18 



V. G. DETHIER 



P 



Ld 



1 



o 2m 

 A Im 



OVIPOSITORS 

 aCI 

 0.5m 



0.25m 



1 



D 0.125m 



14 



16 



18 



8 10 12 



TIME -SECONDS 

 Fig. 17. Change in reaction time with temperature and concentration after stimu- 

 lation of the ovipositor of the cricket GryUus with NaCl. 



one with a negative temperature coefficient is negligible. Of the two alternatives 

 the former seems the more likely. Acceptance of the former would rule out many 

 enzymatic reactions as the limiting reaction. It would argue in favor of the first 

 step {S -\- R^± RS) being one which involved binding forces of the order of van 

 der Waals' forces, which fall off as the seventh power of the distance. The critical 

 importance of molecular configuration is consistent with this idea in that attrac- 

 tion would depend upon close association between S and R with conformity 

 of more than one point. The reaction would not be expected to be an all-or-none 

 one but would be expected to exhibit gradations correlated with the goodness 

 of fit. This expectation is realized in the finding that some sugars are excellent 

 stimuli (low threshold), others good, others poor. 



Another feature of stimulation by sugars is that it is not inhibited by any of 

 the following inhibitors: phlorizin, fluoride, azide, iodacetate, cyanide. This 

 finding indicates that the first step in stimulation probably does not involve any 

 steps in the glycolytic cycle below those blocked by the agents listed. 



Now, if the receptor sites (or substance) become filled and no sugar is re- 

 moved, stimulation might be expected to continue in the absence of externally 

 applied solutions until adaptation sets in. Since this occurrence has not been 

 observed, it must be presumed that the sites are rapidly emptied. The most 

 likely means of removal of a sugar such as glucose would be by a chemical re- 

 action such as phosphorylation. It might be expected that application of a spe- 

 cific inhibitor to block this reaction would not prevent initial stimulation but 



