PHOTORECEPTION 209 



nerve. This component is easily discernible in the ocelli of dragonflies 

 but rarely detected in cockroaches. Ruck (1961 b) has suggested that 

 the postsynaptic potential is not induced directly by the electrical 

 events in the receptor axon, but rather by the liberation of a trans- 

 mitter substance at the end of the receptor axon. In the absence of the 

 hyperpolarizing potential the fibres of the ocellar nerve discharge 

 impulses (component 4). In the dragonfly the ocellar nerve in the dark 

 discharges spontaneously, while the cockroach nerve usually dis- 

 charges only at off. It is inferred that there is spontaneous receptor cell 

 activity which modulates the rhythmic spontaneous discharges in the 

 ocellar nerve. 



The ocellus of the dragonfly is a very sensitive receptor. Corneal 

 illumination as low as 10~^ ft.-candles produces an ERG (Ruck, 

 1958 b). Furthermore, it is to be expected that sensitivity is enhanced 

 by the high degree of convergence of receptor axons on ocellar nerve 

 fibres, multiple synapsing of single receptor axons with ocellar nerve 

 fibres, and the presence of a white tapetum (Ruck, 1961 a). 



The ability of the ocellus to respond to flickering light varies from 

 species to species. The flicker fusion frequencies of Apis, Pachydiplax, 

 and Phormia are very high, being, respectively, 250-265, 200, and 

 +220 per second. For the cockroach maximum flicker fusion fre- 

 quencies range from 45 to 60 per second (Ruck, 1958 a). Thus, the 

 ocelli of those insects with *fast' eyes are *fast' and those with 'slow' 

 eyes, *slow'. As with the compound eyes. Ruck (1958 b) found that 

 there is no general relation between flicker fusion frequency and 

 sensitivity or rate of dark adaptation. Furthermore, since ocelli do not 

 possess a lamina ganglionaris and since in the dragonfly the generator 

 potential can respond to higher rates of flicker (220 per second) than 

 can the receptor axon responses, the postsynaptic potential and the 

 ocellar nerve impulses, the difference between 'fast' and 'slow' ocelli 

 is a fundamental characteristic of the photoreceptor cell itself. The 

 evidence of Antrum and Gallwitz (1951) that the abihty of 'fast' com- 

 pound eyes to follow high rates of flicker depends upon electrical 

 interaction between the receptor cells and neurons of the optic 

 ganglion cannot be extended to ocelH. 



Dorsal ocelli are sensitive to the same spectrum of wavelengths as 

 are compound eyes. The ocelH of Periplaneta have a single sensitivity 

 peak at 500 mi^., while those of the honeybee show sensitivity maxima 

 at 490 mjj. and at 335-340 m[L (Goldsmith and Ruck, 1958). Because in 

 the honeybee the ERG produced by ultra-violet stimulation is quali- 

 tatively different from that produced by stimulation of light of 490 mjjt, 



