Banded Demoiselles in close-up

I was given a special attachment for my camera - called a Raynox DCR250 - which allows extra close-up shots, and have been keen to try it out. When added to my zoom, I can vary the magnification to suit the image you want. You end up about 10cm from the dragonflies, so today's overcast conditions left the Banded Demoiselles on the Ise quite approachable and I was able to get these shots.

Male

Female

 

I've always been intrigued by the pseudo-pterostigma on the female's wing. This shot shows that it is simply a pigmentation, probably similar to the blue on the male's wings. But what is its purpose