60 THE PHYSIOLOGY OF INSECT SENSES 



hollow cylinder both its inner and outer surfaces are covered with 

 epithelium that is a continuation of the hypodermis. In one case 

 (Hymenoptera) every sensillum is individually ensheathed in epi- 

 thelium. In addition to Johnston's organ there are smaller more typical 

 chordotonal organs present in the scape, pedicel, and flagellum of 

 many insects. 



Everyone who has ever looked at Johnston's organs is impressed 

 by the fact that they are ideally situated to respond to movements of 

 the third antennal segment with respect to the second. This means, 

 essentially, movement of the entire flagellum of the antennae with 

 respect to the base, since all antennal muscles are located in the scape 

 and insert on the pedicel. 



The stimuli for motion of the antennae may be many. Johnston 

 (1885) surmised that the organs of Culicidae were sensitive to air- 

 borne sounds, and Mayer (1874) demonstrated that the long hairs on 

 the shaft of the flagellum could be set in motion when a tuning fork 

 was struck near by. Child (1894) decided that in Culicidae and 

 Chironomidae the organ has an auditory function, but otherwise is a 

 tactile organ. The legend has grown up that Johnston's organs in 

 general are auditory organs, but Demoll (1917) and Eggers (1928) 

 went to great pains to point out that the only evidence for an auditory 

 function to date, and that not conclusive, was the observation of 

 Mayer (1874) for Culicidae. They therefore considered Johnston's 

 organs to be organs concerned with movements of the antennae used 

 actively as tactile appendages and with passive movements induced by 

 air currents. 



As might have been suspected from the fact that the Johnston's 

 organ is a compound and complicated organ, the kinds of mechanical 

 stimuli to which it can react are varied. A study of the action potentials 

 produced in the antennae of the blowfly {Calliphora erythrocephald) 

 revealed that the majority of sensilla house phasic receptors, but that 

 some of these respond to torsional movements independent of direc- 

 tion, while others are sensitive only to movements in one of the two 

 possible directions (Burkhardt, 1960). When wind blows on the an- 

 tenna it causes the arista to act as a lever arm which rotates the 

 funiculus outwards around its long axis (Burkhardt and Schneider, 

 1957). A typical electrical response from a single rotation consists of a 

 large (+5 mV) action potential (on-wave) foflowed by a series of 

 decreasingly small action potentials during the course of stimulation 

 and ending with a large off'-wave at the termination of stimulation. 

 The small potentials are released by oscillations superimposed at the 



