98 THE PHYSIOLOGY OF INSECT SENSES 



The function of the non-acoustic unit may be proprioceptive. If the 

 acoustic sensilla are rendered inoperative resting activity may be re- 

 corded from the non-acoustic unit alone, but only so long as the 

 attachment to that structure known as the Bugel is intact (Fig. 57). In 

 this region is located a single Type II neuron (Treat and Roeder, 1959). 

 The rate of discharge of this unit changes as various portions of the 

 tympanic frame are distorted, as they would be during flight or as a 

 result of activity of the two tympanic muscles (Fig. 56). 



The fact that the moth tympanic organ exhibits after-discharge, a 

 form of physiological amplification for stimuli of short duration, and 

 rapid adaptation fits it very well for the reception of short, rapid bursts 

 of sound of the sort given off by hunting bats. Furthermore, its maxi- 



FiG. 67. Polar plot of the distances at which a 1-msec. click of fixed intensity 

 elicits the same response from the right tympanic organ when placed 

 at various angles relative to the median axis of the moth (0-1 80 degrees). 

 The moth was headed towards degree and inclined upward at about 

 30 degrees to the plane of measurements. Distances in metres from the 

 moth are indicated on the 90-270-degree line. Open circles and solid 

 line, Acronycta; broken line, Gmphip/iora; solid circles and solid line, 

 Lucania. (Redrawn from Roeder and Treat, 1961.) 



