Pterostigma

 

The dragonfly's wing spots

One of every dragonfly's characteristics is a pigmented spot on the leading edge of each wing. It is not simply a decorative feature.

A pterostigma

The biologist's name for it is a pterostigma, meaning 'wing spot.' It is proportionally heavier than other areas of the wing and acts as a counterweight in flight. 

Northern Spreadwing

If not for this, the outer leading edge of the dragonfly's wings would be prone to flapping and feathering vibrations known as flutter, which would rapidly stall its flight when gliding at high speed. Such resonance is known to endanger far larger structures, such as aircraft, buildings, and bridges. 

Fragile Forktail

In the case of dragonflies a small spot that accounts for less than 0.1 per cent of its total body weight allows it to safely reach a flight speed 10 to 25 % higher than it would otherwise.

Wing corrugations

A dragonfly's wings are not perfectly flat plates. Their veins form three-dimensional corrugations that make the wing's performance more efficient and prevent them from warping and deforming as a result of resonance.

Pterostigma on a parasitoid wasp

The pterostigma is common among the insect orders as well, although not usually so easily seen. By passive, inertial, pitch angle control, the pterostigma probably makes the wing beat more efficient in slow and hovering flight of small insects, while its raising of the critical flight speeds probably is of more importance to larger insects.

Calico Pennant

The evolved functional details of insect anatomy are obviously of great interest to aeronautical engineers. 

 

Source: R. Ake Norberg, "The Pterostigma of Insect Wings, An Inertial Regulator of Wing Pitch," Journal of Comparative Physiology, vol. 81, March 1972.

Insects are photographed at Halibut Point.