74 



THE EYE IN EVOLUTION 



Gyrinid beetle 



Daphnia 



boatman moves with them, and if they are obliterated as when swim- 

 ming in the dark or between plain white boards, the insect allows itself 

 to be carried j^assively downstream (Schulz, 1931). Gyrinid beetles are 

 similarly disorientated when swimming in the dark or if a sndden 

 change is made in the landmarks on the banks (Brown and Hatch, 

 1929). 



THE DORSAL (ventral) LIGHT REACTION. The Orientation of 

 animals which progress on the earth's surface can be treated as if 

 movement on one plane only need be considered ; but those that swim 

 or fly have three available planes of movement — they can turn as do 

 land animals on a vertical axis, but they can also roll on a longitudinal 

 axis or they can pitch, turning somersaults about a transverse axis 

 (Fig. 48). They must therefore possess a complex means of orientation 

 to maintain the body in a desired position as it travels towards a goal. 

 Because of its relatively greater specific gravity the stability of an 

 animal body in air is greater than in water, and since the attachments of 

 wings are comparatively high making the centre of gravity relatively 

 low, the equilibrium of balance in birds raises no serious difficulties. 

 This does not apply with the same force to insects although some, such 

 as the dragon-fly, Anax, demonstrate a dorsal light response during 

 flight, the effective organ being mainly the compound eye (Mittelstaedt, 

 1950) ; but aquatic animals require to perform constant and active 

 balancing movements to maintain their normal orientation. Many 

 fishes maintain their position optically by keeping one surface (usually 

 the dorsal) perpendicular to the light, using their eyes as receptor 

 organs ; others have evolved a specific statocyst organ to maintain 

 equilibrium, but although this development has assumed the greater 

 importance eventually, the eyes still participate in the orientating 

 reflexes, a collaboration between the senses which survives in the 

 elaborate reflex connections which continue to yoke the visual with the 

 vestibular system in Man. 



The dorsal light reaction was initially recognized in the crustacean, 

 Da])hnia, by Radl (1901), and its wide distribution was first appreciated 

 by von Buddenbrock (1914-37) ; it has since been observed in many 

 groups of aquatic animals of a wide variety.^ In its essentials the 

 DORSAL LIGHT REACTION eusures that when the light is above, the 

 animal swims with the dorsal surface upwards, maintaining itself 

 symmetrically to it and moving (if it does move) in a plane at right 



1 In Medusaj — Fraenkel (1931) ; polychaete worms — Fraenkel (1931), v. Budden- 

 brock (1937) ; in a large number of Crustaceans — v. Buddenbrock (1914), Alverdes 

 (1926-30). Schulz (1928), Seifert (1930-32) ; among Insects in nymphs and larvje— 

 V. Buddf Ml)rock (1915), Wojtusiak (1929) ; in the dragon-fly, Anax, during flight — ■ 

 Mittelsta. t (1949) ; perhaps in the desert locust, Schistocerca gregaria — Rainey and 

 Ashall (1! '.) ; and particularly in Fishes — v. Hoist (1935). 



