168 THE PHYSIOLOGY OF INSECT SENSES 



Electrical Events 



An electrophysiological attack on the action of retinal cells has been 

 pushed with more vigour than the biochemical, but, for reasons that 

 will become apparent, the results so far have a high degree of 

 ambiguity. 



It has been known for nearly a century that retinae give an electrical 

 response to light. This response, the electroretinogram, is usually 

 detected by placing one electrode on the surface of the cornea and the 

 other at any point on the body. Considering the neural complexity of 

 the eye, this is a relatively crude measurement to make. It should be 

 apparent that the electroretinogram (ERG) is a mass effect dependent, 

 as Granit (1955) has pointed out, upon the favourable orientation of 

 certain retinal structures which conduct currents in one direction. 

 Furthermore, the second electrode, the so-called indifferent electrode, 

 is not necessarily indifferent (cf. Granit, 1955). 



For these reasons and also because the profile of the potential 

 changes recorded from the eye differs in form and magnitude with the 

 location of the electrodes, the intensity of illumination, the duration of 

 illumination, the degree of Hght or dark adaptation, the time of day 

 (Jahn and Wulfif, 1941a, 1941b, 1943), the temperature (Ishikawa and 

 Hirao, 1960 c), the age of the preparation (Hassenstein, 1957; Naka 

 and Kuwabara, 1959), the amount of blood lost (Ruck and Jahn, 

 1954), the amount of pressure applied to the body (Autrum and Hoff- 

 mann, 1960), the species of insect, the individual (HartHne, 1928), and 

 the characteristics of the recording apparatus, it is understandable that 

 there is great difficulty in interpreting ERGs and reconciling the results 

 reported by one laboratory with those reported by another. These 

 difficulties will become apparent below. None the less, hidden in the 

 ERG is the code to some of the primary events that occur in photo- 

 receptors when they are stimulated by light, and ERGs have been 

 studied intensively in the hope that they will yield bits of this infor- 

 mation. 



ERGs have been recorded from a large number of insects. The first 

 ever obtained (from Melanoplus differ entialis, M. femur-rubrum, 

 Chortophaga viridisfasciata, Vanessa atalanta, Bombus pennsyh aniens, 

 Musca domesticd) were described by Hartline (1928) as being essen- 

 tially similar to one another and consisting exclusively of negative 

 components. These included a rapidly rising wave (which he desig- 

 nated as A and later workers as b) whose decay was interrupted by a 

 slowly rising wave (his B and the c of others). The off-effect was merely 



