Norfolk Hawker Dragonfly (Aeshna isosceles): Broadland Jewel.
THE SWEATY, BROODING FLAVOUR OF A BROADLAND MARSH -- where humidity literally seeps from the moisture laden turf during the early summer -- is a most appropriate purlieus to be seeking out one of Norfolk's most prized insects. A fizzing, whirring jewel of a creature that lurks amongst the ditches and grazing marshes of our most watery landscapes, the Norfolk hawker dragonfly makes the midge plagued venture into its fenland abode a worthwhile peril: it brings with it an ancestry dating as far back in time as virtually any surviving life form on the Planet. The early dragonflies (of the order Meganisoptera) are known to have rubbed shoulders with the dinosaurs, a time when the coal that we burn on our fires was beginning to form, the Carboniferous period, over 300-million years ago. Their forebears were massive insects with wingspans of 70cm, maybe more, and inhabited saturated swamplands, not unlike the fens and carr woodlands of present day Broadland.
The lifecycles of all dragonflies are interesting, but A.isosceles has a slight twist; it seeks out ditches containing a mysterious aquatic plant: water soldier (Statiotes aloides). Virtually impossible to locate throughout the winter -- having sunk out of sight during autumn due to the onset of calcium build-up on its serrated leaves -- water soldier floats to the surface of suitable drainage channels at the outset of summer (precisely when Norfolk hawkers are on the wing), made buoyant once more by fresh green foliage. Extremely sensitive to eutrophication (the poisoning of waterways through nutrient run-off from arable land), the water soldier of Norfolk has an very unusual dilemma to solve: virtually all of the county's plants are female and rarely set seed, leaving the plant to multiply using runners sent out from its bulging stem, eventually breaking off to form a new plant. Only found in ditches and channels segregated from polluted watercourses, the water soldier draws in successfully mated female Norfolk hawkers, where they may be observed ovipositing amongst the plant's leafage and flower stems, dabbing down a single egg at irregular intervals.
The lifecycle of the larvae is not particularly well known, although it is most likely they spend 2-years in the sanctuary of the channel's basin, feasting upon the ample invertebrate life living under the dense canopy provided by the water soldier's foliage: like all dragonfly nymphs, they are voracious predators armed with elongated and hinged labium (jaws) that thrust outwards and impale unwary victims. Interestingly, emergence of the adult insect is a 2-stage affair, the males materialising in phase-1 (presumably to set up territories), followed by the females shortly after. Emergence begins with the larval nymph crawling out of the water and up a suitable growth of vegetation -- usually water soldier -- before pausing 10cm - 20cm above the water's surface, where it will break out of its larval skin to become the superb flying machine of adulthood: an excellent study carried out by Norman Moore discovered that 80% of emergence occurs on the southern or eastern edge of the chosen plant stem, with no particular part of the ditch favoured, suggesting that catching as much early morning sunshine as possible is a priority for the emerging insect. It is now that the young hawker is at its most vulnerable from predation by other dragonflies (including Norfolk hawkers!) and heads for the safety of the closest treetops, away from the dangers of the battle-ground that is the ditch network; here it will take 2 - 3 weeks to gain full maturity, feeding on midges and flies in the upper canopy.
To stand a better than average chance of viewing Norfolk hawkers zapping about their ditch-side territories, it will first be necessary to identify waterways and drainage channels with a plentiful crop of water soldier apparent. This is easy enough to achieve once it has floated to the water's surface, for it resembles a mass of floating pineapples, spiky leaves protruding well above the aquatic sheen. June is best, a month when the only other likely hawker on the wing will be the hairy dragonfly (Brachytron pratense), a green, blue and black insect very difficult to confuse with the Norfolk hawker. Excellent sites to start your investigations are NWT Upton Fen and RSPB Strumpshaw Fen, both locations holding excellent quantities of clean water and the elusive water soldier plant.
Much as another fabled fenland insect, the swallowtail butterfly, has a symbiotic relationship with milk parsley whilst its continental cousins are far more liberal with their choice of food plant, the Norfolk hawker endures a very similar paradoxical relationship with Statiotes aloides: in its broadland stronghold, it is not known to breed successfully in water bodies without this plant, yet throughout its global range (Central Europe, the Mediterranean and Northern Africa), it readily breeds in channels and ditches lacking this plant species.
As its scientific name suggests, the Norfolk hawker is a member of the family Aeshnidae (hawker dragonflies) of which there are 27 different species to be found across the British Isles (23 breeding residents and 4 non-breeding migrants), living a lifestyle suitable to this appellation with their mastery of the air making them the insect equivalent of birds-of-prey, capable of flying down their midge and mosquito quarry with remarkable ease. The design of the dragonfly's wings is of a primitive nature, a network of veins supporting translucent membranes that shimmer gem-like beneath the rays of aestival sunshine. These delicate structures grant them airborne flight, powered by muscles attached to the thorax, working in a simple up-and-down fashion. (A style copied by the first human pioneers of flight but never bettered and proof perhaps that longevity is born through simplicity of design.)
The apple-green compound eyes and olive-brown abdomen of the Norfolk hawker make it a most distinguishable dragonfly, with the yellow triangle of colour on its second abdominal segment (the isosceles in its scientific name) easily observed in the field and often broader in the female insect. Unlike most of its hawker cousins, Aeshna isosceles spends considerable time perched on bank side vegetation, preferring to monitor the waterways and ditches of fens and grazing marshes from a partially concealed vantage point, lunging out at intruders, prey or potential suitors when needful: it's thought that this imperfect secrecy leads to an unusually high number of male Norfolk hawkers occupying a single ditch, up to 9 per 100-meters is the norm.
Once seen at close quarters, it is difficult to mistake a Norfolk hawker for any other insect. The apple green eyes and yellow triangle on the thorax are very apparent on this adult, freshly settled to roost amongst low-lying ditch-side vegetation.
A peculiar plant in many ways, water soldier is the essential ingredient that keeps the county's Norfolk hawkers in situ. If it were not abundant in well maintained ditches, the insects would almost certainly disappear.
If you find a ditch that looks something like this, you'll almost certainly encounter a Norfolk hawker nearby. Water soldier only grows in waterways free from eutrophication, which are often cut-off from the main rivers.
...the Norfolk hawker dragonfly makes the midge plagued venture into its fenland abode a worthwhile peril...