By James Duncan
Woods Mill Engagement Officer
When wandering around Woods Mill Lake you may well spot an iridescent flash of green and bronze. Commoner species of late summer dragonfly abound, but amongst the Darters and Hawkers may be the rare, slender metallic form of the Willow Emerald Damselfly, Chalcolestes viridis. Damselflies belong to the same order as their larger relatives, both insects in the order Odonata. The smaller, slimmer bodies and lack of overlapping eyes generally identify the damselflies, as do their folded wings at rest - except in a few cases, the Willow Emerald being one. This feature denotes its family group, Lestidae, the 'Spreadwings.' The adults specialise in eating a huge variety of small insects, utilising compound eyes packed full of thousands of individual photoreception units and superb aerial skills to leave their prey little chance of escape. The fossil record shows evidence that damselflies (zygoptera) existed at least 250 million years ago, making them one of the most successful predatory groups on the planet, found on every continent except Antarctica.
In recent years, since the millennium, warming climate has seen a number of both dragon and damselfly species making their way from the continent and starting to colonise UK shores. The Willow Emerald is just one of them. Amazingly there were only three records of this species in the country before 2009 - in 1979, 1992 and 2007. Numbers have since increased dramatically in South East England, though our climatic conditions offer quite the difference from their more typical Mediterranean habitat. This recent colonist is now pretty well established in Norfolk, Suffolk, Essex and Kent, though has spread to nearby counties over the last decade, including West Sussex, where Woods Mill is now a known breeding site.
© Simon Linington
The Willow Emerald is one of four emerald species found in the UK, typically exhibiting a late flight season from August to September/October. However, spells of decent autumn weather can see this damselfly still on the wing all the way into November. One particularly unusual characteristic of the species is its tendency to hang (literally) around in waterside trees. This strategy is different from other emeralds and makes it particularly distinctive. They favour Willow and Alder, though a variety of soft-barked species may be utilised. Particularly helpful for observation is their preference for perching upon bare, leafless branches in the sun. Part of their reason for inhabiting this environment is for reproduction - the female willow emerald will lay her eggs into carefully made incisions in the bark, a strategy unusual amongst most damselflies who lay into submerged vegetation. The egg laying (ovipositing) produces unusual 'horseshoe shaped' galls, scars in the tree bark, and the eggs will then overwinter before the larvae emerge in spring.
The presence of the willow emerald, and indeed all damselflies, typifies an ecosystem of good quality with suitable freshwater. Though different species have different requirements with regard to water depth, flow and pH, they're all vulnerable to the damaging effects of human impact. Drainage for agriculture, clearance of forests, pollution of waterways, lowering groundwater levels and general degradation of wetland areas all play their part in reducing habitat for our majestic Odonata. Please do look out for the willow emerald and report any sightings to your County Dragonfly Recorder british-dragonflies.org.uk