Black Darter Dragonfly (Sympetrum danae): Resident of Valley Bogs.
LUXURIANT PINK AND PURPLE HUES glittering and shimmering under dew drenched cobwebs that stretch across open expanses of lowland valley heath and mire are a rare sight in Norfolk nowadays, as the late summer sunshine's warming rays burn off early morning mist to reveal the complex mosaics of flowering ling and bell heather, dry and wet acid heath, segmented and interspersed with pools and seepages hosting plant and animal communities eking out a living like no other. It's a sorry tale to learn that 95% of this fragile expanse has disappeared to agriculture, housing and the peat industry, but what remains gives a tantalising insight into the world of heathland-dependent invertebrates, especially dragonflies, and one species in particular that's much specialised to the acid waters and soils that this acreage abounds in; the black darter (Sympetrum danae). Only found inhabiting these evocative, open landscapes in eastern England, it is a totem insect of a vanishing land.
Close study of the black darter in Norfolk is restricted to a small number of locations, but without doubt the principle sites must be Dersingham Bog NNR, NWT Roydon Common and NWT Grimston Warren where an escape into the world of valley bog and mire is rewarded with a glimpse of a bygone panorama. Peak emergence of the insects is during the month of August although cool weather conditions can delay this slightly, with activity levels reaching a climax during the midday when temperatures have attained 20°c or more. The adult black darter is a wanderer and once sexually mature will turn its attention to pairing up with a mate. This is done away from the main breeding pools where packs of adult males will seek out and pursue the females -- who, incidentally, hideaway in dense vegetation, governing that only the most adept males will find them -- where they will pair up and return to the pools already mated and in tandem. By sticking with his female to the moment of oviposition (egg-laying), the male darter is ensuring that her last suitor 'wins' and fertilises the vast majority of her eggs.
Interestingly, black darters oviposit en-masse -- presumably a safety-in-numbers response to lessen the chance of predation when in such a vulnerable predicament -- with the male usually remaining in tandem to his female throughout, a trait often practised by the Libellulidae family. The female is a fussy egg-layer, only dipping the tip of her abdomen into the water where the most suitable clumps of sphagnum moss are present, washing off clusters of precious eggs as one. It is here that the egg will go into a state of suspended development (diapause) where it will not hatch into larval form until the following spring, its ultimate development dependent upon water temperature. The female's choice of sphagnum moss is critical here, as the nymph is able to survive periods of draught and severe heat by sequestering itself beneath the thickest layers of the drying moss, waiting until more favourable conditions for growth return.
The distinction of dragonfly families can be quite complex at times, but one very straightforward way of knowing whether you are studying the family Aeshnidae (hawkers) or Libellulidae (darters, skimmers and chasers) is to watch their movements. The Aeshnidae are restless insects, continually flying and chasing down their prey (much like a hawk) whereas the Libellulidae are more sedentary, preferring to ambush foes by darting out from nearby vegetation, where they will loiter in a perched fashion until a meal or a mate passes by.
The black darter is a northern species, northern European and North American to be precise, inhabiting boreal zones as far afield as Canada through to Russia, dipping no further south than the Alps and Pyrenees. In the United Kingdom it is a well distributed dragonfly, particularly in the West and North where greater amounts of its favoured habitat remain intact, yet in the east of England, particularly East Anglia, Norfolk's heathlands remain the only reliable habitat to allow close observation. Its common name is a clue to its evolutionary progress, and favouring to specialise in climactic zones that are far from tropical is no coincidence, for its black markings offer a form of thermoregulation that enables it to absorb heat (essential for insect flight) at a faster rate, in cooler climates, than lighter toned invertebrates requiring heat of a more tropical intensity. Such is the black darter's ability to absorb heat, it will, on getting too warm, descend into shadier zones of emergent poolside vegetation in an effort to avoid the insect equivalent of heat-stroke. If this trick doesn't work, it will point its abdomen vertically, as a needle will point north on a compass, exposing the minimum amount of body mass towards the sun in a manoeuvre know as obelisking, a favourite tactic of the darters, yet one seldom observed in the British climate.
Favouring tracts of valley heath and mire is a risky business for any animal specialising in its resources; such is their ability to revert back to woodland and scrub. However, the black darter and other dragonflies have a definite advantage when it comes to utilising these habitats. The lack of trees and taller vegetation on heaths and mires is a consequence of grazing and exploitation of the land by man and his grazing stock -- an essential activity to keep these landscapes open -- with acid soils and waters providing a lack of nutrients that are readily available in fens and marshes; these acidic conditions and lack of vegetation favour the procreation of the black darter in a quirky twist of nature: shallow pools containing water with a low pH level (anything below 7 is considered acidic) are too hostile for fish to inhabit, thereby removing a potential predator of the dragonfly larvae. Another useful trait of shallow water is its ability to warm quickly after enduring potentially cold nights in such open environments, whilst the sparse covering of trees and tall herbage allows the full effect of solar radiation to raise temperatures in the breeding pools, stimulating a rapid development of the larvae that can emerge as an adult in as little as 2-months, having prevailed as a top predator in its aquatic crèche over the course of the summer.
...much specialised to the acid waters and soils that this acreage abounds in, the black darter is only found inhabiting these evocative, open landscapes in eastern England: it is a totem insect of a vanishing land.
The female's choice of sphagnum moss is critical here, as the nymph is able to survive periods of draught and severe heat by sequestering itself beneath the thickest layers of the drying moss, waiting until more favourable conditions for growth return.
Cold and stiff, yet in his prime, a mature male black darter dragonfly clings to a stem of sedge as he waits for the welcoming rays of late summer sunshine to warm his body. Eventually the mist will burn off to set him free about his daily duties.
Wings shimmering jewel like in the rays of the rising sun, a female black darter dragonfly emerges from her night time roost covered in a coating of dew. Slowly gaining height up the poolside vegetation, she will fly when wing muscles are sufficiently warmed.