130 THE PHYSIOLOGY OF INSECT SENSES 



sucrose, a ratio of 1 : 1-24 (AS IS = 0-25). Frings (1946) and Frings 

 and O'Neal (1946) had found that Periplaneta americana and Tabanus 

 sulcifrons could also discriminate at this level. On the basis of tests 

 with two specimens Verlaine (1927) claimed a slightly greater sensi- 

 tivity for Pieris, 



A technique permitting the measurement of A/// for the complete 

 effective concentration range of a compound was developed by Dethier 

 and Rhoades (1954) and employed with Phormia. The smallest ratio 

 found (0-12) indicates better discrimination than that obtained in 

 earlier studies. An examination of the results tabulated in Table 5 

 shows that discrimination is best over the middle ranges of concentra- 

 tion and is optimum at about the same concentration for which there 

 is a maximum preference. The discrimination factor for the chemical 

 senses of man is about 0-3 according to Moncrieff (1944) but earlier 

 Lemberger (1908) had recorded optimum values of 0-15 for sucrose 

 and 0-11 for sodium-saccharin and higher values at both extremes of 

 the concentration range (cf. also Dahlberg and Penczek, 1941; 

 Schutz and Pilgrim, 1957). 



Rejection Thresholds - Electrolytes 



The number of compounds that can be detected by insects is very great. 

 From a behavioural point of view the only manifestation of stimula- 

 tion by the majority of them is the rejection of acceptable solutions 

 (e.g., sugar or water) to which they have been added. Rejection thres- 

 holds, therefore, represent the concentration of substances necessary 

 to prevent responses to sugar or water. Because the test solutions are 

 always mixtures, the thresholds involve : the sensitivity of two different 

 receptors, possible interactions at the receptor level, and demonstrated 

 interaction at the central level. Despite these complications, it has been 

 possible to estabhsh rather precise relationships between the stimulat- 

 ing effectiveness of many compounds and their molecular properties. 

 Electrolytes were among the first compounds studied. Tests con- 

 ducted with the mouth-parts of the cockroach Periplaneta americana 

 showed that for several series of salts with a common union (acetates, 

 bromides, chlorides, iodides, nitrates, and sulphates) the stimulative 

 efficiency could be correlated directly with ionic mobilities (Frings, 

 1946). Similar results had been obtained earher with the oral receptors 

 of cecropia moth larvae (Frings, 1945). Extensions of these studies to 

 the tarsal receptors of the horsefly Tabanus sulcifrons (Frings and 

 O'Neal, 1946), the ovipositors of parasitic Hymenoptera (Dethier, 

 1947 c), oral receptors of the hQetle Laccophilus (Hodgson, 1951), the 



