150 THE PHYSIOLOGY OF INSECT SENSES 



revealed a spectrum of activity from complete unresponsiveness to 

 extreme sensitivity (Dethier, 1955 a). The most effective compounds 

 are certain pentoses, hexoses, and compound sugars possessing an 

 a-D-glucopyranoside link. In general, the a-form of a sugar is more 

 stimulating than the p-form. Among effective pentoses D-arabinose 

 is more stimulating than L-arabinose. It is clear from these tests that 

 the structural configuration of a sugar is its most important determi- 

 nant as an effective stimulus. On the other hand, no sense can be made 

 of the situation on the assumption that there is one key molecular 

 structure for stimulation. 



The first intimation that there are multiple and different sites of 

 action on the sugar receptor cell arose from a series of studies in which 

 mixtures of sugars were found not to be effective at concentrations 

 predicted from the threshold values of the individual constituents. 

 There were instances of synergism and of competitive inhibition. The 

 inhibitory effect of a given sugar varied, however, depending upon 

 which other sugar it was mixed with at the time. For example, 

 mannose, a weakly stimulating sugar, inhibited fructose, but not 

 glucose; rhamnose inhibited glucose but not fructose. There was 

 some evidence that sorbose inhibited both fructose and glucose 

 (Dethier et aL, 1956). 



Supporting evidence for the idea of multiple sites came from an 

 entirely different kind of study. Evans (1961 b) found that feeding 

 blowfly larvae on a medium containing a specific sugar depressed the 

 sensitivity of the adult to that sugar. Relative sensitivity to glucose 

 or fructose could be enhanced or depressed by rearing in the presence 

 of the appropriate sugar. Evans interpreted these results as indicating 

 that the sugar reduced either the number or the affinity of sites on the 

 receptor cell for that particular sugar. 



The nature of the combining action of sugars, while not understood, 

 can be narrowed down somewhat by a process of elimination. An 

 outstanding feature of stimulation by sugars is that no change in 

 threshold can be demonstrated to take place as the temperature of the 

 stimulus is changed. Nor is any change in threshold observed when the 

 pH of the sugar solution is changed (Dethier, 1956). Another feature 

 of stimulation by sugars is that there is no inhibition by any of the 

 following metabolic inhibitors: phlorizin, fluoride, azide, idoacetate, 

 cyanide (Dethier, 1955 a). This finding indicates that the first step in 

 stimulation probably does not involve any steps in the glycolytic 

 cycle below those blocked by the compounds listed. The absence of a 

 marked temperature and pH change argues against an enzymatic 



