196 THE PHYSIOLOGY OF INSECT SENSES 



various particles in water (Baylor, 1959 a, 1959 b; Smith and Baylor, 

 1960; Kalmus, 1958, 1959; Bainbridge and Waterman, 1957). Some 

 of the evidence upon which this view is based has already been given. 

 Additionally, it is claimed that the ability of insects to respond photo- 

 tacticly to polarized light depends upon whether the substrate is black 

 or white (e.g., Kalmus, 1958). 



Evidence against the reflexion hypothesis has already been stated. 

 In so far as bees are concerned, recent experiments by von Frisch, 

 Lindauer, and Daumer (1960) emphasize the inadequacy of the hypo- 

 thesis. With bees, direct analysis of the sky's light during the dance on 

 a horizontal comb requires use of the dorsal areas of the eyes, while 

 recognition of reflexion patterns of the substrate requires the ventral 

 areas. Masking of the upper portions impairs orientation of the dance, 

 but masking of the lower portions does not. Furthermore, walking 

 bees orient as well on a white substratum as on a black, glossy one. 



Whatever may be the mechanism, the fact remains that many in- 

 vertebrates behave as though they can detect differences in the plane of 

 polarization of light and utilize this abihty in their economy of living. 



FORM PERCEPTION 



The intrinsic characteristics of the stimulus for a photoreceptor are 

 intensity, wavelength, and plane of polarization, but the occurrence of 

 stimuli also varies in space and time. In the visual field there is at any 

 given time a definite spatial arrangement of the kinds and amounts of 

 radiant energy. In other words, there are patterns of light. The fineness 

 of form perception depends upon the accuracy with which photo- 

 receptors can detect this pattern, that is, upon their resolving power, 

 and ultimately upon the faithfulness with which the spatial and tem- 

 poral relations are represented in the central nervous system. 



The capacity of the dioptric apparatus to form images that are 

 reasonably distinct by human standards is easily demonstrated by 

 taking photographs in the focal plane of excised lens systems 

 (Gottsche, 1 852 ; Exner, 1891; and many others). In those eyes in which 

 each ommatidium is optically isolated from its neighbours by envelop- 

 ing pigment cells and in which the rhabdom Hes immediately beneath 

 the crystalline cone, a small inverted image is produced at the base of 

 each cone. Eyes of this type are termed apposition eyes and are 

 characteristic of diurnal insects (Fig. 6). Miiller (1826) proposed that 

 these minute images were physiologically unimportant as images. He 

 visualized each ommatidium of the apposition eye as a device for 



