208 THE PHYSIOLOGY OF INSECT SENSES 



differences in the wave form and sign of the ERG. The most compre- 

 hensive analysis is that based upon extensive studies of the ocellus of 

 the cockroach and dragonfly (Ruck, 1958 a, 1958 b, 1961 a, 1961 b). In 

 these insects the total electrical response to light of high intensity 



oscilloscope 



different 



Fig. 98. Components of the ERG of the model ocellus. One photoreceptor 

 cell {left) with an expanded sensory ending, and an axon which makes 

 synaptic contact with one ©cellar nerve fibre. The two units are con- 

 tained in an electrolyte-filled compartment. A 'corneal' electrode 

 enters at left; a 'nerve' electrode lifts the nerve into air; an indifferent 

 electrode is placed far to the right. Four components of the ERG are 

 shown, each in one repetition of the model. Active sites for each 

 component are shaded. Current at active sites is indicated by arrows. 

 Each component appears at corneal and nerve electrodes. (Redrawn 

 from Ruck, 1961 a.) 



consists of four components, of which two originate in the retinal cells 

 and two in the ocellar nerve (Fig. 101). The first event is a generator 

 potential arising in the distal end of the receptor. It evokes a depolariz- 

 ation in the axonal region of the receptor. This in turn evokes a 

 hyperpolarizing postsynaptic potential in the fibres of the ocellar 



