Blue-tailed Damselfly

Ischnura elegans (Vander Linden, 1820)

Family: Coenagrionidae - Narrow-winged Damselflies

Order: Odonata (Zygoptera) - Damselflies

Class: Insecta

Phylum: Arthropoda

Kingdom: Animalia

Red List status: Not listed

Total length: 30-34 mm

Abdomen length: 22-29 mm

Hindwing length: 14-21 mm

Larval body length: 13-15 mm

Caudal lamellae length: 5-6 mm

Other common names: Common bluetail

DESCRIPTION

Adult: Typically for Ischnura species, the male of this small damselfly is predominantly bronze above, with a yellow flush to the base of the abdomen. The head and thorax of mature males is sky-blue. The eighth abdominal segment is sky-blue, as is the base of the 'tail'. Teneral males have a yellow-brown head, thorax and abdomen tip; the head and thorax become green in immatures before developing full adult colouration. The male pterostigma is bicoloured, with a greyish upper and darker lower section. In most forms, there is a central spine on the male pronotum, best observed in profile. Most characteristically, the male's lower (externally apparent) anal appendages diverge noticeably, while the upper appendages (visible from behind) abdominal segment) have tips that run parallel to each other. The lower appendages are distinctly longer than the upper appendages.

Females are variable in colouration, and as adults may either resemble the male, or are green with black markings on the upper abdomen and a brown rather than blue abdominal segment 8. Immature specimens may have a lilac or pink thorax. Males and some females possess a distinct black stripe along the side of the thorax. Females can be distinguished by the lack of a bicoloured pterostigma, the absence of a central spine on the pronotum, and the absence of divergent anal appendages. Additionally, a short spine is present on the underside of abdominal segment 8.

Larva: A typical small damselfly nymph, most readily distinguished from similarly-sized species by the absence of spotting on the head, a dark band across the femur (upper portion of the leg), and long, narrow caudal lamellae (four times as long as wide) that taper to a point. Colouration varies from green to brown. In living larvae (not exuvia), two dark spots are present along each wing base.

Blue-tailed damselfly larva. Note the single band on the femur

 

Similar species: The distinctive patterning is likely to limit confusion with members of the family Coenagionidae, aside from other Ischnura species. The pterostigma is diamond-shaped and the body is not metallic, in contrast to Lestes species with a similar 'tail-light'. These species also rest with their wings outstrectched, in contrast to the blue-tailed damselfly. Norfolk damselfly (Coenagrion armatum) and Erythromma species have black abdomens - species of Erythromma also have distinctive red eyes, and a blue base to the abdomen. Other Ischnura species lack the central spine to the pronotum, and the base of the pronotum is small and rounded (large and square-edged in I. elegans). The scarce blue-tailed damselfly (I. pumilio) has the blue 'tail-light' on abdominal segment 9 (8 in I. elegans) and the forewing pterostigma is larger than that in the hindwing (equal size in I. elegans). No other species has parallel upper appendage tips, while the oasis bluetail (I. fountaineae) is characterised by lower and upper appendages of roughly equal size. I. elegans females never exhibit black spots on the upper abdomen (present in some other species), but females can most reliably be diagnosed by the shape of the pronotum.

Coenagrion larvae have spots on the head and broader lamellae with less distinctly tapered tips. Scarce blue-tailed damselfly is smaller, lacks a femoral band, and the setae of the abdomen are largely even (compared with longer and thicker at the sides than the underside in I. elegans), I. pumilio's highly specialised habitat requirements, relying on seepages, reliably exclude it in most cases.

 

HABITAT

Most standing water bodies from pools to lakes, commonly including garden ponds. This is a species of eutrophic habitats and may be the most common odonate at sites suffering from water pollution. In common with a number of pool-breeding species, it may also occur and breed along slow-flowing lowland rivers. The blue-tailed damselfly is tolerant of salinity and is able to colonise brackish waters, but cannot tolerate high acidity and is absent from peat bogs. The blue-tailed damselfly is resistant to poor weather, and may be the only odonate present at sites on shaded, north-facing slopes. Larvae appear able to persist in the absence of standing water by sheltering in humid cavities beneath stones (Corbet & Brooks, 2008).

