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
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