The Southern Migrant Hawker (Aeshna affinis) is a dragonfly species that has recently colonised the U.K., mainly since 2010 when there seems to have been an influx into the Thameside marshes of Essex and Kent. Sucessful breeding in the U.K. was first confirmed in Essex the next year.
This year very good numbers have been recorded emerging from sites in south Essex and, since the species favours water bodies that are likely to dry out, they need to have good dispersive tendencies. It therefore wasn't a huge surprise to have one reach Hertfordshire this year (perhaps the first of several?), with a female found hunting along woodland edge and over the Brick Ponds at Hertford Heath this afternoon (08-July-2018).
They are a relatively small hawker, similar in size to the Migrant Hawker (Aeshna mixta) which they are also similar to in appearance. Key features to look for are the very plain, almost unmarked, yellow or yellow-green sides to the thorax in both sexes, the large spots on the abdomen (more large spots than on Migrant Hawker) and the fact that the spots are all blue on mature males. Due to the scarcity of this species in the U.K., away from the core breeding area near the Thames, all records will need to be considered by the national Odonata rarities committee, so photographs or very detailed descriptions would be needed if you are lucky enough to encounter one in our county.