LIGHT AND MOVEMENT 



73 



light of the sky, as do Amphipods when seeking their return to the sea 

 (Pardi and Papi, 1952-53), or making use of the polarization patterns 

 which exist between the air-water interface (Waterman, 1954). Such 

 reactions have been demonstrated in 12 species of Cladocera, water- 

 mites and caddis-fly larvae, which tend to swim so that their direction 

 of movement is at right angles to the plane of polarization (Baylor and 

 Smith, 1953). The crab, Ewpagurus, shows a definite response to a 

 change in the direction of polarization (Kerz, 1950) as also do mosquito 

 larvae. 



The navigation of the small crustacean, Talitrus saltator, as recorded by 

 Pardi and Papi (1952-53) is a fascinating story. These Amphipods normally 

 live in the intertidal zone. Transferred inland, they move towards the coastline 

 whence they came, taking their direction from the angle of the sun ; as with 

 insects and birds their course can be deflected by changing the direction of the 

 incident light by a mirror. If direct sunlight is not available they can orientate 

 themselves by polarized light from patches of blue sky and can be similarly 

 deflected by the interposition of a polarizing sheet ; under a completely overcast 

 sky they are disorientated. As with bees there is also an innate mechanism which 

 allows them to compensate for movements of the sun throughout the day, but, 

 unlike the reactions of the bee, it would seem that the whole mechanism is 

 established by heredity or acquired in early youth and is set in each individual 

 for ever and cannot be changed. Thus specimens on the west coast of Italy move 

 westwards towards the sea, and even when brought to the seaside of the east 

 coast will automatically attempt to travel westward right across country away 

 from the nearby water. The most extraordinary thing about these creatures is 

 that travelling through the night they appear to be able to navigate with 

 reference to the moon. This is the only instance where this has been established 

 and in view of the complication and rapid change of the lunar path across the 

 sky, it would seem to be an extraordinary feat. 



ORIENTATION TO A VISUAL PATTERN SO that its reception on the 

 retina remains constant corresponds closely in its mechanism to 

 orientation with respect to a source of light. Thus insects placed on a 

 turn-table facing a window will move round when the table rotates 

 (Radl. 1902) and if a striped drum is rotated in front of them they will 

 endeavour to keejD in line with a given stripe (an " optomotor response'") 

 (Schlieper, 1927 ; Schulz. 1931 ; Zeiser. 1934)i. Gregariousness in 

 locusts depends on the same reaction ; moving so that it nullifies the 

 movement of images across its retina, each swarming insect travels 

 precisely with its neighbour (" gregarian inertia "), the whole host 

 being guided by a light-compass reaction to the incidence of the sun's 

 rays (Kennedy, 1939-45). Orientation when swimming against a 

 current of water (" rheotaxis ") is in fact a visual response of the same 

 type : the water-boatman, Notonecta, for example, turns upstream and 

 swims with sufficient strength to maintain a constant impression of 

 the nearby bank ; if the landmarks on the bank are moved, the water- 



1 This reaction has been used to measure the visual acuity of insects, see p. 588. 



Eupagurus 



Talitrus saltator 



Locust 



Xotonecta 



