THE SENSES OF INSECTS 107 



a very small part of the visual field, namely, that which 

 is exactly opposite the facet concerned, one continuous 

 image of surrounding objects — made up, as it were, of 

 countless fragmental images — is formed upon what may 

 be termed the retina. Muller's conception of insect 

 vision is usually spoken of as the " mosaic theory," because 

 the separate images formed by the individual facets are 

 believed to combine on the retina in much the same way 

 that little pieces of stone or marble are fitted together to 

 form a mosaic pavement. Clearly the large, globular 

 eyes of such an insect as the dragon-fly must command a 

 very wide range of vision. Their owner must be able to 

 see in almost all directions at once without moving its 

 head. But our knowledge of the actual power of sight 

 among insects is extremely meagre. The experiments of 

 Lord Avebury (better known as Sir John Lubbock) and 

 others have shown that some insects, at least, are un- 

 doubtedly able to distinguish between colours, and that 

 they show a preference for some colours over others. 

 Thus, hive-bees commonly choose blue flowers, white 

 butterflies prefer white flowers, while yellow butterflies 

 appear to alight most frequently upon yellow flowers. 

 Indeed, there is small room for doubt that an insect's 

 perception of colour enables it to find a particular kind of 

 blossom, or to recognise a suitable mate; although we 

 shall see that the majority of insects seem to rely chiefly 

 in these matters upon the sense of smell. But there is 

 little evidence to show that the sight of insects is at all 

 keen, or that they possess a clear perception of form ; nor 

 is this surprising when we are told that the compound 

 eyes, like the ocelli, are of fixed focus. In a word, the 

 compound eyes appear to be especially adapted for trans- 

 mitting sensations of light and motion to the brain ; and 

 although the limit of vision probably varies in different 



