174 DAVID R. EVANS 



acetate). Strongly stimulating sugars (i.e. L-fucose = 6-deoxy-L-galactose) 

 could be fed to an animal confined in a glass vial and absorbed into the 

 hemolymph and then quantitatively recovered 24-48 hr later from the 

 animal and vial without chemical alteration (Evans, cited in Dethier, 1956). 

 L-fucose also has been shown to be unable to sustain life of the adult fly 

 in contrast to utiHzable sugars (Hassett et al., 1950). Hence, in at least this 

 case, the animals lacked enzymatic equipment for chemical conversion of 

 the sugar (see also Evans and Dethier, 1957). On the other hand, this 

 evidence does not necessarily show that the differentiated sugar receptor 

 also lacks the ability to produce a chemical change in the fucose molecule. 



The most convincing evidence for a physical reaction derives from the 

 behavioral tests of Dethier and Arab (1958) which showed that a change of 

 temperature of about 0-35°C in the stimulating solution did not affect 

 stimulation when the solution contacted only the tip of the chemosensory 

 hair. A number of earUer reports had shown that the temperature of the 

 receptor cell body or the whole animal, as might be expected, did influence 

 the behavioral response to sugars. Consequently, the primary process in 

 stimulation by sugars is very likely not influenced by temperature over a 

 very wide range. 



Recent measurements of the latency of the response of the sugar receptor 

 showed that the first impulse can occur at least as soon as 4 msec after 

 application of the stimulus (Browne and Hodgson, 1962). It may even 

 occur much sooner since these authors measured the latency to sugars 

 contained in a saline solution. Since it is a common observation that the 

 initial frequency is greater than any subsequent frequency, this evidence 

 indicated that the stimulus is fully effective in a very short time. In this 

 period it is generally accepted that sugar depolarizes the tip of the receptor 

 cell and that the depolarization spreads electrotonically to the cell body near 

 which the impulse is generated. Morita and Yamashita (1959) have claimed 

 in addition that the impulse is propagated in a non-decremental manner 

 distally as well as proximally from the region of spike generation, but their 

 evidence is not conclusive. In any case, the stimulus is fully effective in a 

 brief period in which a number of events must occur ; it would seem that 

 the receptor sites are immediately accessible to the stimulating molecules, 

 i.e. that they are associated with the cell surface rather than inside any 

 barrier. 



Prospects 



The theories and evidence above are based upon behavioral evidence, 

 and now should be checked by electrophysiological studies of the sugar 

 receptor. Quantitative analyses have been made of the water (Evans and 

 Mellon, 1962a) and the salt (Evans and Mellon, 1962b) receptor cells, 

 and using the recording method of Morita (1959) can now be extended to 



