SENSES of insects: sight 215 



7. Certain insects, the honey-bee and bumble-bee, clearhy distinguish colors and they 

 recognize colors better than forms. With the wasps on the contrary the per- 

 ception of colors appears to be very rudimentary. 



Criticisms of Plateau's views as to vision. — Forel stated (p. 43) that — 



"Plateau began by assuming that the theory of Muller as to mosaic vision is finally 

 rejected and this in consequence of the work of Exner of which we have spoken! 

 This is completely in error. On the contrary we have seen that Exner, as well as 

 Grenacher, has entirely rehabilitated the theory of Muller. Plateau appears to have 

 failed to understand the works of Exner, because he also adds that this author has 

 concluded theoretically that insects do not see the form of objects, which is wholly 

 inexact as well. Plateau says in the text, 'The earlier hypothesis of J. Muller, con- 

 sisting in the production of a mosaic image formed by the juxtaposition of a series 

 of minute partial images each of which occupies the base of one of the distinct elements 

 of the compound eye is definitely rejected.' But this is the theory of Gottsche and not 

 at all that of mosaic vision of Muller. The theory of mosaic vision does not admit 

 of minute images in each facet, but a single image in mosaic formed by the juxta- 

 position of light rays of different quality perceived by each facet." 



As to the experiments of Plateau upon the perception of form by in- 

 sects, Forel declared that these did not in the least demonstrate that 

 they could not distinguish form, but merely that they would fly to the 

 more luminous of the two orifices without regard to the form. He also 

 pointed out that while Plateau first spoke only of the non-perception 

 of form, he concluded by saying that "insects do not distinguish the 

 form of objects or distinguish it very badly." 



Sensibility to color. — Graber said (1875): 



"I understand very well that a brain relatively small and simple in construction 

 is in general less capable than a more highly differentiated central organ and that 

 both the number and the coordination of perceptions is limited by the simplicity of 

 structure, but I can not admit offhand that when an animal, let us say a bee for ex- 

 ample, exhibits actual color perceptions, these must necessarily be different from ours, 

 or that we then know, since the sensory organ for color perception in the bee is essen- 

 tially different than with us, that the nerve tissues concerned can not call forth the 

 same sensations that they do with us. Disregarding the fact that insects possess 

 very complex eyes and that they appear to react clearly to small differences of color, 

 such as orange, yellow-green, etc., other evidence testifies to the great sensibility of 

 their sense of color. I have especially in mind the fact that many insects have no 

 other means of recognizing their own species than certain differences in color and it 

 is well known that related species often differ outwardly only in coloring or marking. 

 Moreover, it is hardly to be assumed that mutual recognition takes place by means of 

 other attractions, imperceptible to us, such as odor or sound. But if I ascribe a very 

 delicate sense of color to certain insects on the basis of these relations, I do not how- 

 ever go so far as Allen, who asserts that the ability of insects to distinguish color 

 may be more marked than with man." 



Perception of form and movement by insects. — In a series of six 

 papers appearing from 1885 to 1888, Plateau dealt with a comprehensive 

 investigation of vision in the Arthropoda. Some of these were made upon 

 other groups than the insects and others had little or no relation to 

 the problem of insect attraction. In consequence, the following account 

 of his conclusions is confined to those that have to do with the perception 

 of form or movement. Diurnal insects require a fairly strong light and 



