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Status and distribution
Orthetrum coerulescens (Fabricius) Keeled skimmer
Description G coerulescens is the smallest of the four libellulids in which mature males
European and world
distribution
possess a blue pruinescence on the abdomen. The lack of a dark tip to the
abdomen distinguishes 0. coerulescens from 0. cancellatum and Libellula fulva.
The lack of dark basal patches on the clear wings separates 0. coerulescens
from male and female L. fulva and L. depressa (this latter species also having a
very much broader abdomen). The pterostigrna is pale yellow-brown in both
sexes, unlike the other large libellulids in which they are dark brown. In
immature males and in females, the abdomen is yellowish brown with a thin
longitudinal line running mid-dorsally along it, and has tiny transverse bars
from segments 3-7. This line, and the pair of pale creamy stripes on the dorsal
surface of the dark brown thorax distinguish 0. coerulescens from females of
Sympetrum stnblatum which are similar in size and coloration. As males
mature, a blue pruinescence develops over the entire abdomen. As with other
blue libellulids, this may be rubbed off in places by the female during
copulation, leaving dark marks dorsally The base of the wings of females is
suffused with saffron coloration which may extend faintly to the whole wing.
Habitat 0. coerulescens breeds in flushes, the boggy margins of runnels and streams,
and bog pools in valley mires in areas of heath and moorland. They are
characterised by the presence of plants such as Sphagnum moss, bog
pondweed, bog asphodel, marsh St John's-wort and common cottongrass.
Breeding biology 0. coerulescens is territorial. Where territories lie along rivulets, highest
steady density is 9 males per 100 m of watercourse. Males patrol their
territories by flying low over the bog surface, often in an erratic manner. They
also spend a lot of time perched on low vegetation, rocks or on the ground.
Males and females will sometimes fly in tandem prior to copulation, the
duration of which varies greatly and may take as long as 25 minutes. Usually
the female rests for some time before ovipositing (Miller & Miller 1989), which
she performs by dipping the tip of her abdomen in the water whilst in flight,
and with the male in close attendance guarding her from other males. Larvae
live in peaty detritus, or muddy silt of runnels, and probably take two years to
develop.
Flight periods 0. coerulescens is on the wing from early June to early September. It can be
seen with other species, depending on their geographical range, such as
Aeshna juncea, Cordulegaster Coenagrion mercuriale, Pyrrhosoma
nymphula and Ceriagrion tenellum.
0. coerulescens is restricted by its habitat requirements to lowland heathland
in southern Britain and the moorlands of western districts from Cornwall to a
few isolated locations in west Scotland, including several islands of the Inner
Hebrides. In eastern England, where it is found very rarely, its few remaining
sites are very small but have survived for many years (Moore 1986). It has
become extinct at several sites as a result of land reclamation and lowering of
the water table. The only site for 0. coerulescens in eastern Scotland was at
Methven Moss, near Perth, where it was recorded in 1908 with Leucorrhinia
dubia. In Ireland, 0. coerulescens is restricted mainly to peat bogs and
seepages in the Mourne Mountains, the Wicklow Mountains, and the western
moorlands of Co Kerry, Co Mayo and Connemara, plus a few sites elsewhere.
There are few records from the lowland pbatlands of the midlands.
0. coerulescens is widespread in western and central Europe, including the
Iberian peninsula, thinning out towards the north in Germany and southern
Scandinavia and east towards Russia.
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