A-Z of na­ture:

Dr Ben Ald­iss ex­plains why the swal­low­tail but­ter­fly and the Nor­folk hawker drag­on­fly have a spe­cial place in the county’s fauna

EDP Norfolk - - CONTENTS - Dr Ben Ald­iss wildlife jour­nal­ist and broad­caster, deputy man­ag­ing ed­i­tor of World Agri­cul­ture

We take a look at some of the county’s spe­cial­i­ties

Ev­ery­body has heard of the swal­low­tail but­ter­fly, but few peo­ple have ever seen one in Bri­tain – un­less they live in Nor­folk. We’re very lucky that this beau­ti­ful but­ter­fly has cho­sen the Nor­folk Broads as its home. Our swal­low­tail is par­tic­u­larly spe­cial, as it is suf­fi­ciently dif­fer­ent from the Euro­pean one to be given the sta­tus of a sub­species and is en­demic – found nowhere else in the world.

Apart from a few dif­fer­ences in coloura­tion and size, the main rea­son our swal­low­tail is so dis­tinct – and so rare – is its de­pen­dence on a scarce plant as food for its cater­pil­lar; milk pars­ley, which is a mem­ber of the car­rot fam­ily and grows in only a few coun­ties in the UK.

Ap­par­ently, the but­ter­fly can only build up a sus­tain­able pop­u­la­tion in lo­cal­i­ties where milk pars­ley is rel­a­tively abun­dant over a wide area, so de­spite the best ef­forts of con­ser­va­tion­ists, at­tempts to rein­tro­duce it to Wicken Fen in Cam­bridgeshir­e have re­peat­edly failed. The Nor­folk Broads, how­ever, still have plenty of milk pars­ley, as I dis­cov­ered when do­ing a botan­i­cal sur­vey of the Bure Marshes for the Nor­folk Wildlife Trust some years ago.

Oddly, the Euro­pean ver­sion of the swal­low­tail is much less fussy about its cater­pil­lars’ food­plant and nor­mally lays its eggs on fen­nel – a com­mon plant of waste ground on the con­ti­nent and also fre­quently seen on road­side verges in Bri­tain. When we lived in Bel­gium we even had swal­low­tails in our gar­den, where the cater­pil­lars fed on pars­ley in our herb patch.

Your best chance of see­ing one of these beau­ti­ful but­ter­flies is to visit the Broads dur­ing June or July – their main flight pe­riod. You’re par­tic­u­larly likely to spot one in the val­leys of the rivers Ant, Thurne and Bure.

The newly-hatched cater­pil­lars re­sem­ble bird-drop­pings – a dis­guise that seems pretty ef­fec­tive at fool­ing preda­tors – but as they grow big­ger and shed their skins, they be­come bright green with black rings dot­ted with vivid or­ange.

So, from us­ing cam­ou­flage when young, their strat­egy changes to the op­po­site as they get older: now they use warn­ing colours to ad­ver­tise the fact that they are dis­taste­ful. And in case that doesn’t stop a hun­gry car­ni­vore, the cater­pil­lar can sud­denly raise a strange, forked, or­ange struc­ture from be­hind its head.

This os­me­terium, as it’s called, not only shocks the predator, but it also emits a strong odour rem­i­nis­cent of rot­ting pineap­ple.

When fully grown, the cater­pil­lar spins a pad of silk on the stalk of the milk pars­ley on which to hook its rear pair of legs. It then at­taches a thin gir­dle of silk around its body and fixes it at both ends be­fore split­ting its cater­pil­lar skin to re­veal the pupa or chrysalis.

Usu­ally be­ing some shade

of pale green, the pupa is cam­ou­flaged and in this rest­ing state it re­mains from late July un­til it emerges the fol­low­ing June as an adult but­ter­fly.

The sec­ond Nor­folk spe­cial­ity is another in­sect – the Nor­folk hawker drag­on­fly. Like the swal­low­tail, this im­pres­sive crea­ture is large and quite colour­ful, be­ing mainly a light tan colour with yel­low mark­ings and enor­mous green eyes.

