10 



V. G. DETHIER 



10 



00 



o 



cr 



o 

 m 



> 



03 

 Q 



LiJ 



CE 



Z 



o 



z 

 o 

 o 



or 

 < 



CD 



o 



-1.0 



-2.0 - 



-30 



-40 



O ALDEHYDES 

 A KETONES 



NORMAL ALCOHOLS 



• SECONDARY ALCOHOLS 



00 



04 



08 



1.2 



LOG NUMBER OF C ATOMS 



Fig. 7. The relation between the rejection thresholds of alcohols, ketones, and 

 aldehydes by the blowfly and the chain length (Chadwick and Dethier, 1949). 



be concerned directly with chemoreception, although this remains to be proven. 

 The proximal processes of all three cells travel directly to the central nervous 

 system without, as far as we know, synapsing. Here, then, is a primary neuron, 

 one end of which is exposed to the stimulus and the other end of which makes 

 its first connection deep within the central nervous system. This is indeed a 

 convenient preparation to have at hand. 



Only the tip of the hair, where the two fibers terminate in a minute papilla 

 (Fig. 14), is sensitive to stimulation. A single hair is sensitive to every com- 

 pound to which the animal as a whole is responsive provided that a threshold 

 concentration can be applied. Stimulation by certain sugars elicits a behavioral 

 response in the nature of a proboscis extension. Stimulation by non-sugar com- 

 pounds elicits withdrawal of the proboscis; or it inhibits extension. A variety 

 of tests, such as simultaneous stimulation of one hair with sugar and another 

 with salt, show that the inhibiting compounds are truly stimulating. The elec- 



