One of the more common dragonflies of heathland ponds and bogs is the splendid four-spotted chaser (Libellula quadrimaculata). It gets its name from the four black spots, one on each of the four wings. It is easy to mistake this insect for the female broad-bodied chaser so spotting those markings on the wings can be important.
In many dragonflies the males and females are very different but in the four-spotted chaser this is not so, they are very similar. This is the male as you can clearly see the 'claspers' which is uses to hold on to the female while mating. The males usually have a preferred perch from where they launch off to deter any intruding male, or mate with any female, that might enter their territory. This is the species you may have seen some time ago on Springwatch where Simon King sat by a pool at Arne holding a twig and the dragonfly came and sat on it!
Flying from late May until August the wet areas of the Purbeck heaths are as good a place as anywhere to see them but they can also be found on other still water ponds and lakes that have a degree of acidity and are fairly shallow and with extensive vegetation.