Pterostigma

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

The pterostigma (plural: pterostigmata) 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.

File:Red veined darter pterostigma.JPG

Pterostigma of red-veined darter

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 show that with the pterostigma, the critical gliding speed is increased 10–25% on one species of dragonfly.

Some female damselflies in the family Calopterygidae possess a feature known as a pseudopterostigma. This is similar in location on the wing to a true pterostigma but is crossed by veins and is only defined by its paler colour compared to surrounding areas of the wing.