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Description
Status and distribution
European and
world distribution
Oxygastra curtisii (Dale) Orange-spotted emerald
This corduliid dragonfly is believed to be extinct in Britain. It can be distinguished
from the three resident corduliids, namely Somatochlora metallica, S. arctica and
Cordulia aenea, by the slender abdomen, on the mid-dorsal surface of which is a
series of elongated yellow markings usually on all segments except 8 and 9. Some
individuals, usually females, have much reduced markings with only a few spots on
the anterior segments (Chelmick 1983). The general body coloration is metallic
green with bronze reflections. In common with other corduliids, 0. curtisil can appear
quite dark in flight, the yellow spots being inconspicuous. Other distingt.iishing
features are the pattern of yellow facial markings and the shape of the anal
appendages. There is a small amber area at the base of each wing which, in
immature individuals, may suffuse the entire wing with a pale tint. The eyes are
greenish.
Habitat On the continent, 0. curtisil breeds on moderate- to slow-flowing tree-lined rivers and
canals with little or no emergent vegetation, and with a silt or mud bottom in which
the larvae live. The only known breeding sites in Britain were such riverine habitat.
Breeding biology Observations on the ecology and distribution of 0. curtisil have been made by Fraser
(1940), Chelmick (1983) and Moore (1991a). The males are aggressive and fly low
over the water chasing off other males and searching for females. Territories appear
to be ill-defined, though, on the shadier parts of rivers, males may vigorously defend
small sunlit areas against other males. Oviposition takes place close to the river bank
amongst the tangle of tree roots where the female dips the tip of her abdomen into
the water so releasing her eggs. The larvae live in the bottom silt and mud, and
probably take two or more years to develop. Emergence occurs on exposed
bankside tree roots, and on the trunks and branches of overhanging trees at heights
of up to 3 m, much higher than those attained by the three resident corduliids
(Chelmick 1983).
Flight periods In Britain, the recorded flying times were from mid-June to the end of July. Whilst
hawking for insects, 0, curtisil will often circle quite high up in a clearing or woodland
glade, disappearing as soon as the sun goes in. Other species which occurred on
the same river as 0. curtisil were Platycnemis penmpes, Calopteryx splendens,
Libellula fulva and Gomphus vulgatissimus.
0. curtisii has the distinction of having been first discovered and described in Britain.
It was found at Parley Heath near Hurn in Hampshire (now Dorset) on 29 June 1820 by
J C Dale. It continued to be recorded intermittently on the River Stour and the Moors
River between Pokesdown and Parley Heath for many years. By the 1950s, however,
suitable habitat on the West Moors River, its sole location, had been much reduced by
excessive shading of the river bank by trees (Moore 1991a). In July 1957, Moore
estimated that suitable habitat along the 1.2 km of river that he visited (which
represented about a third of the total available habitat) was sufficient for only about six
male territories. Such changes must have contributed to its decline, and
over-collecting cannot have helped either, but the cause of extinction is believed to
be the accidental pollution caused by a large sewage works constructed upstream to
serve a new housing estate. The last record of 0. curtisii in Britain was 19 July 1963
when a male was captured by B P Moore at Hurn (specimen in the Natural History
-Museum, London). In July 1946, one female and two males were picked up from
vegetation in a marshy area by the River Tamar, north of Gunnislake Bridge, Devon,
by 0 G Watkins (pers. comm.), and one was despatched to Cynthia Longfield at the
Natural History Museum, London. Despite searches at the time, and in the 1970s by
R Merritt, N W Moore and H P K Robinson, no further individuals could be located.
Nevertheless, it could have been overlooked. There is an unauthenticated record of
0. curtisii from Braunton Burrows in north Devon, c 1830 (Fraser 1940).
Oxygastra is one of the few genera that is virtually confined to western Europe. It is
found most commonly in Portugal, Spain and southern France, with small outlying
populations in neighbouring countries.
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