56 THE PHYSIOLOGY OF INSECT SENSES 



Proprioceptive chordotonal organs are usually much simpler than 

 those concerned with sound reception, although there are some note- 

 worthy exceptions (e.g., the tympanic organ of noctuid moths). 

 Whereas the auditory organs may consist of tens to thousands of 

 chordotonal sensilla, organs subserving a proprioceptive function 

 consist of very few sensilla, sometimes only one. 



Fig. 47. Subgenual organ of Formica sangidnea. A, accessory cell ; C, cap 

 cell ; E, enveloping cell ; S, sense cell ; N, nerve. (Redrawn from Schon, 

 1911.) 



In the water bug Aphelocheirus, Larsen (1955) discovered a single 

 chordotonal sensillum attached to each of the large tracheal sacs of 

 adults and larvae. He suggested that the sensilla are concerned with 

 respiratory behaviour. When air pressure within the animal falls, as it 

 does according to Larsen, and when an animal in poorly oxygenated 

 water has used up its air supply, there is a decrease in the volume of the 

 air sacs. The decrease triggers the chordotonal sensilla, and the animal 



