CHEMORECEPTOR MECHANISMS 



25 



restricted olfactory stimuli by no means indicates a high receptor specificity. 

 While it is tempting to think that different species may be highly specialized 

 at the receptor level in this respect, possibly, for example, in a manner analogous 

 to protein and antigen /antibody specificity, it is equally likely that the specific- 

 ity is at a much higher level corresponding to Tinbergen's (1951) "innate re- 

 leasing mechanisms." 



At the moment there is no evidence from insects either that there are different 

 receptor types or that the receptors are restricted in their ranges of sensitivity. 

 Thus, for example, caterpillars (with no more than six to ten receptors) can 

 respond to a wide variety of odors (Dethier, 1941). A similar situation exists 

 with respect to the water beetle, Laccophilus (Hodgson, 1951). Furthermore, 

 extirpation of as many as 90% of the olfactory receptors as in the case of abla- 

 tion of antennal segments of honeybees (von Frisch, 1919) and dung bettles 

 (Dethier, 1947) does not seem to hamper these insects in their ability to operate 

 normally in their highly olfactory world other than by raising threshold. 



It appears to be quite pointless at this time to theorize on the nature of the 

 transducing mechanism on the basis of correlations of subjective modalities of 

 odors with physico-chemical properties of the stimulating compounds. At any 

 rate the approach has not been profitable thus far. On the other hand, a study 

 of the relation between stimulating efficiency (threshold) and molecular char- 

 acteristics does indeed seem to hold some promise. Ideally, as with taste, one 



0.0 0.4 0.8 1.2 



LOG NUMBER OF C ATOMS 

 Fig. 22. Relation of olfactory thresholds (rejection) of alcohols to chain length 



(Dethier and Yost, 1952). 



