Thanks to Barbara for the initial sighting of the dragonfly at rest on a mop-head drying in the sun. The first image clearly shows the brown coloured pterostigma in each of the outer wings of the dragonfly. These can also be seen in other insect wings. The pterostigma is a thickened, heavier cell whose purpose is to assist the dragonfly when gliding. Apparently, without the pterostigma, self-exciting vibrations would set up within the wing after a certain critical speed and this would make gliding impossible.