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The Large Tortoiseshell Nymphalis polychloros is a butterfly of the family Nymphalidae. although it looks very like the Small Tortoiseshell (Nymphalis urticae), it is more closely related to the Camberwell Beauty. It is now an extreme rarity in Britain, although it used to be widespread throughout England and Wales. Most of the specimens seen in Britain are thought to be captive-bred releases. The adult insect (imago) over-winters in dry dark places, such as hollow trees or out buildings. In early March the butterflies emerge and mate. The females lay their pale green eggs (ova) in a continuous band around the upper twigs of Elm (Ulmus spp.) or Sallow (Salix Caprea) trees. The caterpillars (larvae) are gregarious, and systematically strip the topmost twigs of the tree bare. They seem to have little defence against predation by birds. It is possible that their decline and extinction in the British Isles (late 1970s) was due to the loss of predatory birds, which previously had preyed upon smaller birds if they strayed to the tops of these trees. The full grown larva spins a silk girdle around a twig further down the tree, and hangs from this by means of hooks (cremasta) at its rear end, to pupate. The chrysalis (pupa) is greyish-brown with a slight silvery sheen. There is only one generation per year, and the imagos emerging in mid summer seek out sources high in sugar to feed. Tree sap and damaged ripe fruits are particularly suitable sources. In captivity a solution of 15% honey to 85% boiled water (by volume) gives a very high survival rate. This solution is soaked into cotton wool pads and these are placed on the top of the cage, and changed daily. The survival rate is further increased if the imagos are warmed for a day in January, making them sufficiently active to feed. A cardboard box is an ideal container with one half of the top cut away and fitted with fine netting. About 70 can be over-wintered in each box, which must be kept cool and dry through the winter. In March they will start to move and feeding should commence immediately. Fresh twigs of Elm and Sallow can be placed in the box in small bottles of water. This will stimulate mating and egg-laying within the box. Mating takes place in the darkest top corners of the box and the pair will remain joined for some hours. If a cage is used, it needs to be covered on one side with a black cloth. The related Camberwell Beauty (Nymphalis Antiopa) can be over-wintered in exactly the same way, but will not mate in confinement.
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