588 



THE EYE IN EVOLUTION 



Musca 



Butterfly, Vanessa 



length ; in one response the bee may act as if colour-blind, in another as if 

 partially so, and in a third it may show a wide discrimination of hues. In the 

 same way the hawk-moth, Macroglossa, selects yellow and blue objects when 

 hungry, yellow-green backgrounds for oviposition, and dark surroundings of 

 any colour for hibernation (Knoll, 1925-26). This restriction of a specific 

 response to a few " sign-stimuli " rather than to all possible environmental clues 

 is of wide application ; it is well exemplified in the ajaparent blindness of the 

 water-beetle, Dytiscus, in its hunting reactions ^ and is by no means confined 

 to Insects.^ 



The perception of form in insects appears to be rudimentary. The 

 visual acuity as measured by responses to revolving striped drums is 

 relatively low (Hertz, 1929-39 ; Hecht, 1931)— about 1/100 that of 

 man in the bee, 1/1,000 in Drosophila (Baumgartner, 1928; Hecht and 

 Wolf, 1929 ; Hecht and Wald, 1934 ; Gavel, 1939 ; Roeder, 1953), 

 while in the house-fly, Musca, the narrowest stripe that can be 

 perceived subtends an angle of 5° (Gaffron, 1934) (in man, 1'). These 

 results of behavioural experiments correspond with the theoretical 

 acuity deduced from the structure of the eye (Piitter, 1908 ; Best, 

 1911).3 



As would be expected from their low standard of visual acuity, 

 the capacity of insects to analyse a pattern is relatively poor. It is 

 true that experiments have shown that bees and butterflies can be 

 attracted by broken or checkered figures and divided contours to 

 which they have been trained, a response which confirms the biological 

 value of " honey guides " on flowers (Zerrahn, 1933 ; Hertz, 1935 ; 

 Bolwig, 1938).^ It is also true that the honey-bee can be trained to 

 seek a sugar-container associated with a black disc and avoid one 

 associated with a black cross or can differentiate four parallel lines 

 from a black circle ; but it cannot be conditioned to distinguish 

 between a black cross and four parallel lines on a white surface (von 

 Buddenbrock, 1952). In order to allow the discrimination of patterns, 

 therefore, the differences must be gross. It is probable, indeed, 

 particularly in so far as the " fast " type of eye is concerned, that the 

 response is less to the recognition of the configuration of objects than 

 to the frequency of change of retinal stimulation (Wolf, 1933-37) and 

 that fast-flying diurnal insects resolve the spatial display of a pattern 

 into a temporal display of sequential stimuli. The method of interpreta- 

 tion of slow-moving, nocturnal or aquatic insects is not yet known. 



From these characteristics it follows that moving objects excite 



1 p. 103, Fig. 74. 2 p. 664. ^ p_ m 



^ It must not be thought that all the adult bee's activities in visiting flowei's 

 for honey are determined by vision. At relatively close quarters the sense of 

 smell is important. Bees can be trained to react to scent alone. Moreover, when the 

 insect lands on the flower, taste-organs which occur not only on the inouth but on 

 tlic antennse, labial palps and feet, come into play. In the search for honey, therefore, 

 tlio ■,;yes are the distance-receptors, the organs of smell the intermediate, and of taste 

 the contact-receptors. See Bolwig (1954) and others. 



