Of the five senses, sight is to mankind undoubtedly the most precious. The changes of the seasons, the beauty of scenery, sunset and sunrise, the wonders of nature, and the triumphs of art are only to be appreciated through the eyes, which have aptly been described as the 'windows of the soul.' Yet there are many who pass through life without even realising what we may call the 'gilding' of the world—the delights of colour. Quite a large number of people have no colour-sense, and are unable to tell red, for instance, from green. The writer knows an eminent botanist who is unable to tell the colours of the flowers he so loves to study!

How is it with the little people of the insect world in this matter? Their eyes are constructed on an entirely different plan from ours. What sort of a world is it that they look on? Taken as a whole, it would seem that the insect inhabitants of our world see but very little of it; they perceive it rather through the sense of smell. Only a very few insects, such as dragon-flies, for example, see well, and even their length of sight probably does not exceed six feet or so. They are a near-sighted race. Moreover, they see moving objects more easily than stationary ones.

That many recognise colours there can be no doubt, and many show preferences for certain colours. Bees show a great liking for blue, and ants for violet. White butterflies appear to prefer white flowers, and yellow butterflies yellow flowers. Orange and yellow are also attractive to bees, whilst other colours seem to have no charms for them.

There is no doubt that some insects, however, see much more of the world than others, for the eyes of the insects and their near relations, the spiders and scorpions, are of two different kinds, and both kinds differ greatly from ours in structure. Let us take the simple eye found in the spider or scorpion, for an example, and look at it. If you catch a spider, and carefully examine the front of his head, you will notice a number of bead-like bodies of different sizes, arranged sometimes in the form of a circle, sometimes on a prominent swelling or 'tubercle,' or it may be in some other fashion, according to the kind of spider. These are the eyes. A section cut through one of these eyes and placed under the microscope would show that the surface of the eye was formed by a transparent body like a lens, and that behind this lay a complicated arrangement of rods passing gradually into the nerves of sight. Only ocelli, as these eyes are called, are found in the spider and his kind. But in true insects, like the dragon-fly, or the butterfly, we meet with eyes of another kind, in addition to ocelli. These are known as compound eyes. Where compound eyes are found, the ocelli never exceed three in number, and are arranged in the form of a triangle, and placed in the middle of the head (figs. 2, 3, and 4).

The compound eyes vary greatly in their size. In some insects they are placed one in each side of the head (fig. 1); in others, as in the drone bee, they meet one another at the top of the head (fig. 3, spot marked O) and extend downwards to the mouth. In others, yet again, they may attain a huge size, and occupy even the whole front of the head, crowding over the ocelli to form a little group at the top, as in the head of a species of fly known as the Bibio (fig. 4).

The compound eye is so delicate and wonderful, that great knowledge of anatomy or the science of optics is necessary before it can be really appreciated. Briefly, it is made up of a cluster of simple eyes, in each of which there are several parts. Beginning at the surface we have what is known as the facet, or cornea, which roughly corresponds to the surface of our own eyes. Next we meet with a clear, glassy rod, and this passes downwards into the nerve of sight. Around these rods is a sheath of black colouring matter, so that each eye is cut off from its neighbour. Thus the whole eye may be likened roughly to a bundle of telescopes.

Of what use, it may be asked, are the three little eyes in the middle of the head of insects which have these wonderfully complex eyes? Well, the large compound eyes are used to watch the movements of other animals; thus they are enabled to escape their enemies. Many of you doubtless have tried to catch butterflies, and if so you will know how suddenly and quickly they avoid the master-stroke that is to land them in the net. But the use of the three little eyes seems to be to enable their possessor to see in the dark. By their means the bee (figs. 2 and 3) can distinguish objects even in the darkest parts of the hive; so too the ant can find his way about the galleries of his underground home. Night-flying moths all have these little eyes, whilst in butterflies, which fly in the daytime, they are wanting.

W. P. Pycraft, A.L.S., F.Z.S.