122 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



remains for consideration their reactions to ordinary white Hght 

 and its absence. Positive and negative responses to Hght have 

 been explained by Loeb (1918) on the basis of tropisms. His 

 theory states that when a bilaterally symmetrical animal responds 

 positively to a given stimulus, it approaches that stimulus owing 

 to the continuous action of the latter in provoking symmetrical 

 muscular activity. If the stimulus does not fall equally on the 

 two sides of the body, it provokes asymmetrical muscular activity, 

 which results in the animal orienting itself so that its long axis 

 comes directed towards the source of the stimulus. If the response 

 be negative, the muscles of the side nearest the source of the 

 stimulus are less active, which has the result of turning the 

 animal away from the stimulus. Although many facts relative 

 to the behaviour of insects appear to be in accord with this 

 general theory, a larger number seem to imply the existence of 

 differential sensitivity to the stimulus, rather than a compulsory 

 and irresistible response. In such cases orientation is not wholly 

 of the mechanical nature implied by the theory of tropisms. 

 Thus Urban (1932) found that the hive bee can be so conditioned 

 that it will choose one of two equal light sources and move directly 

 toward it. If the eye of one side be blinded, it may still be 

 conditioned to pursue a straight course toward the light. If one 

 or more legs on either side be removed the bee will still go to the 

 light in a straightforward course. 



There are also insects which move at a constant angle to a 

 source of light, this type of response being termed menotaxis. 

 The light-compass orientation shown by ants and bees has been 

 studied more recently by von Buddenbrock (1931, 1935) and his 

 pupils. With certain Coleoptera it was found that when walking 

 on a plain surface these insects maintain a straight course at a 

 definite angle towards a light some metres distant. If the position 

 of the light be only slightly altered, corresponding to the opening 

 angle of a single ommatidium, the insect reacts by means of a slight 

 change in its direction of walking. Beetles, kept in the dark 

 between experiments, maintain the same angle towards the light 

 for hours and even days. They turn into the exact angle required 

 by the shortest possible movement. 



