LIGHT AND MOVEMENT 



exposed they may take a zig-zig path initially, as if hesitating between 

 the two, but soon the insect goes towards one, usually the stronger, 

 neglecting the other (Figs. 30 and 31) ; and if it is unilaterally blinded, 

 after some initial circus movements it again jjroceeds straight towards 

 the light (Fig. 29). Experimenting with termite larvae (Calotermes), 

 Richard (1948) found that the direction of motion was determined 

 by the direction of the rays rather than' by the intensit}' gradient, 

 but that the latter determined the straightness of the path. The 



57 



Fig. 32. — The Relative Role of the Ocelli and Compound Eyes 

 IN Telotaxis. 



A, B, C, D, the track of the flesh-fly, Sarcophaga, in a darkened room 

 towards a Hght indoors (6-watt lamp, marked by the circle). 



A, a fly with all its eyes uncovered ; B, only the compound eyes un- 

 covered ; C, only the ocelli uncovered ; D, all the eyes covered. It is seen that 

 in C and D the insect is completely at a loss. 



A', B', C" , D' . Movements of the same individuals over the ground out- 

 doors towards the sun. It is seen that the fly with only its ocelli uncovered 

 orientates itself well. The irregularities of the tracks were produced by 

 responses to patches of cirrus cloud passing overhead and do not occur when 

 the sky is clear. 



E. The track of the larva of the sawfly, Neodiprion, indoors, and E' out- 

 doors. It is seen that, in contradistinction to Sarcophaga, the track outdoors 

 is straighter than that indoors. 



The time-marks in all tracks show lO-second intervals (W. G. Wellington, 

 Nature). 



stemmata of larvae generally mediate this activity, but in the adult as 

 a rule the effective organ is the compound eye, the action of which is 

 frequently supplemented by the ocelli which, however, may be quite 

 ineffective by themselves. 



Fig. 32, for example, taken from Wellington's (1953) work, shows the 

 phototactic response of the common dipterous parasite, the fiesh-fiy, Sarcophaga, 

 crawling with clipped wings towards an ordinary (non-polarized) light in the 



