SIGHT — GENERAL 23 



elephant hawk moth feeds almost entirely after dusk, and 

 during the day can be found only down on the bottom stems 

 of willow-herb, which is its food plant. Such behaviour is 

 protective. 



In spiders the eyes are of considerable interest. A spider is 

 not sufficiently well-served merely by a pair of eyes — either 

 six or eight eyes being present. Four families have six and 

 the remainder have eight. W. S. Bristow in his World of 

 Spiders (New Naturalist Series) considers that spiders can 

 alter the focus of their eyes to suit given situations or require- 

 ments. 



All the pairs of eyes may be used in catching prey, but 

 which ones are in operation depends upon the situation of 

 the pairs — these being differently situated in different species 

 — and also on the position of the prey in relation to the spider 

 itself Naturally, those spiders which are hunting species 

 depend more on ocular ability than those with webs of one 

 kind or another where the delicate sense of touch takes first 

 place. 



Among the insects there are two distinct kinds of eyes. 

 There are the simple eyes or ocelli, and the compound eyes. 

 The simple eyes are themselves split into two types, the dorsal 

 ocelli and the lateral ocelli. The former are found in adult 

 insects and in the nymphs of some of them, while the lateral 

 ocelli are, in nearly all cases, confined to the larvae. 



These simple eyes are only capable of perceiving objects 

 which are very near, and are very different from the mar- 

 vellous compound eyes of adult insects, which are made up 

 of a great many lenses or facets — in dragonflies, for instance, 

 the number of lenses may amount to some thousands. There 

 are two compound eyes, one on each side of the head, and 

 the type of vision these provide is called "mosaic vision". 

 This means that the brain of the insect receives "pieces" of 

 the picture of the object in view, and these "pieces" are 

 eventually put together to form a single picture. The word 

 mosaic describes very well the kind of ultimate picture that 

 reaches the brain. This brief description of the optical 

 apparatus of typical insects may help the field worker to 

 understand that part of some insect behaviour in which the 

 eyes are used, and will enable him to interpret much which 



