How they become aware of their surroundings 



In many insects the pigment on the walls of the lens-cyhnders moves 

 in response to hght-intensity. Thus in a strong light we have the 

 cylinder- walls black, and the picture at its sharpest ; in a weak light the 

 pigment retreats and allows the more blurred, night-adapted image to 

 be formed. 



A mosaic image of this kind gives an insect only an imperfect picture 

 of what Hes before it. That is why it is a mistake to think of insects as 

 moving about in a world of soUd objects such as we ourselves see. Their 

 view must be rather hke that we have through the frosted glass of the 

 bathroom window, but without our advantage of knowing what it looks 

 like with the window open. Insects obviously can distinguish shapes to 

 some extent, and the bees and wasps can find their way back to their 

 hive or nest again. But movement is the thing that really attracts their 

 attention. A robberfly lives by chasing flying insects and catching them 

 in the air, but it will equally well follow a piece of thistledown, or a tiny 

 paper ball. 



A single compoimd eye can give little idea of distance, except what 

 can be inferred from the size of the object seen: i.e. the number of 

 facets of the eye that it occupies. In most insects the space immediately 

 ahead comes into the field of view of both compound eyes, and this gives 

 a sort of rangefinding device, rather like our own stereoscopic vision, 

 except that the insect's eyes are fixed, and cannot squint like ours. 

 Predaceous insects seem to be able to wait patiently until their prey is 

 just at the right distance before snapping at it. 



The question of colour sense is too compUcated to discuss here. It is 

 certain that many insects can distinguish some colours, though prob- 

 ably never quite the same spectrum that we can see. When making 

 experiments it is difficult to make sure that the insect is really dis- 

 tinguishing between two colours, and is not being guided instead by 

 shape, or diff"erences in Hght intensity; just as colour-blind people 

 generally behave quite normally, even though they may live in an all- 

 grey world, or one in which all greens are bright red. 



Ocelli are siniple eyes, with only a single facet, though they often 

 occur in groups of three or six. They. are of two kinds, the dorsal ocelli 

 of adult insects, and the lateral ocelli of larvae. 



Dorsal ocelli are to be found at the top of the head, or vertex. Three 

 is the full number, though there is some evidence that the middle, or 

 median ocellus is a double structure. Sometimes only the median 

 ocellus is present; sometimes only the two lateral ones; in a great many 

 insects the ocelH are completely absent. It is beheved that the ancestral 

 insects had ocelli, and that where they are now absent they have been 

 lost in the course of evolution. 



5 6i 



