48 THE PHYSIOLOGY OF INSECT SENSES 



innervation. The resemblance of this receptor to the vertebrate muscle 

 spindle is striking. Both have rich motor innervation, an absence of 

 striations in the central region, and much nuclear material (nucleus in 

 the insect; nuclear bag in the vertebrate). In Crustacea and other 

 insects there is no giant nucleus, but there is, as already described, a 

 richly nucleated sheath. 



Despite morphological differences, the mode of action of all of these 

 receptors is nearly identical. Under minimum tension, a condition 



Time fsec.) 



Fig. 38. A series of curves showing the discharge frequencies from the 

 stretch receptor of the larva of A. pemyi in response to stretch of 

 different intensities plotted against time. Arrows indicate beginning and 

 end of stretch. Figures below the curves denote the stretch in mm. The 

 broken line indicates response to 'over-stretch'. (Redrawn from 

 Finlayson and Lowenstein, 1958.) 



approaching the normal situation, the receptor discharges impulses at 

 the rate of 5-10 a second. Under zero tension it is silent. The basal 

 rate is maintained for hours. When the receptor is stretched there is a 

 large increase in the frequency of discharge (Fig. 38). This drops very 

 rapidly to a new level which is maintained for a long period. Upon 

 release there is a post-stimulatory reduction in discharge. The rela- 

 tively large drop in initial discharge is probably due both to neuronal 

 adaptation and a mechanical accommodation to stretch. Over what 

 may be judged a normal range of stretch there is a linear relation be- 

 tween the intensity of stretch and the frequency of impulses (Fig. 39). 

 Beyond a certain point this relationship breaks down, probably 



