176 THE PHYSIOLOGY OF INSECT SENSES 



and 250 per second. The amplitude is related to the number of illumi- 

 ated ommatidia, the Hght intensity, and the state of adaptation of the 

 eye. As illumination continues the amplitude gradually decreases. If 

 illumination is interrupted before the amplitude has dropped to zero 

 the rhythm is abruptly terminated. The likelihood is great that these 

 rhythms are an important link in the causal chain of central processes 

 which occur when an eye is illuminated. 



One conclusion that can be drawn with certainty from all of the 

 electrical data obtained from the compound eye is that the ERG, even 

 in its simplest form, is the algebraic sum of potentials originating at a 

 number of loci. Some of these loci are non-retinal; nevertheless, it is 

 now doubtful that even retinal potentials are as simple as once be- 

 lieved (cf. Wulff, 1950). The most elegant approach to the whole prob- 

 lem has been made in Limulus. As the classical work of Hartline (1928) 

 showed, this is a rather simple monophasic cornea-negative wave. 

 When a micro-electrode is placed intracellularly in an ommatidium in 

 the dark (HartHne, Wagner, and MacNichol, 1952; MacNichol, 

 Wagner, and Hartline, 1953) there is a resting potential, negative at the 

 recording electrode, of about 50 mV. When the ommatidium is stim- 

 ulated there is a change in the positive direction (with intracellular 

 recording the recording electrode goes positive when the cell is de- 

 polarized). Superimposed on this slow wave are spikes that are related 

 linearly to the magnitude of depolarization. These spikes are synchron- 

 ized with spikes recorded simultaneously from the axons of the om- 

 matidium. Curiously enough, the only cell in the ommatidium which 

 appears to be active is the eccentric cell; the others invariably are 

 electrically silent. There is little doubt that the slow negative wave 

 characterizing the ERG of the Limulus eye is the generator potential 

 (and probably also the receptor potential) giving rise to the action 

 potential in the nerves. There are now two reports of similar record- 

 ings having been made in insects. Burkhardt and Wendler (1960) have 

 recorded with intracellular electrodes the ERG from individual retinal 

 cells of Calliphora. The ERG is a simple negative monophasic wave 

 nearly identical in appearance to that o^ Limulus. Naka and Eguchi 

 (1962) have made similar recordings from the drone honeybee (see 

 also Ishikawa, 1962). 



Many attempts have been made to deduce from ERGs some of the 

 characteristics of the photoreceptors themselves, but as the previous 

 discussion has indicated, the ERGs have been much too complicated 

 and too poorly understood to have contributed greatly to this end. 

 Such attempts have been made by Wulff (1943) and Wulff and Jahn 



