RESPONSE TO LIGHT 113 



ocellus. The Collembola are exceptional in that their so-called 

 ocelli are the functional adult eyes : they occur in a group com- 

 prising up to eight such organs on either side of the head, and each 

 ocellus has the general structure of an ommatidium. It will be 

 obvious that the visual properties of ocelli of these diverse types 

 will vary more or less directly in relation with their structure. 

 Mere groups of pigmentary sense cells can appreciate little beyond 

 variations in light intensity. The more highly developed lateral 

 ocelli appear to have visual properties adaj^ted for near vision 

 only, but their image-forming capacity can only be of a rudi- 

 mentary kind. Certain experiments conducted with lepidopterous 

 larva:* appear to show that they are only able to perceive objects 

 at a distance of about 2 cm. 



Differential Response to Light Rays. Recent research on 

 insect vision substantiates the conclusion that colour perception 

 exists in insects but that it does not apply to the whole range of 

 the spectrum visible to human beings and, at the same time, 

 involves the application of rays entirely invisible to man. In 

 his well-known experiments von Frisch (1914, 1924) has shown 

 that the honey bee, which has been the chief subject of colour- 

 vision experiments in insects, can be conditioned so as to associate 

 certain colours with food. A number of pieces of paper of diverse 

 colours, but of the same shape and size, were disposed in a sort 

 of chequer-board fashion. A watch-glass containing an inodorous 

 sugar solution is placed over one of the colours. After this has 

 been repeated a number of times the food-solution is omitted, 

 but it was found that the bees still went straight to the colour 

 upon which the watch-glass had previously been placed. A 

 series of fifteen shades from white through various tints of grey 

 up to black were laid out, and a blue paper introduced. An 

 empty watch-glass was then placed over each piece of paper and 

 the whole covered with a sheet of glass. Bees previously con- 

 ditioned to blue at once gathered over that colour, notwithstanding 

 repeated changes in its position, when they searched for the 

 non-existent food. Von Frisch showed that bees conditioned on 

 scarlet or black failed to discriminate between those two colours, 

 or dark grey. He therefore concluded that bees are blind to red. 



