54 



THE EYE IN EVOLUTION 



Fig. 26. — Positive 

 Tropotaxis. 



The tracks taken by the 

 woodlouse, Armadilli- 



dium, blinded on the 

 right side, a, b, c, d. The 

 tracks of the louse in 

 darkness. e, /. Circvis 

 movements with the light 

 overhead (after Henke). 



1 i i i i 



Fig. 27.— Circus Movements in a 

 Unilaterally Blinded Noioxecta. 



The animal directs itself towards the light 

 above, indicated by arrows. The illustra- 

 tion shows the path taken in repeated trials. 

 From left to right, the tracks are the 1st, 3rd, 

 35th, 39th, 41st and 43rd attempts. It is 

 seen that the initial attempts are circus 

 movements which gradually straighten out 

 until eventually, after some trials, the track 

 is almost straight (after Clark, 1928). 



Armadillidium 



stantly to deviate towards one side, or in an overhead light to perform 

 circus movements. This deviation towards the seeing side after 

 unilateral blinding is well seen in the case of the woodlouse, Armadilli- 

 dium, a Crustacean which lives under stones or decaying wood 

 (Henke, 1930) (Fig. 26). In some instances these abnormal deviations 

 occur for an indefinite time,^ but in others a process of adaptation sets 

 in so that the circus movements gradually cease and the path eventually 

 straightens out ^ (Fig. 27). An exception to this type of behaviour is 

 seen in the evolutionary development of the tropotactic response 

 whereby each eye becomes regionally differentiated so that each can 

 act as a symmetrical pair of organs. Thus the eyes of some worms and 

 insects possess two functionally different regions one of which initiates 



1 The snail. Helix — von Buddenbrock (1919) ; the millipede, Julus — Muller (1924) ; 

 the silver T'sh, Lepisma — Meyer (1932) ; the larva of the flour -moth, Ephestia — Brandt 

 (1934). 



2 Thf^ water-boatman, Notonecta — Hobnes (1905), Clark (1928), Liidtke (1935-42) ; 

 the robber-fly, Proctacanthus — Garrey (1918); the whirligig beetle, Dineutus assimilis — 

 Clark (1931-33), Raymont (1939). 



