CAUSES OF PATCHINESS 229 



endeavour'/ combined perhaps with that 'little dose of judgement or reason' that, as Darwin 

 in his Origin of Species has said, 'often comes into play, even with animals low in the scale of 

 nature '. 



The behaviour of Gunther's swarm at the jetty (p. 155) does seem to suggest a marked posi- 

 tively rheotactic sense, although whether the reaction of the krill in this instance to the current, if it 

 was in fact to the current, was dependent on a tactile sensitivity to it, or on visual stimulation by the 

 jetty or its shadow, or even by the sunlight itself, perhaps only experiment will show. Certain aquatic 

 insects it is known (Schulz, 1931; Tonner, 1935) show a positive rheotaxis that apparently springs 

 from tactile perception of the current by the antennae or from straightforward vision, or from a 

 combination of both. Other invertebrates, both terrestrial and marine (Carthy, 1957), seem to be 

 attracted to shadow (positive skototaxis), and it would seem that our krill swarm was repelled by it 

 (negative skototaxis). It might be supposed, too, that the individuals of a swarm contrive to keep 

 heading in the same direction (p. 151) by eye alone, as a body of troops does when marching in line 

 abreast. As Thorpe (1956) has written of the remarkable and so far little understood regimentation 

 in schooling fish, 'Whatever may be the function of such extraordinary social organization, there is 

 no doubt that it must be partly learned and based on a series of extremely precise visual orientations '. 

 In the open sea with no landmarks to guide them it is not so easy to see how the krill, if they do, 

 as seems possible, orientate themselves visually on external objects, could maintain themselves for 

 long over a given position, or point themselves in one direction rather than another. But here again 

 sight or a delicate tactile perception of each other would help them at least to keep contact and 

 to point, too, perhaps in the same direction. Even in the open sea, however, there is at least one 

 'landmark' by which they might orientate themselves visually. By day, for instance, there is 

 the sun and recent researches on terrestrial arthropods suggest the possibility that it may exercise 

 some influence on the behaviour and movements of pelagic animals. It has become widely known 

 since the pioneer work of Frisch (1948) on bees that many terrestrial arthropods have eyes 

 that are highly sensitive to linearly polarised light, using this sensitivity to navigate by means 

 of sky polarisation. Waterman (1958) summarises the literature that has recently been published 

 on this phenomenon, observing that the widespread occurrence of polarised-light behaviour in 

 these animals makes it most likely to be present in a variety of aquatic forms as well, adding that, 

 if it is, 'the complex patterns of light polarization known [Waterman, 1955] to be present will 

 be bound to influence the movement of plankton and other pelagic animals in the photic zone'. 

 He continues : 



Although a variety of physical and chemical clues to orientation have been proposed for their environment, animals 

 living in open water would appear to have but sparse information from which to direct their horizontal migrations. 

 Particularly when well below the surface, but still far from the bottom, they might have little or no obvious sensory 

 data permitting them to gauge geographical direction. Yet if they are sensitive to the underwater polarization patterns, 

 a polarized-light compass could be available to them throughout a considerable part of the photic zone and thus 

 function much as a sun or sky compass does for terrestrial animals. 



* The late Sir J. Arthur Thompson in his first-year lectures. A simple definition indeed, yet it embodies the essence of 

 leading ethological thought. I quote from Thorpe (1958). ' Lorenz' main argument is that in each example of true instinctive 

 behavior there is a hard core of absolutely fixed and relatively complex automatism, an inborn movement form. This restricted 

 concept is the essence of the instinct itself; it is now usually referred to as th^ fixed action or the. fixed-action pattern. Such 

 action patterns are items of behavior in every way as constant as are anatomical structures, and potentially just as valuable 

 for phylogenetic and systematic studies. Where such action patterns constitute an end point or climax of either a major or 

 a minor chain of instinctive behavior, they have come to be known as consummatory acts. The internal coordination mechanism 

 of these consummatory acts is assumed to be generating some Specific Action Potential (SAP). This is the internal drive, 

 which is of the very essence of the word instinct.' 



