Darter or Skimmer

In May or June when teneral skimmers and darters appear people come across insects where it is difficult decide what it is.

The wings are milky, the pterostigmas pale, the abdomens are yellow and appear featureless, the eyes brown above and green below.

So is it a Keeled Skimmer or a Common Darter?

One answer, and a good one to learn as it will stand you in good stead abroad where there a lot of small libellulids, is to check the venation.

In the image below (as usual click on the image to bring up a larger version) are two dragonflies. At the top is the whole insect and below it the wing detail. Because they are teneral insects it's a bit difficult to see the veins but you can see that there appear to be more veins coming off the leading edge of the wing in insect B than in insect A.

If you scroll down more there are images taken from slightly older insects where the venation is much more evident.

The node is where there's a kink midwing.

The leading edge of the wing is called the costa

The veins running from the costa are called crossveins and those between the wing base (by the body) and the node are called antenodal crossveins which is usually abbreviated to Ax.

Skimmers (Orthetrum species) have 11-14 Ax while Darters (Sympetrum species) have 6.5-8 Ax. (You get 1/2 an Ax because the crossvein runs from the costa across the subcostal vein to a third vein but some times stops at the subcostal vein.

You will see in the bottom image that the Common Darter has 6.5 Ax and the Keeled Skimmer has 12 Ax.

In the middle images (of the wing detail) I can make out that insect B has 12 or 13 making it a Skimmer (no dark wing-bases so not a chaser). As we only have two Skimmers in the UK and the other - Black-tailed Skimmer - has small pterostigmas and a different thorax pattern then it's a Keeled Skimmer.

It's not easy to count the veins for insect A but it looks like 6 or 7 making it a darter and the striped legs make it a Common Darter.

I would suggest having a look at page 32 and surrounding in Dijkstra to learn a bit about venation. It can be very useful when confronted by a dragonfly that doesn't look like one in the field guides (recently emerged insects or insects which are between the stages illustrated in the field guides).