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From caterpillar to flying machine: Adrian Thomas introduces the magical world of butterflies and moths

Every year, there comes a day in early spring - in March maybe, or even February now - when, for the first time, the sun shines with an intensity that warms the skin and the temperature eases its way up to 13o centigrade.

As if to confirm that the gloom of winter is lifted, this is likely to be the day when the year's first "butter-coloured flies" will be seen flapping gently through gardens and along country lanes.

These are male brimstones, just one of the 60 regularly occurring species of butterfly to be found in the UK, but the one species that, with their bright sulphureous colouring, gave the group their name. Already, these brimstones will have been in this adult stage, known as the imago, for more than six months, but the winter will have been spent hibernating, tucked away in a thick tangle of ivy.

Having been oblivious to females when they emerged from their chrysalises at the end of last summer, hibernation has now flicked a hormonal switch in the males and they set off, "looking for love". Tirelessly, they beat a path through the countryside looking for the paler females who sit, waiting patiently to be found.

Once wooed and mated, the females set off to find either an alder buckthorn or a purging buckthorn. Both these small trees are uncommon in the countryside, and even more so in our gardens, but find them she must, because they are the only plants her caterpillars will eat.

With poor eyesight and no sense of smell, how does the brimstone find her goal? The trick is deep in her sole: alighting on bush after bush, she "tastes" for the right one with special sensory glands on the base of her feet .

She then lays her eggs which after four months will hatch into caterpillars, chomping their way through enough leaves to have to moult their skins several times, and then pupate. Through the magic of metamorphosis, the adult butterflies will be on the wing by the end of the summer.

The life cycle of the brimstone sums up many of the features of butterflies that make them so accessible and rewarding to study. They tend to be conspicuous, often brightly coloured, and approachable - given just a bit of self-control and awareness of your own shadow.

The number of butterfly species in the UK means that most are fairly easy to identify, without being so limited in variety as to be unchallenging.

Each species appears in its adult form according to quite a strict timetable, and so butterflies make good phenological subjects, both allowing the year to be charted species by species and providing indicators of climate change when the shift in emergence times and flight periods is compared over the years.

Further enhancing their reputation for being "user-friendly natural history", butterflies are widespread geographically; a few species will even make it into the most urban of places, especially if an effort is made to attract them. Butterfly gardening at its most simple is about growing flowers; far more challenging, though, is to attempt to satisfy the animal in all its phases rather than just the adult's love of nectar.

The one thing about butterflies that never fails to amaze is their life cycle, and particularly the change from caterpillar to adult. Within the shell of the chrysalis, the caterpillar "dissolves" into a thin soup of cells that reorganise themselves and develop into a gaudy flying machine.

Of course, a moth's life cycle is exactly the same as a butterfly's, yet, for an article about butterflies and moths, the latter have been conspicuous by their absence so far. But perhaps that's an accurate reflection of how the two groups are perceived. If butterflies are showy extroverts, moths seem to be the duller occupants of a rather more hidden and gothic world, all whirring wings and erratic flights-in-the-night.

So now here comes the bit where I try to convince you there are moths that will make you go "Wow!" Let's start with their names. There's the silver cloud, the anomalous, the bedrule brocade - if you ever need to name a pop band turn to the index of a moth book. How about the geometrician, the chamomile shark, the nonconformist? And of course the one called the gothic.

Moths are a curious bunch. Some are very large - the death's head hawk moth is about 13cm in wingspan. In some species, the females are wingless and sit on fenceposts oozing pheromones. There are species that emerge only in even-numbered years, others only in odd-numbered. There are moths which are crinkled like an autumn leaf and one which is commonly mistaken for a bird.

However, as subjects for study, the word "challenge" now enters the frame. While there are only 60 British butterfly species, in the moth world there are about 2,500, with an average garden often attracting several hundred different species. Many are tiny, generally known as the "micros".

Even experienced moth enthusiasts often stick solely to the bigger ones, the "macros".

And even in the macros, identification can be a nightmare. Many are incredibly similar in appearance and yet, within each species, moths can be alarmingly variable. However, if poring over the identification books seems a challenge, then the methods for finding moths are likely to appeal too.

Inevitably, there is a nocturnal element to finding many (although not all) of them. There is also some alcohol involved, and extensive use of ultraviolet light and eggboxes.

The alcohol comes into play in a technique called sugaring. A special brew of sugar, treacle and ale is brewed on a stove (boiling the alcohol first so as to avoid an eruption) and is then painted on fenceposts just before dusk. Attracted by the smell, moths come to feed and can be identified while they sup.

Light, however, is the main mothing tool. It was once thought that moths are drawn to lightbulbs because they mistake the bulb for the Moon while trying to navigate; it seems more likely that the structure of a moth's eye paradoxically makes it see darkness in the centre of the light. The moth thinks it is heading for a safe hiding place.

The easiest light trap is to hang a white sheet on a washing line at dusk and shine a light onto it. Moths will land on the sheet or on the ground below it. It has none of the drawing power or sophistication, however, of one of the three "proper" traps, known as the Heath, the Skinner and the Robinson. They tend to be quite expensive but, with good electrical know-how and great care, it is possible to build a simple model and fit it with the right bulbs for under pound;50.

Each of the three traps uses a light with a large ultraviolet output, the part of the spectrum most attractive to moths. The Heath uses a fluorescent tube; the other two use more expensive mercury vapour bulbs that cannot be run off the mains without a choke. In all the designs, the bulb sits on top of a box or drum. Moths fly in, are intercepted by Perspex screens which they can't see and drop through slits into a holding area below stuffed with old egg boxes.

Once inside the trap, most moths settle down, perhaps thinking that the light is sunrise. When the trap is opened, usually the next morning, the moths don't instantly fly away, partly because some moths need to do "exercises" (muscle vibrating) before they can fly, partly because most moths' instinct is to stay still in the hope they are mistaken for something inedible.

After a good summer's night there might be 1,000 moths ready for identification, counting and photographing. All the moths can then be released unharmed, ideally the following dusk to prevent birds getting an impromptu moth feast.

It is quite possible to find eggs laid in the boxes. These, or indeed eggs or caterpillars found anywhere else open the possibility of rearing butterflies and moths in captivity. It isn't easy and requires far more dedication than, say, keeping goldfish. But to see a caterpillar you have nurtured pupate, and then from it emerge a bedraggled creature that slowly pumps out its crumpled wings to full adult glory, and for it then to fly off into the wild - now that's special.

Adrian Thomas is a wildlife journalist and press officer for the Royal Society for the Protection of Birds

Set up to protect wild butterflies and Features 1,270 of the more than 2,400 recorded species in Britain and The UK Butterfly Monitoring in traps and other equipment

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