( Sympetrum danae )
The black darter is the UK's smallest resident dragonfly, but is common and widespread in the Highlands, including in the Caledonian Forest.
The black darter dragonfly has a circumboreal distribution, meaning that it occurs in northern latitudes all around the world, in Europe, Asia and North America. In Europe it is found in central and northern parts of the continent, from Ireland and Britain to Russia, and from northern Scandinavia to the Pyrenees and Alps, and across eastwards through Romania to the northern coast of the Black Sea. In Asia, it occurs in Russia across to Siberia and Kamchatka, and in Japan. In North America, where it is known as the black meadowhawk, it occurs from Alaska and Canada's Yukon Territory across to Labrador and Newfoundland, and southwards to northern California, and to Maine and Kentucky in the east of the continent.
The black darter occurs throughout Scotland, including all parts of the mainland, the Inner and Outer Hebrides and much of Orkney. It is absent from Shetland. It is less common in the east of the country, perhaps due to the drier climate providing fewer breeding sites. South of the border, it occurs throughout much of England and Wales, but is less abundant in the south and east.
The black darter is a small dragonfly in the family Libellulidae, in the order, Odonata, the taxonomic category that covers dragonflies and damselflies. Adults are between 29 and 34 mm in length and up to 47 mm in wingspan, making it the smallest resident dragonfly species in the UK. Like all dragonflies, the adult black darter has two large, bulbous compound eyes, two pairs of wings and 6 legs. The body has three parts - the head, the thorax (where the wings and legs are connected to the body) and the abdomen, which has ten segments to it. At the end of the abdomen there are two small claspers, which are larger in the male than in the female.
The head is freely moveable, and in addition to the compound eyes it contains three smaller simple 'eyes' and two small antennae. The ability of the head to turn through 180 degrees, combined with the huge eyes, give the dragonfly excellent vision. The jaws or mandibles have teeth on them, and this characteristic gives dragonflies and damselflies their order name, Odonata, which means 'toothed jaw'.
The wings are transparent and have a net-like structure of veins, which provides strength and rigidity while still being lightweight. The pattern of the veins varies between dragonfly species, and can be used as an aid to identification of some species. The forewings are longer and narrower than the hindwings, and each wing has a distinctive coloured patch on the leading edge, near the tip. These are called pterostigma, and because they are slightly heavier than the other cells in the wings, they aid the dragonfly in gliding. In the black darter, the pterostigma are black. The wings are held fully open and often pushed forward at an angle to the body when the dragonfly is at rest, perching on vegetation.
The black darter's legs are distinctive in being entirely black, and are segmented and covered in spines. Because the thorax is tilted upwards, the legs face forwards, enabling the dragonfly to perch on upright stalks. It also means the legs can form a basket-like shape, which the dragonfly uses in catching flying insects.
Male and female black darters are different in their colouration. Females are mainly yellow, with a black underbody and a black triangle on the top of the thorax, although the yellow colour darkens with age to an olive-brown. Immature males are yellow, but their colour changes to black as they mature, with yellow markings remaining on the sides of the thorax and on the lower abdomen. The colour of the males gives the species the distinction of being the only black dragonfly in the UK. Females have straight abdomens and small anal claspers, while males have distinctively waisted abdomens (ie they narrow slightly in the middle) and longer claspers.
Like all dragonflies, the black darter has a two stage life cycle, although it is only the adult that is encountered by most people. It begins life as an egg that is deposited in water by an adult female in late summer while she is in flight, often in tandem with her male mate. The eggs, which can be over 200 in number, are laid in the still, acidic waters of moorland lochans and pools, which are well-vegetated with bog-mosses and rushes. The eggs will persist, or overwinter, until the following spring before hatching.
The black darter larva is known as a nymph (or a naiad in North America), and is predatory, feeding on midge larvae, water fleas and other small aquatic invertebrates. It grows quickly, with the nymph stage lasting from two to six months - in some other dragonfly species, the nymph stage can last for up to 5 years. The nymph passes through a series of moults, in each of which its exoskeleton is shed, as it is outgrown. In its final stage, or instar, the nymph is 1 - 2 cm. in size with its wing buds clearly visible, and it climbs out of the water on to nearby vegetation to shed its last larval skin, which is known as an exuvia.
Emergence takes place mainly in July and August, although the period when adults are emerging can extend from June until early October. The black darter is one of the last dragonflies to emerge as an adult in Scotland each year. The newly-emerged adult will wait until its wings unfold fully and its body expands and dries out before flying for the first time. After an adult has emerged it takes from 10 to 13 days for it to reach full maturity, with its final colouration.
Like all dragonflies, the black darter is an expert flier. Because it can move each wing independently, it has the ability to hover, take off vertically and even fly backwards. This aerial agility enables it to catch its prey in flight, and it feeds on flies, midges and other small airborne insects. Its preferred habitat is areas of wet or boggy ground with low bushy vegetation. This provides perching sites, where it can bask and gain warmth from the sun. If it is disturbed when perching, the black darter will often fly a short distance away before settling again.
When the adults are fully mature, they are ready to mate. The male uses his anal claspers to hold the female just behind the head. The female then curves her body forwards so that her sexual organs, which are at the tip of her abdomen, reach those of the male, which are on the second segment of the abdomen down from the thorax. This puts the pair into the 'wheel' or 'heart' position that is characteristic of all dragonflies and damselflies when mating. Copulation in the black darter takes about 5 minutes, and the pair are able to fly while mated. The female then goes on to deposit her eggs in water. The lifespan of the adults is quite short, probably only 2 or 3 weeks, allowing enough time to mature and mate. However, in some instances adults must hibernate through the winter, as adults with mature colouration have occasionally been recorded in April, well before any will have newly emerged that year.
In its nymph stage, the black darter helps to control the populations of its prey, which includes midge larvae and other aquatic invertebrates. In pools and lochans that are large enough to support fish, they are the main predator of the nymphs. A trematode or flatworm ( Prosotocus confusus ) that parasitises frogs and toads also occurs as a parasite in black darter nymphs. The larvae of some water mites ( Arrenurus spp.) parasitise the nymphs of other dragonflies in the genus Sympetrum , and it's possible they may affect the black darter as well.
The adult black darter plays a role in limiting the numbers of the insects it feeds on, and is also prey itself for a range of predators, including birds, robber flies and various spiders. As a small dragonfly it is also at risk of predation by larger species, which in Scotland include the golden-ringed dragonfly ( Cordulegaster boltonii ). Females are also susceptible to predation by frogs when they are ovipositing (laying their eggs).
With its more widespread occurrence in the north of the UK, the black darter is one of our most common dragonflies and is frequently seen around the pools and lochans of the Caledonian Forest.
Alan Watson Featherstone
Last updated: 13 December 2012
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