It, too, ap­pears in June and July, es­pe­cially in the east of our county. Again, it’s re­stricted to Nor­folk be­cause of a plant

– this time an un­usual species known as the wa­ter sol­dier that gets its name from its straight, reg­i­mented leaves and a unique abil­ity to rise out of the wa­ter like a sol­dier stand­ing to at­ten­tion.

Look­ing rather like the leaves of a pineap­ple, this plant emerges ma­jes­ti­cally as sum­mer ap­proaches then sinks be­low the sur­face again in the au­tumn. Its leaves are brit­tle and spiny and de­pend on a high cal­cium car­bon­ate con­tent in the wa­ter for its sur­vival.

It’s a rare species and is now mainly re­stricted to slow-flow­ing and still, chalky wa­ter in Nor­folk. Nor­folk hawk­ers – like all drag­on­flies – are car­ni­vores, so why do they de­pend on this plant?

The an­swer seems to be that the fe­male drag­on­fly lays her eggs in the leaves. Her ovipos­i­tor, or egg-lay­ing or­gan, can cut a slit in the wa­ter sol­dier leaf, then in­sert an egg, where it re­mains pro­tected un­til it hatches around a month later.

Drag­on­fly eggs hatch into nymphs, whereas but­ter­flies pro­duce lar­vae, the dif­fer­ence be­ing that nymphs look like minia­ture adults, but lar­vae are com­pletely dif­fer­ent. Bee­tles, wasps and flies are like but­ter­flies and moths in this re­spect, hav­ing young stages that bear no re­sem­blance to the adult.

A drag­on­fly nymph looks su­per­fi­cially like the adult, hav­ing six legs and big eyes, but only de­vel­ops wings when it emerges from the wa­ter and crawls out of its fi­nal skin.

The Nor­folk hawker has to change its skin around 15 times over the course of the three years it spends as an aquatic nymph. When near­ing ma­tu­rity, the nymph is a big, fear­some-look­ing beast, but even then it re­lies on cam­ou­flage to avoid be­com­ing fish-food and also so it can creep up on its prey. It’s pre­dom­i­nantly pale brown with darker speck­les, al­low­ing it to blend in very ef­fec­tively against the muddy bot­tom of the pond or broad.

It will eat any small crea­ture it can catch, in­clud­ing tad­poles and young frogs. Con­sid­er­ing the im­pres­sive speed of its par­ents in flight, the nymph is a very slug­gish crea­ture, so it would seem to have trou­ble cap­tur­ing its prey, or es­cap­ing preda­tors.

How­ever, it has two re­mark­able adap­ta­tions. If it needs to es­cape, it can use a form of jet-propul­sion by squirt­ing wa­ter forcibly out of its rec­tum, send­ing it shoot­ing for­wards out of dan­ger. Then, if it’s hun­gry, it has another as­ton­ish­ing trick.

Creep­ing up slowly to within a cou­ple of cen­time­tres of its prey, it sud­denly shoots out a tele­scopic, hinged jaw from be­neath its head. Called a mask, this use­ful at­tach­ment has two claws on the end that grip the un­for­tu­nate prey be­fore drag­ging it quickly back to the mouth.

When ready to emerge as an adult, the nymph crawls up a plant stem out of the wa­ter that’s been its home for so many months, then emerges as a winged drag­on­fly a few hours later by split­ting its fi­nal nymphal skin. Un­like other in­sects, adult drag­on­flies don’t de­velop their fi­nal colours un­til up to a fort­night af­ter emerg­ing and will spend most of this time some con­sid­er­able dis­tance from their pond, be­fore re­turn­ing, re­splen­dent in their

‘Creep­ing up slowly to within a cou­ple of cen­time­tres of its prey, it sud­denly shoots out a tele­scopic, hinged jaw from be­neath its head’

daz­zling liv­ery to hunt over the wa­ter where they spent their youth.

Next month we reach the let­ter O –O for or­chid

Dr Ben Ald­iss is a wildlife jour­nal­ist and broad­caster, deputy man­ag­ing ed­i­tor of World Agri­cul­ture and an ad­viser in bio­di­ver­sity and ed­u­ca­tion on farms. wasp­sand­wildlife.co.uk

ABOVE: The Nor­folk hawker drag­on­fly has yel­low mark­ings and enor­mous green eyes

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