THE NERVOUS SYSTEM 127 



teresting to note that the limit of this perception is, approximately, 

 the lower limit of the solar spectrum after the far ultra-violet has 

 been filtered out by the atmosphere. Other insects arc capable of 

 seeing red (Pieridae), and associated with this is the shift in the 

 apparent luminosity of colours that we have already noted. 



Insects, like man, experience the phenomenon of simultaneous 

 contrast; a grey field surrounded by yellow appears blue to the bee, 

 a grey field surrounded by blue appears yellow - but in view of the 

 fact that ultra-violet is perceived by the bee as a separate colour, not 

 yellow and blue, but yellow and 'bee-violet', blue and 'bee-purple', and 

 blue-green and ultra-violet, are complementary colours. The capacity 

 for colour vision may vary markedly in different parts of the eye. 



Insect behaviour 



Most of the information on colour vision in insects, and a great deal 

 of the more reliable information upon other senses, has been ob- 

 tained by training experiments: stimuli which the insect can learn to 

 associate with the presence of food, or with the location of its nest, 

 are judged to be perceptible. Such experiments, of course, need the 

 greatest care in their interpretation; for the insect may be guided by 

 some other stimulus that has been overlooked by the experimenter; 

 indeed, in the course of a single experiment, it may cease to be 

 orientated by one stimulus and come to depend upon another; and 

 there are many who hold that the whole perceptual experience of the 

 organism is integrated, as it were, into a pattern, and that it responds 

 to general changes in this pattern, and not to the isolated stimuli of 

 which it is composed (Gestalt theory). 



Thus, when we consider the behaviour of insects under natural 

 conditions, we find them gaining their ends by an infinite variety of 

 sensory impressions. Inborn reflexes, and reflexes 'conditioned' by 

 the experience of the individual, certainly occur; but these are 

 masked by inhibitions, and integrated by higher centres, in such a 

 way that they serve constantly the needs and purposes of the insect as 

 a whole. It is this unifying quality, that welds the organism together, 

 and makes the whole something greater than the sum of the parts, 

 which in the sphere of behaviour as in the sphere of growth still 

 eludes physiological analysis. In the absence of a satisfactory physio- 



