46 



THE EYE IN EVOLUTION 



Forficula 



Two members of 

 Polyzoan colony 



Caterpillar 



Any u ilia 



in water and upside-down, one stimulus (the presence of water) thus modifying 

 the influence of another (light). Its geo -negativity drives it to the surface of 

 the sea and if it surfaces in bright light it returns to the water because of its 

 photo negativity ; if it surfaces beneath a submerged rock its positive photo- 

 taxis makes it crawl beneath it in the upside-down position until, reaching the 

 air, its negative phototaxis keeps it in a shaded cleft. Again, when the gardener 

 traps an earwig in a flower-pot containing dry straw inverted on a cane, he is 

 utilizing the fact that Forficula deinonstrates photokinesis, thigniotaxis, hydro- 

 kinesis and negative geotaxis. 



A change in response during the development of the animal is well 

 exemplified in the case of some marine worms ; these are usually 

 photo-positive when they leave the egg so that they come to the 

 surface and swim ; at a later stage they become photo-negative with 

 the result that they burrow in the mud and crawl (Mast, 1911). The 

 larva? of the polyzoan sea-mat, Bugula, similarly disperse under a 

 positive phototaxis, but after a few hours turn photo -negative so that 

 they attach themselves to the bottom and undergo metamorphosis 

 (Grave, 1930 ; Ljoich, 1949). 



These changes may be associated with stages in the development of the 

 visual cells. Thus the larvte of the cat-flsh, Ameiurus, are initially imresponsive 

 to light at a stage when the visual elements are not fully differentiated ; later 

 they become photo -negative, a phase during which the rods and cones are 

 contracted and show no retinomotor reactions ; finally the larvae become 

 photo -positive, a phase characterized by the commencement of retinomotor 

 reactions (Armstrong, 1949). 



A change in response may also accompany a change of habit. 

 Thus young caterpillars of Porthesia are strongly photo-positive when 

 they are hungry, a response which normally leads them upwards to the 

 leaves of their food plant, but the response is lost after feeding ; while 

 male and female ants become temporarily photo-positive at the time 

 of their nuptial flight, a reaction lost when they shed their wings 

 (Loeb, 1918). 



Another interesting example of this type of change to suit a marked change 

 in habit is the common eel, Anguilla. At the stage of sexual maturity in the 

 autumn when it lea\es fresh water to migrate downstream^ on its journey to 

 its mating grounds in the Sargasso Sea, there is a great increase in the size of 

 the eyes and the fish becomes photo-negative. This season coincides with the 

 safety afforded by floods and moonless nights and the fish avoids the light to 

 such purpose that its nuptial journey can be checked and the eels diverted into 

 traps in large numbers by means of underwater lights shining upstream 

 (Lowe, 1952). 



A phototactic response of this type may be so prepotent that, although 

 generally biologically useful, it may driv^e the animal to destruction. Thus the 

 stimulus which leads the moth to fly towards the sun will drive it into the 

 candle-flame ; the same response in the newly hatched larva of Euproctis which 

 normally loads it upwards towards the leaves of its food plant will force it to 



