British Insects: the Odonata


L. Watson and M. J. Dallwitz

Coenagrion mercuriale (Charpentier, 1840)

Agrion mercuriale.

Southern Damselfly.

Adults. Adults about 26–29–32 mm long. Average wingspan 35 mm; hindwings 15–20 mm long.

The eyes lateral and widely separated; blue. The front of the head immediately interior to the eyes pale whitish blue.

The prothorax posteriorly more or less entire. Legs black and blue (blue below). Hind tibia without a featherlike expansion. Thoracic antehumeral stripes present (humerals also conspicuous); blue (in the male), or green (in the female). The wings held vertically in repose; similar in shape and venation; petiolate; unpatterned and clear. The inner wing venation blackish. Discoidal cell a simple quadrilateral, not longitudinally divided (trapezoid, without transverse veinlets). The wings exhibiting 3 postquadrilateral cells between the quadrilateral cell and the subnodus. Antenodal veins in the forewings 2. Pterostigma more or less isodiametric (diamond shaped); pale with a dark centre (with a blackish centre surrounded by a hyaline ring and a black border). The hindwings with only one row of cells distal to the pterostigma between the costa and the radial vein.

Abdomen linear from base to tip (very slender); 22–26 mm long; predominantly blue and black (blue conspicuously patterned with black in the male), or bronze, or black (the greyish ground hidden by the blackish bronze patterning in the female); predominantly transversely banded (in the female), or complexly patterned (the second segment of the male characterised by a black figure resembling the usual sign for the planet Mercury, while that of the female has one shaped like a thistle-head. In the male, segments 3 to 6 have posterior black spots or bands, which are anteriorly prolonged into a lanceolate median line in the first two or three of these; segment 7 is entirely black, 8 is blue usually with a pair of black spots, and 9 and 10 are black edged with blue. In the female, segments 3–10 are not conspicuously patterned, save for narrow, posterior, bright blue rings on segments 7 and 8); without mid-dorsal spots. Abdominal segment 2 of the male blue, with a complex dorsal black Mercury mark. The male abdomen without auricles on segment 2; with paired inferior anal appendages. Abdominal segment 8 of the female without a ventral apical spine.

Nymphs. The nymphs elongate and slender-bodied, gradually tapering posteriorly; when mature, 15–17 mm long.

The head without prominent spots; in dorsal view markedly narrowing from immediately behind the eyes. The postocular lobes curving sharply to the back of the head from some distance behind the eyes. The antennae with the scape considerably shorter than the other segments taken together; with the first flagellar segment longer than the pedicel. The mask narrowed gradually to the hinge; without a median cleft. The prementum bearing 6–8 major setae (3+3 to 4+4, but with a field of short setae proximal to them). The body of the labial palps bearing 5 major setae. The outer margins of the labial palps armed with small spines (but the spinate setae few, and piliform). Distal margins of the labial palps crenate and coarsely toothed. The moveable hooks of the labial palps without setae. The apical combs of the tibiae mainly of trifurcated setae.

The abdomen terminating in three conspicuous caudal gills. The three caudal appendages all lamellar. The caudal lamellae oval; pointed (slightly apiculate); hyaline without coloured markings; hairy on both margins to beyond the middle; nodate, with the marginal hairs coarser proximally to the node and finer beyond it; with much-branched primary tracheal branches leaving the main trunk at an upward angle. The gizzard with 8–16 radially symmetrical folds.

Distribution. Southwest Wales, southwest England, and central southern England. Adults on the wing late may to late August (generally in best mature condition mid-June to early August).

Classification. Zygoptera; family Coenagriidae.

General comments. Hindwings generally with one row of cells between the costa and the radius distal to the pterostigma.

Illustrations. • Coenagrion mercuriale (from Lucas).

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: Dragonglies and Damselflies (Odonata). Version: 1st January 2012.’.