THE VISION OF ARTHROPODS 



579 



sensory bristles which are distributed all over the body and appendages, 

 particularly the antennae. These are of two types, being sensitive to 

 touch or chemical stimuli, and are present in enormous numbers ; in 

 the lobster, for example, there are said to be 50,000 to 100,000 on the 

 pincers and walking legs alone. 



A considerable aptitude to training is evident among the Malacostraca but 

 it is based on the tactile sense rather than on vision ; the feat of mastering a 

 T-maze is easily acquired by those species which have been investigated but the 

 aptitude is based on the texture of the walls (Agar, 1927 ; Gilhousen, 1929 ; 

 ten Cate-Kazejewa, 1934 ; and others), and is equally showTi by the blind 

 Isopod, Asellus (Bock, 1942). 



Asellus 



THE \t:sio]S" of arachnids 



The function of the eyes of arachnids is very variable and often 

 crude. The smaller species (Acarines) merely respond to the intensity 

 of light, and training experiments with water-mites (Hydracarina) 

 utilizing any sense have been unsuccessful (Agar, 1927). The larger 

 representatives, however, have more fully developed visual functions. 

 The jerrymanders have relatively good vision ; but with the exception 

 of spiders the other Arachnids probably only perceive variations in the 

 intensity of light and movement ; the optics of their ocelli is poor and 

 the number of visual cells small, while visual impressions seem to play 

 an insignificant part in their behaviour. 



THE VISION OF SPIDERS has received more attention than that of 

 any other type (Petrunkevitch, 1907-11 ; Homann, 1928-53 ; Millot, 

 1949 ; Drees. 1952). It is true that the web-spinners with their 

 rudimentary ocelli of a short effective visual range are not particularly 

 visually conscious, for their behaviour is dominated essentially by 

 their exquisite sense of touch ; any tremor on the web caused by an 

 alighting insect excites their immediate attention, probably while the 

 object causing the tremor is still out of the range of their vision. It 

 is interesting that this sense of vibrotropism is purely reflex, for photo- 

 graphy has showT;! that the waiting spider orientates itself so that the 

 vibrations of the web stimulate the legs on each side equally and then 

 sets out in a straight path for its victim. Similarly, ripple-spiders sit 

 at the water's edge resting their forelegs on the surface waiting to 

 appreciate the ripples set up by an alighting insect. In the same way 

 the vibrations of a tuning fork on the web or in the water will excite 

 the spider to run out as if to capture prey. The more active hunting 

 types, however, which move abroad to chase their prey, base their 

 behaviour progressively^ upon vision, each element in the ocellar 

 system having a particular function and the whole acting in a curiously 

 reflex manner. 



Jeri'S'maudsr 



'eb-spinner, 

 Arunea 



^v 



olf-spider, 



Lycosa 



