Downy Emerald
Cordulia aenea (Linnaeus, 1758)

Family: Corduliidae - Emeralds

Order: Odonata (Epiprocta) - Dragonflies

Class: Insecta

Phylum: Arthropoda

Kingdom: Animalia

Red List status: Least Concern

Total length: 47-55 mm

Abdomen length: 30-39 mm

Hindwing length: 29-35 mm

Larval body length: 22-25 mm

 

DESCRIPTION

Adult: A moderately-sized, robust, typical member of its family. Males have a constricted 'waist' which gives them a strongly club-tailed appearance; the seventh and eighth sections of the abdomen are the thickest. The thorax is metallic green and distinctly hairy, accounting for the common name. Females have a compact, robust abdomen without a slender waist. In both sexes the wings are clear, with a yellow patch at the base of each that becomes more pronounced with age. In mature specimens the eyes are bright green; in newly-emerged animals they are light yellow-brown. The abdomen is metallic, appearing dark bronze in males and the same green as the thorax in females. The upper surface is unpatterned; however there is a large yellow or whitish marking on the underside of each

abdominal segment, extending onto the sides. The segments closest to the thorax have larger markings. The front of the head

is dark green, without yellow markings.

 

Downy emerald emerging.

Teneral male

View of the same teneral specimen showing the distinctive thorax.

Larva: The larva is fairly small with a broadly oval shape, bulging eyes and long hind legs, which extend well beyond the tip of the abdomen; this form is typical of larvae in this family. Viewed head-on, a row of serrated 'teeth' runs down the centre of the face below the eyes (actually the inner margin of the labial palps, part of the feeding apparatus). There is a distinct row of dorsal spines along the centre of the abdomen,

                                      extending from the fourth to ninth segments; that on segment 9 is reduced and may be absent. The

legs are banded in cream and black. Antennae are very long and the sides of the thorax are patterned with two dark diagonal stripes. Important: These latter two features are not always visible in exuvia, as antennae readily break off and patterns fade with age.

Larval exuvium

Similar species: The genus Cordulia has only two species; the American Emerald (C. shurtleffii) is restricted to the Americas, so cannot be confused with the similar C. aenea in the field. Within Europe, the species can be distinguished from members of the genus Somatochlora by the absence of yellow markings on the face, and by the more strongly club-shaped tail of the adult male. Species of Oxygastra and Macromia have yellow-spots along the dorsal surface of the abdomen. No other European dragonflies have both green eyes and a hairy, metallic green thorax. Female Somatochlora have a spine projecting below the penultimate abdominal segment (the vulvar scale); the vulvar scale does not project below the abdomen in the downy emerald. The anal appendages of the male (protruding from the final abdominal segment) are short and differ structurally from those of other emerald genera.

Larvae superficially resemble those of darters and chasers (Libellulidae); in these species the labial palps are often straight-edged or less strongly serrated, the abdomen is generally broader, many lack dorsal spines, and the hind legs are shorter, with the final joint extending no further than the tip of the abdomen when outstretched (this joint is beyond the tip of the abdomen in corduliid larvae). This latter feature reliably distinguishes the downy emerald from the similarly-sized broad-bodied chaser. The brilliant emerald (Somatochlora metallica) lacks dark stripes on the thorax, has larger dorsal spines, and has a prominent dorsal spine on the ninth (penultimate) abdominal segment. The northern emerald (S. arctica) altogether lacks dorsal spines.

Facial details of an exuvium. Note the serrated palps and long antennae.

 

 

HABITAT

Downy emeralds are most closely associated with woodlands, where they occur in standing or occasionally slow-moving waterbodies ranging from small ponds to lakes; in particular, mature deciduous woodland is favoured, and the species prefers to use sunlit waterbodies with dense emergent vegetation. Populations may also occur in heathland and bogs, and in southern Europe in alpine lakes. The animal's disjunct distribution in some heavily-developed areas of its range, such as Britain, may reflect historical fragmentation and pollution of these habitats. However, populations may also occur in heath and marshland.

Reproductive habitat: Downy emeralds favour pond margins exposed to the sun, and the long larval development period     

                                            requires a pond with permanent standing water. Areas with a leaf litter substrate and emergent vegetation are likewise necessary 

for larval development. Larvae appear able to tolerate waters with pH ranging from 4.5-7.5, and so are sensitive to pollution that increases or reduces acidity too far. The abundance of larvae is positively correlated with the presence of fish (Corbet & Brooks, 2008), i.e. ponds containing fish promote the persistence of this species.

Example of a downy emerald emergence site.

