16 V. G. DETHIER 



be assumed that four steps may be involved in the process of stimulation: (1) 

 combination of sugar (S) with receptor substance (R), (2) initiation of polariza- 

 tion, (3) discharge of impulses in the sensory nerve, (4) removal of sugar. Four 

 alternative hypotheses may be advanced by way of checking the mechanisms 

 enumerated in the basic assumptions. 



The simplest hypothesis is that sugar combines reversibly with receptor sub- 

 stance to form a complex in or as part of the receptor membrane. Formation of 

 the complex results in depolarization of the membrane. Removal of the external 

 source of stimulation causes removal of combined sugar: 



S + R ^ RS — » depolarization (1) 



A variant of this process would involve combination of sugar with a receptor 

 substance which is thereupon activated to elaborate a product capable of de- 

 polarizing the membrane: 



S + RX ^± RSX ^X^ depolarization (2) 



Two alternatives of these hypotheses envision the removal of sugar by a side 

 reaction involving chemical degradation of sugar. 



(3) 



(4) 



R + Si 



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. 

 Frings and Cox (1954) have observed significant threshold changes when the 

 temperature of the entire organism (the fly, Sarcophaga bullata) is altered, but 

 even under these conditions the changes are small. The values are: 0.14 M at 

 19-21°C, 0.06 M at 27-29°C, 0.17 M at 37-39°C. Dethier (Figs. 16 and 17) 

 was able to demonstrate in the case of the chemosensory areas of the ovipositors 

 of crickets and certain parasitic Hymenoptera that there was very little change 

 in reaction time with temperature. Hodgson and Roeder (in press) have shown 

 that the frequency of impulses in the afferent nerve fiber increases rapidly with 



