that in these insects 

 the whorls of antennal seta?, so highly developed in the males, are 

 caused to vibrate when stimulated by certain tones. It is believed 

 that in such cases the vibrations of these seta; are transmitted to 

 the conjunctival plate and hence to the sensilla^, and that insects 

 possessing this special type of organ are able to appreciate sound 

 waves of certain qualities. The mere fact, however, that certain 

 hairs of specific insects can be caused to vibrate by different 

 pitches of sound may be of little significance. It is well known 

 that a cord of a one- stringed instrument responds to that of 

 another instrument set in vibration in a similar key ! 



Pumphrey and Rawdon-Smith (1936) have shown that the long 

 sensory hairs on the cerci of the cricket {Gryllus domesticus) and 

 cockroach (Periplaneta americana) are sensitive to auditory 

 stimuli. The sensitivity of the cercus to low frequencies is much 

 greater than that of the human ear, a marked response being 

 obtained to a stimulus completely inaudible to man. It is 

 possible that the acoustic function of the cerci is merely incidental. 

 The difficulty of distinguishing between acoustic and vibratory 



