Southern Damselfly - Coenagrion mercuriale

Adult Size

Overall length: 27 - 31mm   Hindwing length: 15 - 20mm

Distinguishing  features

The Southern Damselfly is one of the ‘blue’ damselflies of which there are 4 species in Dorset and special care must be taken with identification. They are the smallest of the group, weak flying and restricted in habitat. 

Identification - for help with terms see glossary

Mature Male

Like all the other ‘blue’ damselflies has mainly blue and black stripes running down the abdomen, the shape of the black marking is distinctly spear like. Has 2 stripes on the thorax when looked at from the side. The distinguishing feature of this species is the distinctive ‘mercury’ mark found on segment 2. This mark, however, is variable and confusion could occur with identification.
 
 

Immature males

Develop as females a pale brown base colour which gradually develops into the mature blue and have brown eyes.

Mature Female

Mature females as with other blue damselfies  have two colour forms, blue and green. They have a base colour with thick black markings on segments separated by thin stripes of base colour. Unlike other Coenagrion species, Southern damselflies have a more exaggerated flared shape to the black markings.  They have a variable ‘thistle’ like shape on segment 2, which can be difficult to distinguish. Blue females have green eyes. The green from is a dull bottle green with an almost completely black abdomen, and similar ‘thistle ‘marking. The thick black segments are separated by thin stripes which go from green to pale blue towards the last few segments.

Habitat

In the UK the Southern Damselfly is primarily a species of base-rich runnels and streams often within acid heathland areas. However, it also occurs on water meadows in the flood plains of chalk rivers, a habitat said to be more typical of many continental sites. At breeding sites the water is typically shallow and slow-flowing over a gravel bed. Not usually found with other blue damselflies.

Behaviour

Has delicate and weak flight, and rarely travel far from their breeding sites. Males can be territorial and fly over open patches of shallow water & perch on emergent low bankside vegetation. Can be easily overlooked.

Status and distribution

The Southern Damselfly is a rare species in the UK. Although it is living on the extreme north-western fringe of its European range, it is believed that up to 25% of the global population of this species occurs in the UK. Its two main strongholds are in the New Forest, Hampshire and the Preseli mountains, Pembrokeshire. Smaller colonies are found in Devon, Dorset, Anglesey, Gower, Oxfordshire and on the flood plains of the Test and Itchen rivers in Hampshire. Has a patchy distribution in Dorset as restricted to suitable sites on chalk fed heathland streams mainly in the Purbecks.



Flight period

Main flight period is mid May to Mid August, peaking in June & July.

Similar species

This species is easily confused with the other members of the genus Coenagrion, that is the Common Blue Damselfly, Azure Damselfly & Variable Damselfy. Look at Segment 2 (on the males) for distinguishing characteristic markings. 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 of the blue damselflies can be particularly tricky to distinguish apart.

More photographs

Click on the photos below to enlarge...

All photographs by kind permission & © of Ken Dolbear unless otherwise stated. All rights reserved.