Chemoreception 459 



TABLE 65. COMPARISON OF TASTE THRESHOLDS 



(from Dethier and Chadwick^) 



Compound Threshold Concentrations 



Sucrose man, 0.02 M; bee, 0.06-0.125 M; butterfly (Pyrameis), average ca. 0.01 



M, in starvation as little as 8 X 10"' M, or for Danaus, 9.8 X 10"" 

 M; horsefly {Tahanus), 0.005-0.11 M. 



NaCl man, 0.009 M; bee, rejects ca. 0.24 iM in 0.5 M sucrose; various cater- 



pillars reject at 0.2 M, while others accept over the full range up to 

 and including 5.0 M. 



HCl man, 0.00125 M; bee, rejects 0.001 M in 1.0 M sucrose; various cater- 



pillars reject at 0.01-0.2 M. 



Quinine man, 1.5 X 10"' M; bee, rejects at 8 X 10 ' M in 1.0 M sucrose; 



various caterpillars reject at 0.002-0.033 M; aquatic beetles were 

 conditioned to respond to 1.25 X 10"" M. 



Not all gustatory end-organs of insects are equivalent in sensitivity. Thus, 

 in the horse fly, the average threshold for sucrose is 0.021 M. for the labella, 

 and 0.060 M for the tarsi. The receptors on the antennae of bees are more 

 sensitive than those on the legs, and those on the proboscis are more sensitive 

 than those on the antennae. 



Pumphrey, "- using the frog's tongue and the technique of recording elec- 

 trically from the appropriate nerve, demonstrated a reduction of sensitivity, 

 after intense stimulation which lasted for sexeral minutes. Although adapta- 

 tion of gustatory receptors is slow, it is apparently of sufficient magnitude to 

 produce changes in sensitivity. In addition to these variations in sensitivity, 

 it has been reported that the scnsitixity of the same sense organs may vary 

 from day to day. 



The Cuunuon Cheiu'ical Sense. I he concentration' of ethanol necessary 

 to stimulate the sense organs located in the mucous membranes of man is 

 5 to 10 M. No comparable figures for the insccis are available. Numerous 

 experiments show, howexer, that substances v\'hich attract insects at one 

 concentration may repel at a higher concentration. Indeed, most substances 

 are optimally attractive at a particular concentration and decrease progres- 

 sively as the concentration increases (Fig. 149). Substances which are re- 

 pellent at low concentrations usually elicit a more violent repellent response 

 at high concentrations. I hese changes in response suggest the participation 

 of a new group of receptors, perhaps those mediating the common chemical 

 sense, at these high concentrations. 



Modalities of Chemoreception. Olfactory Receptors. The sense of smell 

 in man not only is the most sensiti\c of the chemical senses but also gives 

 rise to the greatest variety of sensations. It has been estimated that the odors 

 which a normal person can discriminate number between one thousand and 

 four thousand. All efforts to reduce the olfactor\' sensations of man to some 

 system, whereby mixtures of certain fundamental odors will reproduce all 

 odors of experience, have met with notable failure. Ihese odors are neces- 

 sarily classiBed on a strictly subjecti\e basis. It is hoped that such a classi- 



