458 



Comparative Animal Physiology 



long, 100 meters wide, and 50 meters high, very unpleasant. It is estimated 

 that man, with one sniff of air containing mercaptan at the threshold con- 

 centration, would draw in about 1 X lO'*^ molecules. Consideration of the 

 fact that there is only one molecule of mercaptan to every fifty thousand 

 million molecules of air leads to some appreciation of the sensitivity of ol- 

 faction. However, it should be noted that the surface activity of mercaptan 

 and of odorous materials in general is such as to enhance the possibiHties of 

 increasing the concentration at the receptor surface. 



Among insects comparable sensitivity of olfactory sense organs has not 

 been demonstrated, although the fact that the odors (lipoid, protein, or ester) 

 emanating from the body of the female gypsy moth attracts male gypsy moths 

 from distances greater than two miles attests to a great sensitivity. Threshold 

 measurements with other insects show that the dung beetles (genus Geo- 

 trupes^ respond to the odor of skatol at concentrations of 2.3 X 10~® M 

 and the tent caterpillar, Malacosoma americana, is repelled by a mixture of 

 benzaldehyde at a concentration of 4.1 X 10"^ M.^^- ^^ These values rep- 

 resent the response threshold level, and it is a safe assumption that the thresh- 

 old of individual sense organs would be significantly lower. Von 

 Frisch,^^' ^-' ^^ in his classic experiments conditioning the feeding response 

 of bees to the odors of essential oils, demonstrated that the minimum per- 

 ceptible odor is approximately the same for the bee as for man (Table 64). 

 Gustation. One of the most effective taste substances for man is quinine 

 hydrochloride, which elicits an extremely bitter taste that can be detected 



TABLE 64. COMPARISON Of OLFACTORY THRESHOLDS OF 

 CERTAIN INSECTS AND MAN (from Dethier and Chadwick'") 



in concentrations of approximately 1.5 X 10" ^ M. Other substances which 

 most effectively stimulate the other tastes are: hydrochloric acid, 1.25 X 

 10-3 ]vi. NaCl, 9 X 10--^ M; and sucrose, 2 X lO"- M. The sensitivity 

 of insect contact chemoreceptors is equal to or greater than those of the 

 human (Table 65). Aquatic beetles may be conditioned to quinine hydro- 

 chloride at concentrations of 1.25 X 10"" M. Stimulation of the tarsal re- 

 ceptors of some butterflies with sucrose at 9.8 X 10"*"' M and of some flies 

 with sucrose at 3.9 X 10~-'* M. elicits a response. 



