584 



THE EYE IN EVOLUTION 



Vespa 



Dytiscus 



Cockroach 



reverse its direction of movement, a reaction repeated with dramatic 

 precision on each occasion and in rapid succession on repeated stimuli. 

 If the field is kept stationary a moving insect shows the same type of 

 response to the shift of the retinal image produced by its own move- 

 ment (v. Buddenbrock and Moller-Racke, 1952). 



The electro-physiological characteristics of the visual mechanism 

 have recently been applied with considerable success to the physiology 

 of the compound eye. Depending on the type of electrical response 

 on stimulation by light, two distinct physiological types have been 

 differentiated by Autrum and his co-workers (1948-53). 



(1) FAST 'EY'E.s, found in rapidly flying diurnal insects (the blow-fly, 

 Calliphora, the bees, A2ns and Bombus, the wasp, Vespa, and so on). 

 On stimulation by light the electro -physiological characteristic of such 

 an eye is a diphasic wave made up of an initial positive response 

 indicating the on-effect, followed by a terminal negative response 

 indicating the off-effect ; on prolonged stimulation the initial positive 

 response subsides rapidly. In such an eye there is a high temporal 

 resolution with a response to intermittent stimulation in the form of 

 flicker up to 250 or 300 stimuli per sec. The absolute threshold of 

 sensitivity to light is, however, high ; the reaction is little affected by 

 light- and dark-adaptation ; and the optomotor response shows an 

 ability to discriminate between stimuli of 200 per sec. 



(2) SLOW EYES, seen in nocturnal, aquatic or slow-moving insects 

 such as the grasshopper, the water-beetle, Dytiscus, and cockroaches 

 (as well as Limulus). Such an eye is characterized by a low threshold 

 of flicker to intermittent stimulation up to 40 to 50 per sec. ; the 

 absolute threshold of sensitivity is low ; the reaction changes markedly 

 in light- and dark-adaptation ; and the subjective optomotor response 

 can be obtained only by stimuli up to 5 to 10 sec. 



The experimental evidence makes it probable that the characteristic 

 properties of these two types of eye are attributable more to the central neurones 

 than to the end -organ, particularly to the first optic ganglion ^ (Autrum, 

 1951-54 ; Autrum and Gallwitz, 1951). The optic lobes of both types are the 

 source of spontaneous electrical oscillations ^ elicited by the onset or cessation 

 of stimulation ; in the slow type of eye the frequency of these rhythms lies between 

 20 and 35 cycles/sec; in the fast type, between 120 and 160/sec. (Adrian, 1937 ; 

 Boeder, 1939-40 ; Crescitelli and Jahn, 1942 ; Massera, 1952 ; Autrum, 1952 ; 

 Burkhardt, 1954), and it is noteworthy that the fast type can be converted into 

 the slow type by the surgical removal of portions of the optic lobe (Autrum and 

 Gallwitz, 1951 ; Autrum, 1951-52). 



In general, insects respond to the short waves of the spectrum 

 rather than to the long. The cornea (of the bee, Apis, and the flesh-fly, 

 Sarcophaga) is transparent to wave-lengths as short as 253mft, the 



1 p. 524. 



p. 524. 



