178 THE PHYSIOLOGY OF INSECT SENSES 



filtered than out of normal sunlight. At the opposite end of the spec- 

 trum Buck (1937) had observed that the firefly Photinus pyralis could 

 respond to red flashes (approximately 560-690 mjji) emitted by other 

 individuals. What has not always been clear is whether or not the eye is 

 equally sensitive to all wavelengths and can distinguish one wave- 

 length from another when the energies of the various coloured 

 fights are equalized; in other words, whether insects possess colour 

 vision. 



A large number of experiments designed to provide information on 

 these points have been undertaken. They fall into two categories: 

 behavioural and electrophysiological. In both instances some selected 

 response is measured at different wavelengths of fight whose spectral 

 purity and intensity had been controlled with various degrees of rigor. 

 In so far as response to different wavelengths is concerned, the results 

 of the majority of tests, regardless of their sophistication, agree in a 

 general way in showing that insects as a class are especiaUy sensitive to 

 ultra-violet and to blue-green fight (Fig. 88). 



Behavioural experiments have been based on phototactic responses 

 (Peterson and Haeussler, 1928;Bertholf, 1931a, 1931b;Sander, 1932, 

 1933; Cameron, 1938; Weiss, Soraci, and McCoy, 1941, 1942, 1943; 

 Weiss, 1943a, 1943b, 1944, 1946; Weiss, McCoy, and Boyd, 1944; 

 Milne and Milne, 1945; Fingerman, 1952; Fingerman and Brown, 

 1953; Wolken, 1957; Wolken, Mellon, and Contis, 1957; Heintz, 

 1959; Goldsmith, 1960), optomotor responses (Schlegtendal, 1934; 

 Rokohl, 1942; Moller-Racke, 1952; Antrum and Stumpf, 1953; 

 Schone, 1953; Resch, 1954; Schneider, 1956), training (von Frisch, 

 1914; Koehler, 1924; Kiihn, 1927; Kuhn and Pohl, 1921; Bertholf, 

 1931a; Use, 1949; Kuwabara, 1957; Hertz, 1939; Daumer, 1956, 

 1958), and simple observation (Lubbock, 1886; Hamilton, 1922; Use, 

 1928, 1937, 1949; Buck, 1937). Phototactic responses have been em- 

 ployed exclusively for studying spectral sensitivity, while other be- 

 havioural techniques have been employed in the search for evidence 

 of colour vision. 



The relative efficiency of different wavelengths in eliciting photo- 

 tactic responses has been measured for a number of species. Bertholf 

 presented to Drosophila and Apis two sources of light, one of which was 

 the test wavelength and the other, a white standard. In one series of 

 tests the intensity of the test colour was kept constant and that of the 

 standard varied until both sources were equally attractive. In another 

 series of tests each coloured light was matched with a white fight of 

 fixed intensity. After the ratio of attractiveness had been found a white 



