32 THE PHYSIOLOGY OF INSECT SENSES 



While it is convenient to speak of all sensilla trichodea as a class of 

 sensilla responding to deflexion, the variations in structure, hence in 

 physiological characteristics, impart to the different ones quite 

 different functions in the economy of the insect. One of the salient 

 differences encountered is the rate of adaptation. Generally, the 

 smaller, more delicate hairs adapt rapidly during prolonged deflexion 

 while the stouter spines adapt more slowly and incompletely. The 

 nature of the process is still unclear. 



6 



Receptor Potential MV 



Fig. 22. Receptor potential amplitude plotted against frequency of 

 impulses for a mechanosensory hair on the anal plate of a female 

 Phormia. Each point is the average of 2 seconds. (Redrawn from 

 Wolbarsht, 1960.) 



Evidence has been presented to show that the generation of impulses 

 follows depolarization of the neuron membrane. The rate of adapta- 

 tion appears to be closely related to the rate of decay of the depolariz- 

 ation. The generator potential of the rapidly adapting Pacinian 

 corpuscle of the cat (Alvarez-Buylla and de Arellano, 1953 ; Gray and 

 Sato, 1953), the fast adaptory crustacean muscle receptor (Eyzaguirre 

 and Kuffler, 1955), and the sensory hairs on the wings of flies (Fig. 17) 

 (Wolbarsht, 1960) fall rapidly below threshold. The slowly adapting 

 frog-muscle spindle (Katz, 1950), the slow muscle receptor of Crust- 



