THE NERVOUS SYSTEM 117 



the adverse stimulus increases, it once more begins to turn. These 

 two factors, of random turning and adaptation, will lead the insect 

 to a favourable environment. Klinokinesis is a mechanism that is 

 used by many insects for orientating themselves in diffuse types of 

 stimuli without steep gradients, such as temperature, smell, humi- 

 dity or the texture of a surface. 



Where a more complex system of sense organs is present the insect 

 can make use of directed reactions or 'taxes*. Sometimes the insect 

 will compare the intensity of stimulus on the two sides, either by 

 moving the whole body or by swinging the antennae first to one side 

 and then the other. It then moves in the preferred direction. A strik- 

 ing example of this is seen in the maggots of flies which retreat from 

 the light. As they move they swing the anterior end of the body, 

 where the light-sensitive organs are, to right and left ; they then pro- 

 ceed in the direction in which the organs are most shaded. This pro- 

 cedure of making successive comparisons is termed 'klinotaxis'. 

 Sometimes the insect may be able to compare the intensity of stimu- 

 lus on the two sides simultaneously, without movement of the sense 

 organs or of the body; this less common mechanism is termed 

 'tropotaxis'. It can be strikingly illustrated in the honey-bee. The bee 

 can be trained always to turn towards the scent of aniseed when it is 

 exposed to this scent in a Y-shaped osmometer. But if the antennae 

 are crossed and fixed down in this position, the trained bee always 

 turns in the wrong direction. 



In the case of vision an object or a point of light is perceived from 

 a distance. The image of such an object may be 'fixed' upon the 

 central point of the retina of the eye and the insect thus enabled to go 

 straight to it. This response is termed 'telotaxis' or movement to- 

 wards a definite goal. The response seems to be made up of a series 

 of reflexes specifically related to the region of the eye that is stimu- 

 lated (like the fixation reflex in man); so that the insect finally turns 

 until the image of the source of stimulus is received upon the region 

 of clearest vision (in the insect with a single eye) or upon corres- 

 ponding points in the two retinae (in insects with both eyes intact). 

 So exact has this reflex become in such insects as the fire-fly Photimts, 

 that the single flash of light emitted by the female will cause the male 

 to turn accurately in her direction. 