Reproductive habitat: Females may potentially lay eggs in any freshwater habitat. Eggs are injected directly into aquatic plants, which may be emergent, floating or submerged.

 

BEHAVIOUR

Being unusually tolerant of poor weather conditions, blue-tailed damselflies are sometimes on the wing even in fairly strong winds. Adults thermoregulate through perch selection, preferring to move from shaded to sunlit areas and pressing their underside against warm vegetation. While this behaviour appears typical of odonates in cool climates, I. elegans is active earlier in the day than other species, at least in Britain, and maintains its body temperature at the lowest recorded level for any British odonate (as low as 12C) (Corbet & Brooks, 2008).

Larvae are territorial and defend patches of cover that can be used as perches for foraging and as shelter. Competition with other I. elegans larvae takes the form of a 'staring contest', following which the defeated animal withdraws.

Diet: All dragonflies are predacious at all stages of the life cycle, and in common with most I. elegans probably has a generalised diet made up of opportunistic captures. This species has, however, been observed preying on spiders by 'gleaning', grabbing them from their webs while avoiding becoming entangled. The larval diet is likely to be similarly diverse, but in contrast to many species intraspecific predation (i.e. cannibalism) appears to be very rare. Adult blue-tailed damselflies may sometimes capture prey at a faster rate than they can consume it; in such cases they have been known to hoard captures in a 'meatball' below their mouthparts for later ingestion.

Breeding behaviour: Males intercept females at basking or foraging sites away from water, following which they guide the female to the oviposition site in tandem. The blue-tailed damselfly has an exceptionally long copulation period, lasting up to six hours, during which the male will first attempt to dislodge the sperm of rival males before inseminating the female. However, the exact reasons for the duration of mating, the longest known in any dragonfly, are uncertain. Once mating is completed, females oviposit alone, without an accompanying male as is commonly the case in other species. Unusually, female blue-tailed damselflies aggressively defend oviposition sites from males, a behaviour unknown in the related scarce blue-tailed damselfly.

Emergence: Emergence takes place between early May and late September, with a peak in late May. Larvae emerge into vegetation at the pond/stream margin.

Flight season: Late April to late September, longer in the southern part of the range.

 

BIOLOGY

Life cycle: As far north as southern Britain, larvae complete development in a single year. More northerly populations take two years to complete development. In parts of southern Europe, two or three generations may be completed in a single year. The maturation period of newly-emerged adults appears to vary with habitat type, and ranges from 3 to 10 or more days. In contrast to many species, blue-tailed damselflies appear to remain at the emergence site throughout the maturation period.

 

TAXONOMY

Ischnura elegans belongs to a complex of very similar species. The described forms include I. genei (an island species that overlaps with I. elegans only on Elba and Giglio), I. graellsii (Iberian Peninsula and north Africa), and I. saharensis (which replaces I. elegans in the Sahara). Hybridisation is known to occur between I. elegans and I. graellsii. Southeastern forms may warrant specific status, but variation is less well-understood. The most distinctive eastern subspecies include I. e. ebneri (southern Italy to the Middle East), in which the upper anal appendages are crossed, and I. e. pontica (Hungary and the Balkans east to central Asia), in which males lack a spine on the pronotum.

 

STATUS

One of Europe's commonest and most widespread damselflies, and very abundant at nutrient-rich sites.

 

REFERENCES

Cham, S. 2009 Field Guide to the larvae and exuviae of British Dragonflies Volume 2: Damselflies (Zygoptera) The British Dragonfly Society: 76 pp.

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

Immature male blue-tailed damselfly, Buckinghamshire, UK

RANGE

Temperate Eurasia, from northwest Spain east to Japan. Absent from Portugal, central and southern Spain, northern Scandinavia and several European islands, although the species occurs throughout Britain and Ireland, but is local in Scotland.

Range        Description        Habitat        Behaviour        Biology        Taxonomy        Status        References