Thursday, 4 August 2011

The Quest for Common Darter

Common Darters will be on the wing through late summer and early autumn, some may even be seen as late as November, so no hurry to record them officially, but knowing they were out at Far Pasture now I wanted to get them photographed and filed.
Far Pasture has always been (in my opinion) the best site for this species. Once they appear they come in good numbers and are easy to photograph as they perch openly on the access road, the fences and any tall vegetation, and can also be quite approachable for excellent close-ups at different angles.
The only confusion species here is Ruddy Darter and there seems to quite a few of those around at the moment too, but once you have your eye in they aren‘t so difficult to tell apart, and if you‘re not sure, do like I do and take a photo.
On my visit still only small numbers present, I counted five males and two females (Common that is) and also three male Ruddy Darters.
But to show how similar the species can be, here are two males taken from a similar angle, one Common and one immature Ruddy. Can you tell which is which?

They are both an orangey red colour, the abdomen appears to bulge at the tip of both, and the thorax appears to show yellow stripes on both specimens too.
One thing I have noticed over the years is that in profile, the abdomen of the ruddy seems to curve down at the tip and that of the common curves up, but the biggest clue here is the colour of the legs. Ruddy Darter legs are uniform black, a diagnostic trait of this species which tells them apart from other red darters, the Common Darter has a pale (sometimes yellow) stripe running the length of the leg, not always easy to tell but on a half decent photo like this a dead giveaway. Therefore in this photo the top specimen is the immature Ruddy and the bottom is the Common Darter.

To be honest, all would be a lot simpler with a change of viewpoint. Below we see the same Ruddy Darter from above, the abdomen is nipped in and bulges at the tip.

The Common Darter from this angle shows the abdomen has more or less parallel sides, slightly tapering at the tip.

And once you get accustomed to both species, even in flight they are distinguishable as the Ruddy Darter is a smaller insect and the abdomen appears a lot shorter in comparison, also in more mature individuals the deeper red colouring of the ruddy is another factor, the Common Darter in the composite photo won‘t get much darker than it is now but look at these three Ruddy Darter individuals, all in different stages of maturity, showing how the colour evolves from orangey, to pink tinged and finally the deep red of the mature male. The mature Ruddy Darter will also lose the yellowish striped appearance on the thorax to take away another confusion issue.

Now a similar comparison between the females :

The abdomen are generally a similar yellow/brown in colour, though the Common Darter female pictured here (top) is a more mature individual and shows an orange/red tinge, and like the male, the Ruddy female looks shorter and stubbier. Study the black markings along the abdomen, on the Common they run above the mid-line, but on the Ruddy the black line runs along nearer the underside. And again, the leg colouring is the telling factor, with the pale striped legs of the female Common being more conspicuous than even in the male.

Below is a photo of an ovipositing pair of Common Darters (taken at Kibblesworth last weekend by Steven Fryer) which shows well the distinction between the sexes. Most of the ID points we’ve already covered, but note the abdomen of the female (behind) is a lot thicker than that of the male.

Species Recorded : Common Darter Sympetrum striolatum

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