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Status and distribution
European and world
distribution
Coenagrion mercuriale (Charpentier) Southern damselfly
Description The males of this small and dainty damselfly are predominantly blue and black, in
common with other species in this genus. Typically they possess a 'mercury' sign
On segment 2 of the abdomen, but this mark is subject to some variation and may
superficially resemble C. pulchellum and C. scitulum. The pattern of black
markings on the dorsal surface of the abdomen is usually quite distinctive,
however. The shape of the anal appendages is diagnostic. Females are dark with
pale olive-green or blue sides to the thorax and abdomen, extending to the dorsal
abdominal surface as thin inter-segmental rings. Females of C. mercuriale can be
distinguished from C. puella and Enallagma cyathigerum, with which they may be
found, by the markings on the sides of the thorax and on the head (Welstead &
Welstead 1983b), and from female C. pulchellum by the almost straight hind
margin of the pronotum.
Habitat C. mercunale breeds in base-rich runnels and streams, often in heathland areas
but it is not confined to these (Merritt 1983a). Mayo and Welstead (1983) discuSs
its occurrence on water-meadow ditches on the floodplains of two chalk rivers.
Colley (1983) describes its presence on a spring flush in a calcareous valley mire
on Anglesey. Winsland (1985) discusses its habitat requirements on the New
Forest heathlands. Water at breeding sites is usually shallow and slow-flowing over
a gravel or marl bed overlaid in places with organic detritus.
Breeding biology The males are not territorial. Populations are usually small but densities of 250
per 100 m of stream can occur. Jenkins (1991) reviews a population study at a site
in the New Forest. Oviposition occurs in tandem. The eggs are inserted into the
tissues of aquatic and emergent plants such as marsh St John's-wort, black
bog-rush, bog pondweed, and fool's water-cress. Larvae usually take two years to
develop. Details of larval development are given by Corbet (1957).
Flight periods C. mercunale has a relatively short flying season from early June to mid-August. It
has a slow, weak flight, low down amongst emergent vegetation. In heathland
localities, it may be seen with or near colonies of Ceriagnon tenellum, Ischnura
pumilio, and Orthetrum coerulescens. On water meadows it may be found settled
amongst lush vegetation with Calopteryx splendens, Coenagrion puella and I.
elegans.
C. mercuriale is confined to a few southern and western counties in England and
Wales. It has not been recorded from Ireland. Its strongholds are the New Forest
heathlands and Mynydd Preseli in Pembrokeshire. Elsewhere it breeds at a few
sites on the Dorset heaths, east Devon pebble-bed commons, Gower peninsula,
Anglesey, and the floodplains of the River Itchen and River Test in Hampshire. In
July 1991, a single adult male was captured at Cothill Fen, Oxfordshire (too late to
be included in the maps). C. mercunale is subject to many threats, principally the
cessation of grazing by stock animals (Evans 1989) resulting in the smaller runnels
and streams becoming completely overgrown with rank vegetation such as purple
moor-grass. Other threats include excessiVe nutrient enrichment from the runoff
of nitrogenous fertilizers from adjacent agricultural land, drainage due to
agricultural and forestry pressures and, in the case of water meadows,
over-extraction of water by water companies resulting in lowering of the water
table. C. mercuriale has become extinct in Cornwall, having last been seen at its
site at Trevorgans near St Buryan in 1957, and has been lost from several of its
former sites in Devon during the 1950s and 1960s. It has also declined in Dorset.
The species does not wander far from its breeding sites, and this apparent lack of
dispersal ability could hinder its spread to suitable habitat in neighbouring areas.
It needs rigorous protection.
C. mercuriale has a restricted range on the continent, centred on south-west
Europe and North Africa, becoming rare further north and east. It is threatened
throughout most of its range, and is the only British resident dragonfly species to
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