 

BEHAVIOUR

Adults often disperse into woodland to forage following emergence, and will feed on the day they emerge. Mature males will maintain and patrol territories along the water's edge, typically in unshaded spots, although they rarely cross open water. As a result, large numbers of males can hold territories in quite small ponds, each patrolling a break in the shoreline vegetation. The size of each territory declines with increasing numbers of males. Individual territorial patrols generally last for less than half an hour before animals leave the water to forage, and multiple males may defend the same territory one after another on a 'time-share' basis; returning foragers will reclaim the same territory. Territorial patrols serve primarily to locate females, and encounters with other males during patrols are non-aggressive, with the 'intruder' retreating if it encounters another male. Females likewise return to water only to breed.

Unlike many dragonflies, adults are often active in overcast weather. Animals rest away from water, often in the forest canopy.

Larvae are nocturnal, hunting for invertebrate prey beneath leaf litter and other debris using their well-developed sense of touch. Individual prey items can be consumed in as few as 5-6 seconds. They rest clinging to the underside of fallen leaves, and often congregate in shallow water. When threatened, they may feign death for up to 330 seconds (Corbet & Brooks, 2008).

Breeding behaviour: Males return to water to await females. They will identify a preferred rendezvous site within two minutes of arriving at the pond, which they will then patrol as a territory, sometimes displaying by raising the colourful abdomen. Females undertake a similar 'pre-mating' flight, though with the abdomen held at a different angle. Males appear unable to identify females of their own species by sight, and have been observed approaching other species as though preparing to mate (Corbet & Brooks, 2008). Animals generally refrain from foraging while searching for partners. Mating can last for up to 90 minutes, and takes place in the canopy after the male has grasped the female and carried her to a suitable location. Females then oviposit alone, often at dawn or dusk when few males are present over the waterbody, depositing eggs into submerged or emergent vegetation in fairly open, shallow areas of the pond edge.

Emergence: Larvae typically emerge onto rushes and other emergent vegetation; points where two stems cross are particularly favoured, but the insects may occasionally even use the bodies of previously-emerged larvae, behaviour which is fatal to the 'support'. Animals may emerge near the water's edge, or may leave the water altogether and climb into adjacent woodland vegetation as far as several metres away to complete emergence. When a suitable perch is located, larvae will 'test' the immediate area by extending their limbs, behaviour which establishes whether the site is free of obstructions that may hinder wing expansion. If disturbed before the adult stage emerges, animals locate another support. Emergence is typically synchronised, with many animals completing development simultaneously.

Flight season: April to July, extending into early August. This is the earliest member of its family to emerge in Europe. Most abundant in May and June, later in the farthest north parts of its range.

Newly-emerged downy emerald

 

BIOLOGY

Life cycle: The larval period lasts two to three years. Very few larvae survive to develop into adults, a figure that may be as low as 0.2% (Corbet & Brooks, 2008). Newly-emerged adults mature over 9 (males) to 16 (females) days (Corbet & Brooks, 2008), when they return to water to breed.

 

STATUS

 

The predominantly northern (and, in southern areas, alpine) distribution of this species in Europe may make the downy emerald vulnerable to the effects of global warming, as it will have little opportunity to migrate further north if conditions become unfavourable in its present range. Populations may also be susceptible to land clearance in the surrounding terrestrial habitat, to water pollution and to the loss of woodland ponds. The localised nature of C. aenea populations in parts of Europe, such as the UK, may render them particularly vulnerable to thise forms of disturbance. In Britain, the downy emerald is a scarce species and its presence is listed as a criterion for designating Sites of Special Scientific Interest (SSSIs). Nevertheless, habitat loss appears to be limited across the species' global range, and populations are able to persist in man-made habitats (Claustnitzer, 2007)

 

 

TAXONOMY

 

Two subspecies of the downy emerald are recognised; the widespread C. a. aenea and the east Asian C. a. amurensis, which replaces the typical form in Japan.

 

REFERENCES

 

Asia Dragonfly

British Dragonfly Society (2004) Draft Management Fact File: The Downy Emerald (Cordulia aenea) (L.)

Cham, S. 2007 Field Guide to the larvae and exuviae of British Dragonflies Volume 1: Dragonflies (Anisopera) The British Dragonfly Society: 76 pp.

Clausnitzer, V. 2007. Cordulia aenea. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.1

Corbet, P. and Brooks, S. 2008 Dragonflies New Naturalist Series 106, Harper-Collins, London: 456pp

Dijkstra, K-D.B. and Lewington, R. 2006 Field Guide to the Dragonflies of Britain and Europe British Wildlife Publishing: 320 pp

Newly-emerged (teneral) downy emerald male, with the larval exuvium below. Epping Forest, Essex, United Kingdom.

RANGE

Throughout northern Europe and Asia, east to Siberia and Japan Through much of this range it occurs continuously; however, isolated, scattered populations occur in southern Europe, in the UK and Scandinavia, and at the western edge of the species' range in southwest Ireland and southern France. The range extends south to Algeria and the Caucasus, but the species is absent from the Iberian Peninsula.

Range            Description            Habitat            Behaviour            Biology            Status            Taxonomy            References

Cordulia aenea emerging. This individual appears to have fallen prey to a spider in the process of metamorphosis.