Chetnoreception 465 



there is evidence for considerable interaction among the several modalities. 

 Frings-** has advanced the hypothesis that the several taste modalities may 

 merely be points in a continuous spectrum oF taste sensation which may be 

 related to penetrating power or some other surface-active property of the 

 chemical agent. The facts that some salts and alkalis at low concentration 

 and all the sugars, which penetrate cells slowly, taste sweet to man and are 

 acceptable to insects, and that there exists a gradual transition from salty 

 to bitter in the series of inorganic salts as their ionic mobilities increase, 

 lend credence to this hypothesis. However, the additive stimulating effects 

 of certain compounds and not of others and the similarity of response of 

 conditioned insects to substances which taste alike to man are observations 

 difficult to reconcile with the theory of Frings. 



Although there seems to be some similarity in the taste of insects and man, 

 there appear also to be some differences. Von Frisch-^-"^ tested bees with thirty- 

 four naturally occurring sweet substances and found they responded to only 

 nine, and that these nine were found in the natural food of bees. Thus 

 many pentoses, sugar alcohols, and true sugars are not accepted by bees. Sac- 

 charin is not accepted by bees at low concentrations and is repellent at high 

 concentrations. Acetylsaccharose, which is very bitter to man, is not rejected 

 by the bee. 



Intensity Discrimination. Intensity discrimination has not been well 

 worked out in either insects or man. Von Frisch'^^ demonstrated that meas- 

 urable differences existed in the response of bees to 1/8 M and to 5/32 M 

 sucrose. Similar experiments show that the American cockroach and the 

 horsefly, Tahanus, can discriminate near threshold between two solutions 

 whose concentrations differ by no more than the above factor. These data 

 do not, of course, establish the lower limit of discriminative ability. 



The relation of the intensity of stimulation and the magnitude of the re- 

 sponse in chemoreception has received relatively little attention. Krinner,^^ 

 Crozier,^'^' ^^ and Hasama'^'^ have demonstrated a rough linear relation be- 

 tween magnitude of response and logarithm of the concentration of the stimu- 

 lating agent, using a variety of different techniques. The technique of re- 

 cording spike potentials from single nerve fibers innervating chemoreceptors, 

 used by Pumphrey^^' and Pfaffman,""^ has yielded results which indicate that 

 the frequency of the nerve spike potential burst (Fig. 151), which begins 

 shortly after application of the stimulating solution, increases with increasing 

 concentration of the stimulating agent. A graph relating the frequency of 

 the nerve spike potential discharge to the concentration of the stimulating 

 agent results in a curve that is roughly logarithmic, although no quantita- 

 tive expressions have been derived. It appears, therefore, that the response 

 magnitude or intensity is related to the logarithm of the stimulating inten- 

 sity for chemoreceptors as well as for the other senses (see Chap. 11, p. 415). 



Mechanism of Chemoreceptor Activation. Olfaction. Chemoreceptors 

 which mediate the sense of olfaction in the vertebrates and those of insects 

 possess hairlike processes which project from the epithelium. It is assumed 

 that the odorous substance acts on these processes to initiate nerve impulses 

 in the neurones. The manner in which this act is accomplished is not known. 

 Attention has been in the past and is at present concentrated on the chemi- 



