Wing of a dragonfly of the family Gomphidae, showing the pterostigma

The pterostigma is a cell in the outer wing of insects which is often thickened or coloured and so stands out from other cells. It is particularly noticeable in dragonflies, but present also in other insect groups, such as snakeflies, hymenopterans and megalopterans. The purpose of the pterostigma, being a heavier section of the wing in comparison to nearby sections, is to assist in gliding. Without the pterostigma, self-exciting vibrations would set in on the wing after a certain critical speed, making gliding impossible. Tests by R. Åke Norberg, Department of Zoology, University of Göteborg, Göteborg, Sweden, show that with the pterostigma, the critical gliding speed is increased 10-25% on one species of dragonfly.

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