Southern Damselfly
Coenagrion mercuriale
The Southern Damselfly is a small, delicate blue damselfly found mainly across southern England and Wales.
- Southern Damselfly at Cothill Fen, Oxfordshire. Photo copyright Alan S.L. Leung.
Description/identification
A small, delicate blue damselfly, about 29–31 mm long, usually flying low and weakly over its breeding channel. Males have a distinctive dark marking on the second abdominal segment often likened to Mercury’s helmet. They also show narrow blue stripes on the thorax and a fine coloured line between the two eye spots. Females are darker, with green or blue markings. The aquatic larvae are small (about 11–14 mm), with three short, unpatterned tail gills.
Habitat
In Britain, breeding depends on shallow, slow-flowing, base-rich water in open, sunny places. Most colonies occupy spring-fed runnels, flushes and small streams through wet heath or mire, while others use vegetated side channels of chalk rivers and managed water-meadow ditches. Fen channels can also support the species, including at Dry Sandford Pit in Oxfordshire.
Life Cycle
Adults usually fly from June to August and the best time to see or count them is June and July in warm, dry, relatively still weather. After mating, females insert eggs into submerged plant tissue. Larvae hunt small aquatic invertebrates within vegetation and shallow channel sediments, overwinter in the water, and take around two years to mature. They then climb emergent plants to shed their larval skin and emerge as adults. Timing varies with site conditions and weather.
Distribution and threats
The British range is local and fragmented, found mainly across southern England and Wales such as in the New Forest and the Preseli area of Pembrokeshire, with some smaller colonies in Devon, Anglesey and Oxfordshire.
The main pressures are the loss of lightly grazed, open channels to dense vegetation and scrub, altered flow from drainage or water abstraction, prolonged drying, and nutrient or other pollution. Channel deepening and heavy clearance can remove aquatic vegetation and larvae. Small, isolated colonies have limited opportunities to recolonise after local losses.