Red Admiral Butterfly Vanessa atalanta

I am an amateur naturalist trying to learn something about everything living in my garden.

Taken last summer, photo 1 shows a butterfly basking in the sun on my garden table. There's no mistaking it as a Red Admiral (Vanessa atalanta).

What I have learnt about Red Admirals I have got from reading my newly acquired copy of The Butterflies of Britain and Ireland (Thomas and Lewington, publ. British Wildlife Publishing). This is a major new work that I can't recommend too highly for the interested amateur. All 72 'properly recognised' species of UK butterfly, each copiously and beautifully illustrated as egg, adult, caterpillar and chrysalis, and each with a full and scholarly essay on its natural history.

In common with the Painted Lady I blogged here, the Red Admiral undergoes a remarkable migration. Red Admirals overwinter in Southern Europe, not as adults, but as caterpillars, maturing slowly in the cool Southern winters. In early spring the (by then) adult Admirals start to fly North. Some will fly as far as Scandinavia.

When they arrive at a suitably Northern destination, the males set up territories on high ground where they mate with females which go on to lay their eggs, singly, on plants such as nettle (see my posting here). Eggs hatch after about a week and the emerged caterpillers mature over about a four week period. The caterpillars carry a dozen or so sets of bristles along their bodies and come in two forms: black and green. They have the evolved the remarkable trick of constructing a 'tent' of leaves, sewn together with silk (there's a picture here). Sitting inside they can munch away out the sight of predators. The caterpillars pupate in an attractive grey and yellow-spotted chrysalis to emerge to later as the beautiful butterfly of photo 1. Around October, as the weather cools, these adults fly to back to Southern Europe and the cycle begins again.

The book above mentions a fascinating puzzle for a population of Scandinavian Red Admirals. Leaving Scandinavia at the end of summer the adults start out by flying due South. After a time however, they reach the coast in Southern Sweden. At this point the butterflies 'cleverly' turn West in order to minimise the distance they need to fly across open sea before reaching land again at Denmark. From Sweden, the coast of Denmark is 24km away and only barely visible to human eyes in a fine day. How the butterflies are able to detect it and know to turn West therefore is a puzzle. It is speculated that they may be making use of an ability of many insects have to see the 'polarisation state' of light. 'Polarisation' is a property of beams of light that humans can't see, but suffice to say land and water can affect it in different ways. It is theorised the migrating Scandinavian Red Admirals are using this to aid them in detecting land at a distance. This appears to be unproven however. Another of nature's mysteries!