THE INSECT WORLD 



important parts of the body are indicated. The skeleton of 

 insects is almost all laid down as an external armour. This 

 largely limits their size, for the internal organs of a really 

 large insect would be crushed under their own weight. 

 Moreover, insects breathe by means of a series of fine tubes 

 which ramify into every organ of the body. Oxygen passes 

 in mainly by diffusion and respiratory movements, corres- 

 ponding to our breathing, are less developed and only aerate 

 the larger tubes. This method of obtaining oxygen is very 

 efficient in small animals but becomes less so as size increases. 

 The blood of insects, also, is not in general enclosed in veins 

 and arteries and does not circulate rapidly round the body 

 like ours, and this again would be a drawback in a large 

 animal. 



An insect has a pair of compound eyes made up of a large 

 number of small facets or transparent lenses. The whole eye 

 works on a different principle from ours and there is no way 

 of focusing the vision on objects at different distances. 

 Insects cannot see nearly so far as we can nor form such clear- 

 cut images, but their eyes may be very efficient for certain 

 specialised needs, such as catching quick-moving prey. So 

 much can be deduced from a careful study of the micros- 

 copical structure of the eye. The visual powers of an insect 

 like the honey bee can be tested by a process of training. 

 Small glass dishes, one of which contains sugar and water and 

 the others water only, are exposed on small squares of 

 coloured paper. Foraging worker bees can be trained to 

 associate the dish containing sugar with a particular colour. 

 If the dish placed on the colour used for training is replaced by 

 one containing water only, the trained bees continue to visit 

 it for some time, though they cease to feed from it. The 

 postions of the various squares on the table can be frequently 

 altered so that it is clear that the bees are not learning merely 



b 17 



