THE NERVOUS SYSTEM 125 



is obtained only under optimal conditions of illumination. With poor 

 illumination, both the visual acuity and the discrimination of dif- 

 ferent light intensities are so greatly inferior to that of man, that the 

 resolving power of the eye must be extremely feeble. None the less, 

 these insects can recognize different forms, and can learn to associate 

 with these the presence of food. The bee and other Hymenoptera can 

 find their way over miles of country by means of landmarks, and can 

 recognize the entrance to their own hive or nest by visual memory. 



According to Exner's elaboration of the mosaic theory there are 

 some insect eyes (such as those of Orthoptera, Hymenoptera, Dip- 

 tera) in which the light received in one facet illuminates only a single 

 retinal element to produce an 'apposition image'. Whereas in other 

 insects, notably in nocturnal Lepidoptera in the dark-adapted state, 

 the light from a single facet can spread out and reach a group of 

 adjacent retinal elements to give a brighter but more diffuse 'super- 

 position image'. More recently it has been shown that even in eyes 

 of the apposition type there are diffraction images comparable with 

 those of superposition type. Perhaps these are concerned in particu- 

 lar kinds of pattern or movement perception. 



Another property which is characteristic of the insect eye is its 

 ability to detect the plane of polarization of light. It might be 

 thought that this ability would be of no great practical importance; 

 but in fact the light coming from a blue sky shows a characteristic 

 pattern of polarization which is related to the position of the sun. 

 Bees are able to orientate themselves by means of this pattern, pro- 

 vided they can see part of the blue sky, even if the sun is concealed. 

 Indeed, it has been found that many insects pursue a perfectly 

 straight course when allowed to walk in the open under a clear sky. 

 It was long believed that this was a sun compass reaction, an example 

 of 'menotaxis' (p. 118) related to the position of the sun. But it 

 appears more often to be a menotactic orientation to the pattern of 

 polarized light in the sky. This ability to recognize the plane of polar- 

 ization resides both in the compound eyes and in simple eyes, such 

 as the stemmata of caterpillars. The physical mechanism involved is 

 not fully understood; but it seems likely that the retinal rod, or 

 'rhabdom', contains dichroic visual pigments orientated in a regular 

 fashion characteristic of each radial segment, or 'rhabdomere'. That is. 



