SECT. 4] LIGHT AND ANIMAL LIFE 465 



surface towards the light ; in the second, the animal can maintain some definite 

 angle between the direction in which it moves and the direction of light incident 

 on it. 



A single animal may display responses of several different kinds, and it is a 

 formidable task to analyse the complex of responses which make an animal 

 undertake extensive diurnal vertical migrations. These migrations are more- 

 over modified in the different seasons of the year and within one species by 

 maturity, sex and brood (Russell, 1933; Moore, 1958), Indeed, lunar and 

 seasonal changes in light intensity and changes in the length of day may 

 greatly affect the general behaviour of animals, determining, for example, the 

 periods at which they breed (Korringa, 1957). 



Not least of the difficulties in experimentation is that of the scale of vertical 

 migration, which is very often of tens of metres and sometimes of hundreds of 

 metres. One ingenious approach is that of Harris and Wolfe (1955). They put 

 the freshwater crustacean, Daphnia magna, into a column of water which con- 

 tained Indian ink, and in which light intensity changed rapidly with depth. 

 In response to changes in the intensity of light incident on the top of the 

 column of inky water, the Daphnia were shown to make vertical migrations 

 and to distribute themselves in patterns very similar to those found for many 

 marine animals in the sea. It was found that the duration of the cycle of 

 vertical migration was related only to the variation in overhead light intensity 

 and that a whole cycle of a "diurnal" migration could be compressed into a 

 few hours. At the lower levels of illumination used, the Daphnia had always an 

 orientation such that, on making swimming movements, they would rise, and 

 at these levels their activity increased with light intensity. In the dark the 

 animals were inactive and, being denser than sea-water, passively sank (the 

 "midnight fall") whilst on increase in intensity they became more active and 

 automatically rose towards the surface (the "dawn rise"). This is an example 

 of a photokinetic reaction, for an increase in light intensity produced an up- 

 ward movement of the animal no matter from what direction the light came. 

 At higher light intensities, the animals gave phototactic responses sometimes 

 swimming for brief alternating periods first towards, and then away from, the 

 direction of the light (the net movement depending on which of these periods 

 was the longer) and sometimes, at close to the optimal light intensity, swimming 

 horizontally with the dorsal side towards the direction from which the light 

 came (a dorsal light reflex). Harris and Wolfe (1955) showed also that, at the 

 higher levels of illumination, the simple mechanism just described was modified 

 so that whilst in response to slow changes of light intensity the animals 

 moved to remain at an optimal intensity, their position was little affected by 

 rapid changes in illumination such as might be produced in nature by a cloud 

 going over the sun. 



Many responses of animals to polarized light have been described and great 

 interest has been aroused in the possibility that such responses may help 

 animals to navigate in the sea (Waterman, 1958 ; Jander and Waterman 1960). 

 One great difficulty has been that of deciding whether animals could detect 



16— s. I 



