THE VISION OF ARTHROPODS 



589 



attention and stationary objects tend to be neglected. This tendency- 

 is borne out, as we have already seen ^ in behavioural experiments 

 involvmg the optomotor response to a striped drum which shows a high 

 flicker-threshold up to 200 per sec. in the bee,^ the corresponding 

 figures in man as measured by the fusion frequency of flicker being 

 50 to 100 depending on the intensity 

 of illumination and the size of the 

 fleld stimulated (Collins and 

 Hopkinson, 1954) ; similarly, the 

 fusion-frequency as measured by the 

 changes in the electrical potential of 

 the retina in many insects, particu- 

 larly of the rapidly flying diurnal 

 type, may reach very high values,^ a 

 capacity doubtless correlated with 

 the need to resolve succeeding im- 

 pressions during flight. It would 

 thus seem that in their activities 

 insects depend much more on the 

 primitive faculty of the appreciation 

 of movement than of form. The 

 widely over-lapping visual fields of 

 the compound ej^es allow jjerception 

 of distance, a power of judgment 

 which is impaired if one eye is 

 obscured (Homann, 1924): and be- 

 havioural experiments show that a 

 high degree of spatial appreciation 

 and localization is possible (Tinbergen, 

 1932-38 ; Wiechert, 1938). The 

 extraordinary capacity of some 

 insects for memorizing and recogniz- 

 ing landmarks in their territory has 

 already been discussed at length.^ 



The dependence of insects on visual stimulation by moving objects is seen 

 in the every-day behaviour of the ordinary house-fly which neglects stationary 

 objects but uTimediately absconds on the first suggestion of movement. It is 

 also exemi^lified in a striking way by the habits of the j^raying mantis (Fig. 734) ; 



1 13. 583. 



2 60 stimuli per sec. in Aeschna nymphs, Salzle (1932), and in Anax nymj^hs, 

 Crozier et al. (1937) ; see also Autrum and Stocker (1952), Autrum (1954). 



^ 95 per sec. in the ocellus of the bee (Ruck, 1954) and of the order of 165-300 

 stimuli per sec. in the compound eye of this insect, or 265 per sec. in the blue-bottle 

 CalUpliora (Autrum and Stocker, 1950 ; Autrum, 1952). Corresi^onding measurements 

 ill man with the electroretinogram are 25-30 for the scotojDic and 70 for the photojjic 

 fusion frequencv ( Wadensten, 1956 ). 



* p. 78. 



Fig. 734. — The Praying Mantis, 



MaXTIS RELiaiOSA 



Sitting on a leaf. Note the large 

 and prominent eyes and the " praying " 

 position of the front legs. The ter- 

 minal part of the bent fore -leg with 

 its powerful joint resembles a pen- 

 knife, normally held half open ready to 

 snap shut against its " sheath " with 

 the prey trapjaed between (photograjDh 

 by ^Michael Soley). 



