124 INSECT PHYSIOLOGY 



evidence to support it, is that they are stimulatory organs (p. 121) 

 which accelerate phototaxis by increasing the sensitivity of the brain 

 to light-stimuli received through the compound eyes. Thus the res- 

 ponse of Drosophila to light is appreciably more rapid and more 

 persistent if the ocelli are intact than if they are blackened. It must be 

 realized that the light sense organs have two functions; a phototactic 

 or orientating function, such as we have already discussed, and a 

 photokinetic or stimulatory function. Now the phototactic response in 

 Drosophila is essentially an escape reaction, manifested only when the 

 insect isalarmed. The photokinetic ocelli increase its efficiency. Honey- 

 bees with their ocelli intact start to forage earlier in the morning 

 and continue later in the evening than bees with the ocelli covered. 



But whenever a faceted compound eye is present, no matter how 

 few may be the retinal elements it contains, these will inevitably re- 

 ceive different intensities of light from different parts of the visual 

 field; and some kind of visual pattern must always result (Johannes 

 Miiller's 'mosaic theory' of insect vision). Consequently, any move- 

 ment in the visual field will at once be perceived by the changes in 

 pattern it will produce. We have seen that many insects will turn 

 towards an object that is moving; will turn, that is, until corres- 

 ponding points of the two retinae are stimulated by it. Now the 

 closer the object is to the insect, the nearer to the mid-line will be 

 these corresponding points. Consequently, the insect may enjoy 

 some of the advantages of binocular vision, and be able to judge 

 accurately the distance of objects, even though its perception of 

 images be rudimentary; and like the larva of Aeschna or the bug 

 Rhodnius (when deprived of its antennae), it may attack with pre- 

 cision any object that moves within range. 



In forms like the adult dragon-fly and the bee, image perception 

 reaches a higher plane, and specific objects can be recognized; so 

 that Sphingid moths, for instance, will endeavour to extract nectar 

 from the flowers of wallpapers or from models of flowers. By cal- 

 culation and by experiment it has been estimated that the visual 

 acuity of the bee, in the vertical axis, is about one-fiftieth that of man 

 (in the horizontal axis the acuity is only about one-third of this 

 because the eye is more curved and each ommatidium has to cover a 

 wider field - the bee, in fact, is astigmatic). But this degree of acuity 



