58 



THE EYE IN EVOLUTION 



Sarcophaga 



Honey-bee 



Mysid 



Eupaguriis 



Photinus 



laboratory ; its path towards the light with all its eyes uncovered is straight ; 

 with only its compound eyes uncovered, relatively straight ; and with only 

 its ocelli uncovered, quite indeterminate. 



The compound eye of the average adult insect is well equipped to 

 respond accurately to a telotactic stimulus of this type, and may be 

 specifically differentiated for the purpose. In the honey-bee, for 

 example, the rapidity and accuracy of the response are due to the 

 functional arrangement of this organ wherein tropotactic as well as 

 telotactic elements are found ; the anterior median units of the eye 

 (ommatidia) initiate reflex turning 

 movements to the contralateral side, 

 the lateral ommatidia to the ipsilateral 

 side, while the central ommatidia, which 

 alone are used for fixation, initiate none 

 (Fig. 33). The animal is thus provided 

 with a very efficient mechanism of 

 orientation, the peripheral parts of 

 which can initiate turning in either 

 direction so that the stimulus is rapidly 

 directed to the important central area, 

 a reflex mechanism which is analogous 

 to the fixation reflexes in man. 



A more plastic mechanism is seen 

 in some aquatic Crustaceans such as 

 the tiny mysids of aquarium tanks 

 {Hemimysis — Franz, 1911 ; Fraenkel, 

 1931) or the hermit crab, Eupagurus 

 (von Buddenbrock, 1922 ; Alverdes, 

 1930). The latter animal goes towards 

 a single light, and even although it con- 

 tinually changes its method of progression, now walking forwards, now 

 sideways or at an angle, it invariably walks straight towards one light in 

 the environment, a directness of path unaffected by the removal of one 

 eye. It would seem that, unlike the bee, any part of the crab's retina 

 can act as a fixation area, and that it must be endowed with a more 

 plastic degree of visual coordination. 



The orientation of the fire-fly, Photinus pyralis, is even more interesting 

 (Mast, 1912 ; Buck, 1937). If a male glows ^ in the neighbourhood of a female, 

 she raises and twists her abdomen so that its ventral surface is directed straight 

 towards him no matter in which direction he may be, and produces a momentary 

 glow ; he thereupon, no matter in which direction he is going, turns through 

 any required angle between 0° and 180"' towards the spot whence the glow came 

 and pr ceeds in total darkness straight towards her. These responses, which 



Fig. 33. — The Telotactic Turn- 

 ing Response in the Compound 

 Eye. 



When / is the line of fixation the 

 arrows show the direction of turning 

 induced by iUumination of different 

 regions of the eye (after Kiihn). 



p. 742, 



