Just skimming the surface: dragonflies of the genus Orthetrum

Following my post about the Hornet Robberfly, I thought I'd focus on one of the groups that Hatchet Pond is justly famous for - dragonflies and damselflies (Odonata). The site consists of a large central pond surrounded by grassland, scrub and heathland, plus a series of much smaller ponds scattered around the area. Although there was an unseasonally stiff, cool breeze, the scrub does provide some shelter and a range of species were seen, especially in the afternoon when the sun came out and the breeze subsided a little (it's true, we British do talk about the weather a lot...). Anyhow, to the insects - here I will focus on the genus Orthetrum (skimmers), starting with one individual that was particularly obliging - a male Black-tailed Skimmer Orthetrum cancellatum.

O. cancellatum - a male basking

This male clearly had a favourite basking spot - a small curved indent in the pond edge which presumably formed an effective heat-trap. The green eyes, brownish thorax and blue abdomen with a black tip are unmistakeable even though the yellow spots on the abdomen of mature males have not developed in this specimen. Aside from being an impressive beast (the abdomen here is about 30-35mm long), it's a good indication of how important habitat (and micro-habitat) structural diversity is when trying to ensure species diversity. Males of this species (as is often the case with Odonata) are highly territorial and patrol regularly, sometimes occupying 50m or more of bank, while females spend most of their time feeding away from water. They hunt from perches and often prefer larger prey such as butterflies, grasshoppers and damselflies - they are able to swivel their head to fix on targets visually, and often return to the same perch to feed.

A male O. coerulescens

Superficially similar, but smaller with darker eyes and thorax, and less black at the tip of the abdomen, the Keeled Skimmer Orthetrum coerulescens maintains a smaller territory - around 5m diameter - and there may be about 15 territories along a 100m stretch of bank.

In both species, females entering a male's territory may be quickly seized - in O. cancellatum, mating usually takes no more than 30 seconds (and may occur without landing), while in O. coerulescens, the process (involving a mating wheel) may take anywhere from a few minutes to over an hour. In both species, egg-laying (oviposition) behaviour depends to some extent on the density of males and the resulting levels of 'harassment' the female may experience (females can be seen waiting for an opportunity to lay eggs unmolested!).

O. cancellatum is found throughout much of the southern half of England and has extended its range northwards in recent decades (at least as far as Durham) following creation of wetlands from flooded mineral and peat extraction sites. O. coerulescens has a more south-westerly distribution being most widespread in Ireland, west Wales and Cornwall/Devon, but is found at a scattered range of sites elsewhere in the country and can colonise new sites quickly - a useful ability given the losses of some areas of its wet heathland habitat due to development and peat extraction.

So, only a brief taster of the species found at Hatchet Pond - more to come, including patrolling Emperors and a few rather more uncommon species...


References

Brooks, S. (2004). Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland (revised edition). BWP, Gillingham.

Smallshire, D. & Swash, A. (2004). Britain's Dragonflies. WILDGuides, Old Basing.