580 



THE EYE IN EVOLUTION 



Evarcha blancardi 



The behaviour of the jumping spider, Evarcha blancardi, the arrangement 

 of the ocelli of which is shown in Fig. 216, may be taken as an example. It sees 

 its prey (or mate) with the posterior lateral eyes which, situated far back on the 

 head, have a wide field of vision and respond to moving stimuli only ; a 

 stationary object excites no reaction. As the image of the moving object 

 crosses the retinae of these ocelli, the spider reflexly turns its body in the direction 

 of the object with the result that the image falls on the retina of one of the 

 anterior lateral ocelli, whereupon a further turning movement throws the image 

 on both anterior lateral ocelli and the two central ocelli. If the former ocelli 

 are covered, this second turning movement does not occur. It would seem that 

 the function of these ocelli is to judge distance binocularly, that of the central 

 ocelli, which have a small field and a short range, to perceive the form of the 

 prey ; in each the lens is capable of forming sharp images. A male, for example, 

 acts as if it can distinguish between a female of its own species or a male of its 

 own or another species at a distance of 2 to 3 cm. At a distance of 1-5 cm. it 

 leaps upon its victim with accuracy, but if the lateral anterior ocelli are covered 

 the distance of the leap is misjudged. The posterior lateral ocelli therefore act 

 as the peripheral retina of man, collecting impressions from the whole visual 

 field ; the front row of four eyes acts together as the human fovea, the lateral 

 pair being most useful binocularly at a short distance, the central pair being the 

 chief agent for visual analysis. The small jaosterior median pair of ocelli, on the 

 other hand, are used for the detection of movement behind the sj^ider. 



The reflex nature of the response is illustrated by the automatic movements 

 of the limbs following retinal stimulation. Homann found that on covering the 

 two median ocelli the first pair of legs was held up by the contraction of the 

 femoral muscles and as the animal ran forwards they merely clawed the air 

 instead of touching the ground ; if one of these eyes were covered the foreleg 

 on the blind side alone was held up and the body was tilted sideways. 



Despite the apparent automatism of this reflex response, however, 

 spiders display a very considerable degree of visual intelligence. 

 Nowhere is this more aptly illustrated than in the stalking of a fly on a 

 creviced wall by a jumping spider. Spying a fly settled on the wall some 

 distance away, the spider, knowing that the attention of the fly will be 

 excited at once by a moving object, creeps with the greatest care to the 

 nearest crevice in the brickwork. Arrived there, knowing that the fly 

 will soon take wing, it scampers rapidly along the crevice hidden from 

 view until it comes within range of its victim ; thereupon, anchoring it- 

 self by a life-line of silk to the brickwork, it leaps upon its victim with 

 incredible rapidity, hoisting itself back to safety by the silken cord. 



Moreover, in their visual activities a considerable degree of sensory 

 analysis exists, for jumping spiders can be negatively conditioned to 

 unpalatable prey, and Drees (1952) found that their form vision is 

 sufficiently effective to allow negative conditioning by means of an 

 electric shock to a response acquired by training to visual stimuli such 

 as triangles and crosses. It is also of interest that the jumping spider 

 has been shown by its response to the optomotor reaction to have a 

 ;-elective sensitivity to orange (Kastner, 1949), a response which may 

 ijvlicate some degree of " colour vision " on a reflex level. 



