66 THE EYE IN EVOLUTION 



proven, appears at present to be the most probable explanation of the observed 

 facts ; it may well seem so complicated an automatic calculation by a creature 

 with a proverbially small brain as to appear fantastic ; but the ability of a bird 

 released in America to orientate itself immediately for its flight to a particular 

 and very precise locality in Europe is fantastic — it occurs within 40 seconds of 

 viewing the sun. It would seem that on the basis of its structural potentialities, 

 the avian retina should be capable of such a feat.^ It has also been suggested 

 (again without proof) that the pecten ^ may play some part in the analysis by 

 acting as a fixed point when taking observations (Mermer, 1938 ; see also 

 Crozier and Wolf, 1943 ; Griifin, 1952). 



ORIENTATION TO POLARIZED LIGHT. Arthropods as widely different 

 as the king-crab, the sand-hopper, the ant and the honey-bee possess 

 the abihty to respond to the plane of polarization of light, and by this 

 means may orientate themselves in skylight out-of-doors. This faculty 

 can be investigated experimentally by observing the response to the 

 rotation of the axis of a sheet of " Polaroid " glass. Sensitivity of 

 this type was first demonstrated in bees by von Frisch (1949) and has 

 since been confirmed in behavioural experiments involving a number 

 of Arthropods, both larvae and adults,^ and has also been proved by 

 electroretinographic responses.* 



Light from the blue sky (not directly from the sun) has been scattered from 

 particles in the atmosphere which also partly polarize it, that is, more of the 

 light-waves vibrate in one transverse direction than in others. The plane of 

 maximum polarization is different for each patch of blue sky, and the proportion 

 of light polarized also varies, being greatest at 90° from the sun. Thus each 

 patch of blue sky has its own plane and intensity of polarization, differing from 

 every other patch. A " Polaroid " glass is a submicroscopic crystalline grid trans- 

 mitting chiefly light vibrating in one particular direction ; it can be used to 

 analyse the plane and intensity of polarization of light since, on rotation, light 

 polarized in other planes is cut off. 



We shall see presently ^ that insect larvae have simple eyes 

 (stemmata) while adults, in addition to simple eyes (ocelli) are usually 

 equipped also with two large compound eyes. The stemmata of the 

 larvae respond both to direct light and alterations in the plane of 

 polarization, while in adults the ocelli sometimes show little or no 

 phototactic response to non-polarized light, but aid the compound 

 eyes in their response to polarized light. In these cases the former are 

 thus supplementary in function so that the intact animal reacts more 

 quickly and accurately than one deprived of its ocelli. 



1 p. 417. 2 p. 416. 



* Larvtp of the sawfly, Neodiprion — Wellington et al. (1951) ; mosquito larva? — 

 Baylor and Smith (1953) ; adult insects — Vowles (1950-54), Menzer and Stockhammer 

 (1951), Cartiiv (1951), Stephens et al. (1952-53), Wellington (1953), de Vries et al. 

 (1953) ; oth; Arthropods— Waterman (1950), Kerz (1950), Pardi and Papi (1952), 

 Baylor and vSi ,li (1953), and others. 



« Autrun. id Stumpf (1950). Waterman (1950-51). 



s p. 222. 



