SEEING IN SECTIONS: THE INSECTS’ MANY-SIDED EYE

THE eyes of insects are usually very prominent, and, if looked at under a lens, are seen to be divided into numerous six-sided areas or facets. Each facet is supposed to focus on its visual cells a portion, say of a flower, in such a way that the flower is broken up into as many parts as there are facets and the separate images so made combine to form the whole. It is as though the pieces of a jig-saw puzzle were uniform, each piece corresponding to the image of a facet and fitted together to give the complete picture. As a result of the minute pictures supplied by the cells of each facet, extremely keen vision obtains, and the slightest movements in the insect’s environment are detected. Every novice in the art of insect collecting knows this only too well.

The larger the facets the less detailed will be the combined image. In some insects, the eye is built up of large facets dorsally-placed (that is, on top), which probably indicate differences in illumination and movement, and small ventral ones which give detailed vision—a shadow falling across an insect sitting on a leaf will be registered by the dorsal or upper half

of the eye with its large facets, while the insect perceives in detail the surface of the leaf with the smaller faceted lower half of the eye. An eye composed of numerous facets is known as a compound eye in contrast to another eye structure in insects called the simple eye or ocellus, which consists of a single undivided area of modified skin or cuticle forming a lens which can register a simple picture; this type of eye is found in insect larvae where the compound eye is usually absent. Both types of eye occur in the adult where the ocelli are borne dorsally, but in the larva they are found at the sides of the head.

The whole question of vision is a complicated one. Before leaving the subject, it should be pointed out that clarity of vision not only infers an efficient eye but a well-developed nervous system as well.

It is often assumed that flowers have evolved attractive colours for purposes of pollination by insects, from which it would follow that insects themselves possess an aesthetic appreciation of colour. Such an assumption is extremely doubtful, and it is much more probable that a flower which stands out from its fellows by contrast will be the one which attracts the insects—brightness is probably more important than shade of colour. Reds and purples attract some butterflies more than whites. Experiments in this connection have been performed on bees, and one series, designed by C. H. Turner, may be described.

A number of coloured discs fixed to sticks were set up amongst flowers which were constantly visited by honey bees; the bees still visited the flowers, in spite of the huge amount of honey with which the discs were covered. But if the bees discovered the honey on a coloured disc, they would still visit the same colour even when the honey had been removed, and this despite the fact that a honey-laden disc of another colour was in the vicinity. Such experiments show that neither scent nor colour alone are sufficient to guide bees to a food supply. Turner concluded that both vision and scent are necessary to guide the bees to a source of honey.

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