THE VISION OF ARTHROPODS 



583 



changes in polarized light and thus aid in orientation. In the locust, 

 illumination of the compound eye produces on- and off-spike potentials 

 in the ventral nerve cord, of the ocelli off-responses only (with perhaps 

 a very brief on-response, Hoyle, 1955) ; the former responds to move- 

 ments of an external object while the latter does not (Burtt and Catton, 

 1954-56). It would thus seem obvious that the function of the ocelli 

 of Insects varies in different types depending on such factors as the 

 degree of development of the compound eye and the habits of the 

 species. 



Locusta 



THE COMPOUND EYES OF INSECTS, on the Other hand, possess 

 functional attributes of a high order which have been extensively 

 investigated ^ ; their appreciation of light and colour as well as form, 

 movement and spatial relationships compares well with that of many 

 tjrpes of Vertebrates. Moreover, in some insects the compound eye, 

 occasionally in addition to the ocelli, can appreciate changes in the 

 polarization of light. ^ 



More study has been devoted to the function of the compound eye 

 of Insects than to the eyes of any other Invertebrate. The two 

 classical methods of apjJroach ^ have been adopted — behavioural 

 experiments and reactions based on the electro-physiological charac- 

 teristics of the eye on stimulation by light. The first is the more 

 informative in that it gives some idea of the sensations appreciated 

 by the insect concerned, but insofar as many insects are untrainable 

 perhaps because of their automatism, perhaps because of lack of 

 intelligence, the method is by no means universally applicable. It 

 is always to be remembered, of course, in interpreting the results 

 of the second method, that physiological responses on a reflex level 

 need not necessarily ascend into the level of consciousness and can 

 only be translated with the greatest reserve into terms of sensation. 



Behavioural eiyeriinents depending on the laying down of con- 

 ditioned reflexes can be made available for the investigation of the 

 responses of many insects ; the honey-bee, A2}is, for example, can be 

 trained to go to a container with sugar placed beside a black disc and 

 avoid one marked with a black cross (v. Buddenbrock, 1937). 

 Unconditioned reflex responses such as the optomotor reaction to black 

 and white stripes on a moving drum are also readily elicited in many 

 insects. Again, the honey-bee is very sensitive to stimulation of this 

 type, responding if stationary by a reflex sideways movement of the 

 head and thorax ; if it is crawling it makes a sudden change of direction 

 opposite in sign to that of the movement of the environmental pattern. 

 In similar circumstances the fruit-fly. Drosophila, will completely 



1 See among others, Eltringham (1933), v. Frisch (1950), Wigglesworth (1953). 



2 p. 66. » p. 568. 



Apis 



Drosophila 



