Guiding BeeThe Evolution of Flight


The most accepted theory of the origin of wings suggests that they grew out of a gill-like apparatus present in the very earliest insects. Some of these gills may have grown over time, after they were supplanted in the adults by tracheae, to form little flaps. Initially, these proto-wings would have been useful for little more than jumping, perhaps adding a little to the distance over which an insect could leap. Gradually these wings would have grown larger, until they could be used for controlled diving, gliding, and then even flapping flight. The Megasecoptera, a very old order related to the Odonata, the wing span reached as much as one metre, could flap its wings and even soar (the most advanced flight mode, practised by the birds and insects with the largest wing spans, involving extended periods of gliding and requiring great control over the wings and only occasional flapping). Most modern insects have functional wings as adults, and every order has at least some winged species, so that the principal criterion for distinguishing insects at all hierarchical levels is the wing morphology.

Compound Eyes

One of the very earliest identifying features of insects is the compound eye, found only among the insects, the centipedes, the crustaceans, and the horseshoe crabs. The compound eye is composed a large number (generally a few hundred to thousands) of facets, each of which faces a slightly different direction than its neighbours. Each facet records a general impression of the colour and intensity of the light which comes from the direction in which it faces, but does not produce a complete image. Every facet is bound by is own optic nerve to the insect's brain and contributes one spot of light to the image, much as the pixels (picture elements) in the monitor in front of you are doing now or tiles do in a mosaic. The quality of the picture which an insect sees is determined by the resolution (number of facets) of its eyes, as is the case with a computer monitor. Shapes are much more clearly defined on a 1024x768-pixel monitor than are equal size shapes on a 320x240-pixel monitor. Most insects can see fairly well to a few feet, but not much beyond that. The farther away that an object is, the fewer facets that it covers and the poorer the resolution of the object. The compound eye system does not require and cannot incorporate a mechanism for focusing. The clarity or fuzziness of the image is determined by the number of facets, which is fixed, in the eye and by the insects distance from the object. The sequence of bee head below show, from left to right, the way a human sees a bee's head, the way a bee might see the same head from a short distance away, and the way a bee might see it from farther away.

There is, however, a great advantage to the compound eye system. Image processing is so much more efficient than is the case with, for example, a human's eyes, that the compound eye offers a much greater flicker fusion rate. This means that an insect can assimilate changes in what it's seeing many times more quickly than we can. A prestidigitator's hand-tricks are transparent to insects, because the hand is not faster than the compound eye, and a Hollywood movie would look to an insect like a series of still photographs. The advantage for insects is that compound eyes allow them to fly at high speeds through dense woods and marshes without hitting anything and to chase other quick-flying insects. They are also excellent for detecting motion, essentially just recording slight changes in the image over short intervals. A side-effect of this, however, is that an insect cannot spot a mate from far away. It detects something moving, flies over hoping to find a mate, sees when it's close enough that it has found an enemy, and has to fly as fast as possible away.

The Evolution of Flight...1, March 1996