CHEMORECEPTION 139 



trehalose ; (monosaccharides) fructose = glucose > fucose > galac- 

 tose. All other sugars fail to stimulate. For Calliphora legs the order is : 

 (disaccharides) sucrose = maltose > trehalose = cellobiose > lactose ; 

 (monosaccharides) fructose > fucose > glucose = xylose = galactose 

 = arabinose = mannose. For Calliphora mouth-parts the order is: 

 (disaccharides) sucrose = maltose > trehalose = cellobiose > lactose ; 

 (monosaccharides) fructose > fucose > glucose > arabinose = xylose 

 = galactose = mannose. For Phormia legs the order is : (disaccharides) 

 sucrose = maltose > trehalose > cellobiose > lactose ; (monosacchar- 

 ides) fructose > fucose = glucose > sorbose > xylose = galactose = 

 arabinose > mannose = ribose > lyxose. 



For these three species of insects it is true in general that the most 

 stimulating disaccharides are the a-glucosides and the most stimulat- 

 ing monosaccharides are fructose, glucose, and fucose. From studies 

 with Phormia, in which 75 carbohydrates, 8 polyhydric alcohols, 53 

 amino acids, and 18 monobasic and dibasic acids have been applied to 

 individual labellar hairs, some of the limiting factors in stimulation 

 have been revealed (Table 7) (Dethier, 1955 a). For example, trioses, 

 tetroses, heptoses, and octoses are uneifective. This finding indicates 

 that there is an optimum chain length. The size of the molecule as a 

 whole appears to be critical only within certain limits. Thus, at the 

 smaller extreme some pentoses stimulate while at the other extreme 

 certain trisaccharides stimulate. However, polysaccharides are un- 

 effective. For additional details the work of Dethier (1955 a) should be 

 consulted. 



Categories of Receptors 



Ultimately, an understanding of the mechanism of action of the re- 

 ceptors themselves had to await the application of electrophysio- 

 logical techniques. Studies of this sort have been confined to a few 

 carefully selected species : the flies Phormia regina, Lucilia caeser, and 

 Calliphora vomitoria; the butterfly Vanessa indica; and the Colorado 

 Potato Beetle, Leptinotarsa decemlineata. 



The most intensively investigated and well-known receptors are 

 those occurring on the tarsi and labellum of the black blowfly, 

 Phormia regina. They have been the subject of co-ordinated be- 

 havioural, histological, and physiological study and may be taken as a 

 model for discussion. The multiplicity of neurons associated with these 

 hairs complicated for a while studies of the mechanism of chemo- 

 reception. Because of the specificity of the neurons, however, it was 

 originally possible to arrive at a considerable understanding of 



