David Robinson explains how creatures left the land, and took to the skies

At some time during the Carboniferous period, around 300 million years ago, a series of evolutionary steps took place that produced the first flying insects. Probably it is this single development that has led to the enormous diversity of insect species that are found today.

It is appropriate to call this a single development, because the structure of the wings of all orders of flying insects appears to be derived from a common ancestral form.

Fully-developed wings are only found in adults or last larval stages. A typical wing is a thin sheet of cuticle supported and strengthened by a number of tubular veins. The space between the outer layers of the wing where they expand to form a vein carries blood.

Usually, insects (for example butterflies and dragonflies) have two pairs of wings. However, there are many modifications to this basic structure.

The true flies, like the common house fly, have only the anterior pair of wings and the posterior pair are reduced to small balance organs, the halteres. There are only two stable positions of the house fly’s wings, fully up or fully down. Through the elasticity of the thorax, the wings flip from one state to another, with any intermediate position being unstable. Thus, the flight muscles within the thorax only have to contract a small amount to flip the wings from one state to another. This mechanism is called the ‘click’ mechanism and it amplifies the changes in length of the flight muscle up to 600 times in the wing movement.

In other groups of insects the wings have changed during evolution. Grasshoppers and cockroaches have thick, leathery forewings while the hind-wings are larger and much thinner. The forewings of beetles are hardened and form protective covers (the elytra) over the more delicate hind-wings, when the insect is not flying.

Examples of insect wings:

The left forewing of a fly

The left forewings and hindwings of a grasshopper

The left elytrum of a beetle

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