68 



THE EYE IN EVOLUTION 



Ant 



reaction is the essential determinant of behaviour in laboratory condi- 

 tions with artificial light, in natvu^al surroundings at night and in cir- 

 cumstances during the day when the sun is largely obscured, but these 

 latter workers believe that on a clear day the sun acts primarily as a 

 source of heat. Wellington (1955) concluded that in full sunlight, insects 

 in open places orientate themselves primarily by solar heat when it is 

 available and maintain their orientation to the sun or their straight -line 

 travel in its absence by polarized light from the overhead sky ; if 

 as may happen when smoke or cirrus cloud of varying densities 

 passes overhead, the plane of polarization changes rapidly, the response 

 may completely break down and the insect remains stationary even 

 although the sun remains exposed (see Fig. 32). This sometimes makes 

 its behaviour appear irregular and difficult to interjDret, particularly 

 in the neighbourhood of industrial centres where haze and smoke are 

 plentiful. Wellington considered that during overcast weather travel is 

 probably also aided by light gradients (tropo-menotaxis). In general, 

 when an insect is cool it is thermo-positive and travels towards the sun ; 

 when it is warm it is thermo-negative and orientates itself away from 

 the sun, and if it becomes overheated and the plane of polariza- 

 tion changes rapidly the insect becomes disorientated and is incapable 

 of travelling so that it often circles aimlessly until it succumbs to heat- 

 stroke (W^ellington et al., 1951-54 ; Sullivan and Wellington, 1953 ; 

 Wellington, 1955). The same complex interaction between thermal and 

 visual stimuli is seen in the locust which postures at right angles or 

 parallel to the sun's rays depending on the temperature (Volkonsky, 

 1939). Occasionally, as in the ant, the evidence suggests that other 

 stimuli such as gravity are also effective in orientation in such a way 

 that the geotropic and phototactic elements are correlated in a single 

 central mechanism of taxis (Vowles, 1954). 



From the historical point of view, the homing of the ant provided 

 the classical example of this type of activity. The purposive behaviour 

 of these insects, particularly when returning to the nest laden with 



>N 



Menotaxis. 



The ant was returning to its nest, N, with the sun on its left side. On 

 four consecutive places, 1, 2, 3 and 4, it was shaded from the direct light of 

 the sun and the image from the sun was projected from the animal's right 

 by iii^ans of a mirror. On each occasion the animal preserved its initial orienta- 

 tion n-lative to the sun or its image by turning round (Santschi). 



