PHOTORECEPTION 173 



negativity of the ERG increased. When the medulla interna, medulla 

 externa, and all or part of the lamina ganglionaris were removed the 

 ERG became a negative monophasic curve resembling that of 

 Limulus (Fig. 86). 



In the dragonfly Aeschna cyanea the optic ganglia of the young 

 nymph are remote from the retinal layer. As the nymph develops, 

 the distance between the ganglia and the retina decreases, until in the 

 adult eye the usual situation obtains (Fig. 87) (Viallancs, 1884). 

 The ERGs of nymph and adult are distinct - that of the nymph is 



A B 



Fig. 86. A. Simplified scheme of the optic lobe of Calliphora. AK, centri- 

 petal cells ; IK, centrifugal cells ; R, retinal cells ; a, ERG from isolated 

 retina; b, ERG from retina plus lamina ganglionaris; c, ERG from 

 intact lobe. B. Detail of centrifugal cells. (Redrawn from Autrum, 

 1959 after Cajal and Sanchez, 1915.) 



the *slow' type, that of the adult, the 'fast' type. The ERG of the 

 deganghonated retina of the adult is the *slow' type. 



As a result of these experiments, Autrum (1958) has concluded that 

 there is a negative component of the ERG of all insects assignable to 

 the retina; it is the receptor potential. He has pointed out that in 

 Calliphora under certain conditions (e.g., green or blue light) the posi- 

 tive on-wave is preceded by a negative deflexion. This negative wave, 

 assignable to the retinal cells, triggers a positive electrical response in 

 the centrifugal cells of the lamina ganglionaris. This positive wave 

 is considered to be of great importance in connexion with adaptation. 

 Dark adaptation has been measured for intact insects (e.g., the honey- 

 bee) by behavioural criteria, such as optomotor responses, and has 



