CHEMORECEPTION 121 



ments the acuity of olfaction decreases only slightly. After extirpa- 

 tion of seven segments a pronounced decrease occurs, and amputa- 

 tion of eight segments abolishes all response. A comparison of the 

 frequency distribution of sensilla on the antennae and the threshold 

 of response reveals a logarithmic relationship which Dostal relates 

 to the Weber-Fechner law. An examination of the data which Schanz 

 (1953) obtained by measuring thresholds in the potato beetle after 

 differential amputation of antennal segments reveals similar relation- 

 ships. 



Not all olfactory receptor fields of an individual are equally sensi- 

 tive. As a rule response thresholds are lowest when the antennae are 

 intact. Many insects lose their ability to respond to attenuated odours 

 when the antennae are rendered inoperative, but can respond to 

 higher concentrations as long as the palpi are still operative (Warnke, 

 1931; Frings, 1941; Dethier, 1941, 1947 a, 1952 b). 



The Stimulus 



As with man the number of gaseous materials, natural and synthetic, 

 to which insects respond is very great. Many attempts have been made 

 to relate stimulating effectiveness to some molecular property, and 

 many data have been collected by workers interested in insect attract- 

 ants and repellents. When these data can be translated into terms of 

 threshold some interpretation is possible. One of the earliest of such 

 studies was that of Cook (1926). It was designed to ascertain the re- 

 lation between the optimum attractiveness to flies and the physical 

 properties of aliphatic alcohols from Q to C5 and esters from acetates 

 to valerates. Within a homologous series acceptance threshold de- 

 creases with increased boiling point. The relationship is logarithmic. 

 Instead of ascertaining the relative stimulating effect of members of 

 a homologous series by testing each member separately over a concen- 

 tration range as Cook had done, one may derive the same information 

 by testing a number of compounds simultaneously and employing the 

 number of insects attracted as an index for efficiency. Employing this 

 method, Speyer (1920) found that homologous alcohols and esters 

 become more attractive as the chain length is increased. Increasing the 

 chain length on the acid side of the ester molecule results in a more 

 pronounced and uniform increase in effectiveness than do similar in- 

 creases on the alcohol side. Comparable experiments with codling 

 moths yielded similar results (Eyer and Medler, 1940). Olfactometric 

 experiments with the blowfly Phormia regina have shown that the 

 rejection threshold to homologous alcohols decreases logarithmically 



