The butterfly with a woodland empire

In the latest stage of his butterfly hunt, Michael McCarthy goes down to the woods to see the purple emperor in action

It is the most magnificent of all our butterflies: the purple emperor. And His Majesty has no more devoted a servant than Matthew Oates.

The National Trust's butterfly adviser, Matthew has spent years studying and following Apatura iris, a creature whose dazzling purple sheen is unparalleled in the natural world, but whose habits make it far from easy to see.

For this beautiful insect, which sent Victorian butterfly collectors into paroxysms of desire, spends most of its time high in the canopy of oakwoods, only rarely and briefly descending to the ground, and when it does so it is paradoxically attracted not to flowers, but to puddles, animal faeces or carrion.

To see it we enlisted the help of Matthew, an exuberant character unbounded in his enthusiasm for the species and his knowledge of how to find it. He took us to Alice Holt Forest in Hampshire, a purple emperor stronghold, where after an hour's searching a big butterfly appeared overhead and Matthew cried: "It's Himself! It's His Imperial Majesty!"

We saw several in the course of the day, none descending to the ground. They hunted through the tops of the oak trees for females and then took perches and drove off rival males from their territories, sometimes in spectacular aerial combats when the purple gloss on their wings would flash when caught by the sun.

Alice Holt is also a stunning site for the other big butterflies of the woodlands in high summer, such as the white admiral and the silver-washed fritillary, which were flying in profusion: at one stage there were so many (and a total of 13 species) that it felt like walking through a tropical butterfly house.

Two other fascinating species of the treetops are the purple hairstreak and the white letter hairstreak (the latter perhaps the hardest of all our species to see). To find them we sought the expert help of Liz Goodyear and Andrew Middleton of Butterfly Conservation's Hertfordshire branch.

They found the white letter hairstreak high in an elm tree on the edge of Cheshunt Golf Course, and the purple hairstreak in Broxbourne Woods, where its deep purple wings were catching the sunlight – like a junior version of the Emperor. Liz and Andrew are co-ordinating a survey of the white letter hairstreak and think this may be, as well as the most elusive, one of the most under-recorded of our butterflies. They have found it almost anywhere they have looked where there are elms, even in central London.

The Great British Butterfly Hunt: Species 40-44 (of 58)

40. Purple emperor

Apatura iris

The most magnificent British butterfly (if the swallowtail is the most glamorous), the purple emperor is both dazzling in the purple sheen of the male, and hard to see, as it spends most of its time in the woodland canopy, usually around the tops of tall oak trees. Unusually, it does not visit flowers to feed but prefers rotting matter and even animal faeces; butterfly lovers try to bring it down by placing overripe food on the ground. The effort is worth it.

Larval food plant: Sallow, and some other species in the willow family, growing along rides in old established woodlands.

Where seen: Mature broadleaved woodlands, mainly oak woods. in southern England.

Current conservation status: Difficult to record because of its treetop existence, but if anything, is expanding. Enthusiasts are finding it in more and more places.

41. White Admiral

Limenitis camilla

This lovely butterfly is a creature of broadleaved woodlands like the purple emperor and the silver-washed fritillary, and its wing-shape gives it something of the exotic about it, with a resemblance to the tropical species of the Heliconidae family often on display in butterfly houses. Formerly common in southern woodlands, but now in a worrying decline.

Larval food plant: Honeysuckle, trailing down from oak branches in the woodlands.

Where seen: Shady woodland edges and rides, especially where there are patches of bramble to provide nectar.

Current conservation status: 58 per cent down since 1976. It is thought this decline may be caused by the rising numbers of deer in British woodlands, eating much of the foliage of the understorey – including the caterpillar's honeysuckle food plants.

42. Purple Hairstreak

Neozephyrus quercus

Almost a miniature purple emperor, this is also a butterfly of the woodland canopy, exquisite in the purple wings of both male and female but requiring a lot of effort to see, and strong neck muscles for gazing upwards for long periods at a time. Its treetop existence means it is often overlooked.

Larval food plant: Oak leaves.

Where seen: High in the tops of oak trees, but not necessarily in woods. It can be found in parks and gardens and even in central London.

Current conservation status: Stable.

43. White-letter hairstreak

Satyrium w-album

Most of the hairstreaks are hard to see, but this is probably the most difficult of all, spending most of its life in the tops of elm trees, although individuals occasionally come down to nectar on flowers. One was caught on camera on the grass of a tennis court at this year's Wimbledon championships.

Larval food plant: Elms of various species, such as English elm and wych elm.

Where seen: Wherever elms occur, in hedgerows or woods or parks. Frequently found on young elm shoots.

Current conservation status: 81 per cent down since 1976, largely as a result of Dutch elm disease wiping out most English elms. But this very elusive butterfly may be under-recorded and far more numerous than is commonly thought.

44. Silver washed fritillary

Argynnis paphia

One of the loveliest and most elegant of Britain's butterflies, the silver-washed fritillary makes a wonderful sight swooping through mature woodlands in high summer. One of a trio of spectacular woodland butterflies, with the white admiral and the purple emperor, it is unusual in that it likes dark woodlands, whereas most woodland fritillaries require sunny glades to breed.

Larval food plant: Common dog violets, although the eggs are laid on oak trees, and the tiny caterpillar climbs down to the ground.

Where seen: Mature broad-leaved woodland in the south, especially oak woods.

Current conservation status: Doing well, with a 71 per cent increase since 1976.