166 DAVID R. EVANS 



Structure in man was that of Oertly and Myers (1919), and that theory is 

 inadequate if only because it does not encompass all of these effective 

 chemicals. 



Responsiveness to carbohydrates has been studied extensively also among 

 insects, in several cases with a view to relating stimulating effectiveness to 

 chemical structure. Most of this work, like that in mammals, has employed 

 behavioral criteria of stimulation. Two especially thorough studies were 

 those of von Frisch (1935) on the honeybee and Dethier (1955) on the 

 blowfly, Phormia. There are a number of striking differences between the 

 " sweet " taste in these insects and in mammals, most important of which 

 is the extreme specificity of the insect receptors. Only certain carbohydrates 

 are effective (water was erroneously thought to be, but see Mellon and 

 Evans, 1961, and Evans and Mellon, 1962a). Although numerous chemi- 

 cals have been assayed behaviorally and also electrophysiologically, amino 

 acids, inorganic salts, many organic compounds including saccharin and 

 dulcin, and even most carbohydrates are not effective stimuli. 



With regard to stimulation by various carbohydrates then, Dethier (1955) 

 agreed with von Frisch (1935) that only a few limiting generalizations were 

 possible. The main conclusions for the blowfly were : (1) The size of the 

 molecule is important. Shorter carbon chains than pentoses were in- 

 effective ; some mono- and di-, and tri-saccharides were effective ; and the 

 few polysaccharides tested were ineffective. No oligosaccharides larger 

 than tri-saccharides were examined. (2) Linear polyols are ineffective ; 

 for example, D-glucitol is tasteless in contrast to glucose. However, 

 Haslinger (1935) found that another blowfly, Calliphora, responded to 

 polyhydric alcohols when starved ; and Evans (see below) obtained res- 

 ponses of starved Phormia to saturated solutions of D-glucitol. Linear 

 polyols probably are, therefore, weakly effective. (3) The carbonyl group 

 is not required for stimulation. Myo-inositol and non-reducing glycosides 

 are stimulating. (4) Alpha glucosidic derivatives (carbohydrate or other) 

 are generally much more effective stimuli than are otherwise similar beta 

 derivatives. 



Both Dethier and von Frisch attempted unsuccessfully to discover con- 

 figurations that were common to the stimulating carbohydrates and were 

 absent in the tasteless ones. Even when only the 5 and 6 carbon aldoses 

 were taken into account, there was no apparent relationship between stimu- 

 lating effectiveness and configuration. 



The present paper represents a re-evaluation of the earlier data of Hassett 

 et al (1950) and Dethier (1955) for Phormia and presents some new data 

 and a theory that attempts to explain why stimulating effectiveness could 

 not formerly be related to chemical structure. In addition, some evidence 

 is discussed as it pertains to the mechanism of stimulation by carbohydrates, 

 or better, polyols. 



