158 THE PHYSIOLOGY OF INSECT SENSES 



(Pielou, 1940; Pielou and Gunn, 1940), Pediculus humanus corporis 

 (Wiggles worth, 1941), Agriotes osbcurus and A. lineatus (Lees, 1943), 

 Ptinus tectus (Bentley, 1944), Choristoneura fumiferana (Wellington, 

 1949), Tribolium spp. and Sitophilus (Willis and Roth, 1950; Roth and 

 Willis, 1951a, 1951b, 1951 c), Aedes gracilis and Blethisa multipunctata 

 (Perttunen, 1951), Blatella germanica and Aedes aegypti (Roth and 

 Willis, 1952), Drosophila (Perttunen and Syrjamaki, 1958; Perttunen 

 and Erkkila, 1952; Syrjamaki, 1962), Neodiprion americanus bank- 

 sianae and A^. lecontei (Green, 1954), Oncopeltus fasciatus (Andersen 

 and Ball, 1959), Melanoplus binttatus (Riegert, 1958, 1959, 1960), 

 Schistocerca gregaria (Aziz, 1957 a, 1957 b). The strength of reaction 

 and the ability to discriminate differences in humidity varies among 

 the species. With Locusta migratoria migratorioides, which aggregates 

 in dry microclimates, more individuals respond as the gradient be- 

 comes steeper. With Culex fatigans, Tenebrio molitor, which avoid 

 high humidities, and Agriotes spp., which avoid low humidities, the in- 

 tensity of the reaction is related to the highest humidity. Ptinus tectus 

 reacts most intensely at low humidities. Culex discriminates 1 per cent 

 differences in the range near 100 per cent relative humidity, but reacts 

 only to 50 per cent differences in the range of relative humidity from 

 30 to 85. rr/Z>6>//wm c<3^/<3«ewm discriminates between humidities differ- 

 ing by 15 per cent over the entire range. Reactions to differences of 

 5 per cent are weak between R.H. and 5, absent between R.H. 40 and 

 45, 45 and 50, and 50 and 55, but strong between R.H. 95 and 100. The 

 louse also discriminates better at higher humidities. Wireworms detect 

 the difference between R.H. 100 and 95-5. Normal undesiccated 

 Drosophila melanogaster of both sexes exhibit an intensity of reaction 

 that is correlated with the degree of the higher alternative rather than 

 with the difference. When alternate humidities lie between 100 and 

 87 per cent R.H., the drier is preferred ; when they He between 77 and 

 20 per cent the wetter is preferred (Perttunen and Syrjamaki, 1958). 



The reactions of some species to a given relative humidity have been 

 studied at different temperatures. The results have been interpreted as 

 indicating that the reactions of Culex and Tenebrio are more in accord 

 with relative humidity than with saturation deficiencies, while the 

 reverse is true of Agriotes. What this may mean is difficult to know. 

 Although the results are interpreted by some as indicating that the 

 receptors act as evaporimeters, the lack of correlation between satur- 

 ation deficit and evaporation weakens the argument (cf. Thorn thwaite, 

 1940). On the other hand, in studies conducted with spruce budworm 

 larvae (C. fumiferana) and sawfly larvae of the genus Neodiprion a 



