LIGHT AND MOVEMENT 



67 



The interesting experiments of Wellington (1953) will make the matter 

 clear. Fig. 42 shows the abrupt changes of direction associated with rotation 

 of the axis of a sheet of " Polaroid " held over larvie crawling over the ground ; 

 the intact animal responds most markedly, but an adequate response is obtained 

 if either the ocelli or the comijound eyes are functioning alone. 





SUN 



Fig. 42. — The Effects of Alterations in the Plane of Polarizai'ion 

 ON THE Orientation of Insects. 



The plane of polarization was changed by rotation of the axis of a sheet 

 of " Polaroid '" held between the insect and the sun as it crawled over the 

 ground. The circles show the point at which tlie sheet of Polaroid was placed 

 over the moving insect or rotated or withdrawn. The bar inside the circles 

 shows the orientation of the axis with respect to the sun, and the shading of 

 the circle indicates wliether or not the sky was appreciably darkened when 

 viewed through the "Polaroid" with the axis set as shown. 



A, B, C. The path of a fly : A, with all its eyes functioning ; B, with only 

 its compound eyes uncovered ; C, with only its ocelli uncovered. 



D. A fourth-instar larva of Neodiprion (drawn on a different scale). 



The marked convolutions in the path of B show the response to alterations 

 in the polarization of the skylight when a patch of cinus cloud passed over- 

 head (W. G. Wellington, Nature). 



The Orientation of Insects out-of-doors 



It would thus apjDear that the orientation of insects in natural 

 conditions in daylight is a very complicated matter. Wlien these 

 questions first received attention in the classical observations of 

 Santschi (1911) and Brim (1914) on the behaviour of the ant.^ its 

 conduct was interpreted as being regulated by a light -compass reaction 

 alone. At a later date the experiments initiated by von Frisch 

 (1949-51) introduced the complicating factor of a response to the j^lane 

 of polarization of light. Finally, the experiments of Wellington and 

 his co-workers (19.^)3-55) have stressed the importance of a thermal 

 response. There is complete agreement that the light-compass 



1 p. 6S. 



5—2 



