34 



THE EYE IN EVOLUTION 



King-crab 



Whip-tail scorpion 



Lamprey 



Cockroach 



The response may involve a change of velocity (orthokestesis) 

 {opdos, straight) or a change in direction (kllnoklnesis) {kXlvco, turn). 



ORTHOKESTESIS, wherein random movements are accelerated or 

 decelerated according to changes in the intensity of the illumination, is 

 seldom the sole mode of response of any organism to light but usually 

 reinforces reactions of another type. In its most dramatic form the 

 organisms move while the stimulus acts, that is, so long as an intensity 

 gradient exists ; when the intensity becomes constant they come to 

 rest. Viewed superficially this elementary response gives a false 

 impression of orientation. Thus if the locomotor activity of an 

 organism is increased by light and diminished in darkness, it aggregates 

 preferentially in the shadowed region even if its movements continue 

 to be random, just as the density of vehicular traffic increases as it is 

 slowed in towns and decreases when speed is regained on the trunk 

 roads ; an organism with this reaction of a high kinesis in the light 

 thus appears to show a negative phototaxis but can be said to be 

 negatively phototactic with as much logic as the average motorist may 

 be assumed to delight in traffic-jams. 



This response of activity in a light -gradient and rest in the shade 

 giving rise to an apparently photophobic tendency to aggregation in 

 the dark is relatively common ; it is seen typically in the Bacterium 

 photometricum which, as its name implies, becomes active only under 

 the influence of light, in many flat-worms,^ in the maggot larvae of 

 various flies, ^ in certain Arthropods such as the king-crab ^ or the 

 whip-tail scorpion,* in primitive Vertebrates such as the lamprey ^ 

 and in the larvae of certain fish such as the herring, Clupea, and plank- 

 tonic animals as a means of depth-control.^ The converse reaction is 

 less common but is well exemplified by the inactivity of the cockroach 

 in daylight and its activity in darkness.'^ 



In higher forms these simple kinetic responses are less evident but stimula- 

 tion of the eyes by light frequently has a dramatic effect on general activity. 

 This is especially seen in Insects : thus in the cockroach, Periplaneta, exposure 

 to light considerably reduces the threshold of response to other stimuli (Brecher, 

 1929), and as the intensity of light is increased the beetle, Popillia, walks more 

 quickly (Moore and Cole, 1921). 



KLiNOKESTESis is of much wider application and interest ; in it a 

 change of direction is involved, so that turning tnovements, normally 



^ Planaria—Pe&T] (1903), Walter (1907) ; Leptoplana—Kovey (1929) ; Plagio- 

 s/omwm— Welsh (1933). 



« Mast (1911), Herms (1911). 

 = Limulus— Cole (1923). 



* Mastigoproctus giganteus — Patten (1917). 

 ' :i!,^petra — Young (1935). 



* Vvoodhead and Woodhead (1955). 



' ^iuma orientalis — Szymanski (1914), Wills (1920). 



