CHEMORECEPTOR MECHANISMS 



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20 



LOG NUMBER OF C-ATOMS 



Fig. 6. The relation between the rejection thresholds of alcohols and glycols by 

 the blowfly and the chain length (Dethier and Chadwick, 1948b). 



With insects one tremendous advantage has recently been granted us. It is 

 now possible in the blowfly, Phormia regina, to work behaviorally and electro- 

 physiological^ with a single-receptor preparation. The histology of the recep- 

 tors has been studied in great detail (Dethier, 1955). It is possible to -stimulate 

 a single receptor and elicit a complete normal behavioral response (Dethier, 

 1955) and a clear electrical response (Hodgson, Lettvin, and Roeder, 1955). 

 The receptor organ concerned is analogous to a single mammalian taste bud in 

 that it contains more than one receptor. Two receptors have been demonstrated, 

 but each is innervated by its own nerve fiber and each can be stimulated singly. 

 The receptor organ is a hair (sensillum trichodeum) containing three bipolar 

 neurons two of which send distal processes to the tips (Figs. 13 and 14) where 

 they are clearly invested with a cuticular covering which possesses, however, 

 permeability characteristics radically different from those of the cuticle on the 

 rest of the hair. Furthermore, the cuticle at this point has a very low electrical 

 resistance as compared to the rest of the hair which is nearly perfectly insu- 

 lated. The process of the third neuron ends at the base of the hair and may not 



