Nature · Insects & other invertebrates

Small tortoiseshell

On the wing March–October in two or three broods · wingspan 45–62 mm

Aglais urticae — Nymphalidae

Britain's most familiar orange butterfly, and the one most often confused with the painted lady. It is hot orange with black and creamy-yellow blocks along the front edge of the forewing and — the clincher — a complete row of blue crescents round the margins of all four wings. It hibernates as an adult, often indoors, and lays its eggs in large batches on the young growth of common nettle. It declined sharply in southern England from the 1990s, a fall not yet fully explained.

How to spot them

▸A full row of bright blue crescents round the outer margin of every wing
▸Alternating black and creamy-yellow blocks along the leading edge of the forewing
▸Hot brick-orange ground colour, far richer than the painted lady's salmon
▸Underside dark bark-brown and grey, with a paler outer band
▸Black spiny caterpillars with two yellow side-stripes, in a dense communal web on nettle tips

How to know it

A complete row of blue crescents round the margin of all four wings, over hot brick-orange

Small tortoiseshell — the blue crescents separate it at a glance from the painted lady.

Black and creamy-yellow blocks alternate along the leading edge of each forewing

Small tortoiseshell; the costal blocks are diagnostic.

Lays a heap of a hundred or more eggs on the tender tips of a sunny nettle bed

Small tortoiseshell, whose batch-laying gives the dense communal larval web.

Found hibernating behind a curtain or in a cold corner of a bedroom in January

Small tortoiseshell — the butterfly most often found overwintering inside British houses.

Its specific epithet is the Latin for the nettle its caterpillars strip

Small tortoiseshell — Aglais urticae, from Urtica, the nettle.

Its southern English decline has been linked to a parasitoid fly first found in Britain in 1998

Small tortoiseshell and Sturmia bella, a tachinid fly whose spread coincided with the crash.

Britain has a second, far larger and much rarer relative of it that breeds on elm

Small tortoiseshell contrasted with the large tortoiseshell, Nymphalis polychloros, long extinct as a British resident.

Quick-fire facts

•Aglais urticae — the same genus as the peacock, Aglais io
•Larval foodplants are common nettle and small nettle
•Butterfly Conservation reports a long-term decline, sharpest in southern England, with losses of the order of three-quarters since the 1970s
•One suspect is the tachinid parasitoid Sturmia bella, first recorded in Britain in 1998, whose eggs are swallowed with the nettle leaf
•It hibernates as an adult, frequently in sheds, garages and houses; the others that do are the peacock, brimstone and comma
•Females lay in batches of up to about a hundred eggs, so larvae are found in conspicuous communal webs
•The large tortoiseshell, Nymphalis polychloros, is bigger, lacks the white forewing spot pattern of the small, and is no longer a British resident
•Two or three generations a year in the south, one in the far north

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Sources

Butterfly Conservation — Small Tortoiseshell
Paul D. Brock — Britain's Insects: A Field Guide to the Insects of Great Britain and Ireland