Coenagrion mercuriale (Charpentier, 1840), the Southern Damselfly (Coenagrion mercuriale), is a species found in Western Europe and Northwest Africa, and is mainly centred on the western Mediterranean. In Great Britain it reaches the north-western extreme of its range. The specific part of the scientific name, mercuriale, is because of the distinctive markings on the second segment of the abdomen that resembles the astrological symbol for the planet Mercury. Subpopulations in Italy are sometimes regarded as a distinct subspecies, Coenagrion mercuriale castellanii, but the so-called differences between the two taxa are not significant. 

The species has a body length 23-26 mm. Males are sky-blue and black in colour, with blue eyes and two small eyespots. They can be distinguished from the males of similar species by the blue 'mercury mark' on the second segment of the abdomen, but detailed examination of the anal appendages is the only reliable method. Females do not possess these anal 'accessory genitalia' but have an ovipositor, which is not always easily visible. Females are generally green or blue and slightly lighter than males in colour, becoming brown as they age. They tend to have similar markings to males on the head and thorax but have darker abdomens. In both sexes the wings are clear with small black marks towards the tips.

This species is easily confused with the other members of the genus Coenagrion and with the Common Blue Damselfly Enallagma cyathigerum. Look at S2 (on the males) for distinguishing characters. Southern Damselfly is quite small compared with other similar species and will only be found flying in full sunshine in the hottest part of the day. The females can be particularly tricky to distinguish apart.

The southern damselfly breeds mainly in heathland streams as well as chalk streams and calcareous mires. They require areas of open vegetation, mixed with slow flowing water in which to lay their eggs. Adults can be seen flying between mid-May to August, the flight is weak, and they tend to stay level with grasses and other vegetation. As with all odonata, males and females fly linked together in tandem whilst mating, forming the 'wheel position'. When female southern damselflies lay their eggs, they often remain in tandem with the male and drag him below water where the eggs are laid on submerged or emergent vegetation. The larvae, which are voracious predators, hatch soon after the eggs are laid, but development to the adult stage takes 2 years.

This species is mainly an Atlanto-Mediterranean species, found in Algeria, Austria, Belgium, France, Germany, Italy, Liechtenstein, Luxembourg, Morocco, the Netherlands, Portugal, Slovakia, Slovenia, Spain, Switzerland, Tunisia, and the United Kingdom. It is thought that 25% of the worlds population exists within the United Kingdom but it has declined by 30% since 1960 due to changes in grazing, land drainage and water abstraction. This species decreases in density and frequency in other climatic areas. The species is one of Europe's most threatened odonate species. It is on the edge of extinction in seven countries and is declining in three others including the UK. C. mercuriale is the only British resident odonate to be listed in the European Habitats Directive that requires member states to designate special areas of conservation for its protection. 

Coenagrion mercuriale is classified as Near Threatened (NT) on the IUCN Red List 2006, and Rare under the GB Red Data Book. Listed under Appendix II of the EC Habitats Directive and Annex II of the Berne Convention, and is protected (in 1998) under the Wildlife and Countryside Act of 1981 of UK and features in the UK's Biodiversity Action Plan.