Butterfly eggs are protected by a hard-ridged outer layer of shell, called the chorion. This is lined with a thin coating of wax which prevents the egg from drying out before the larva has had time to fully develop. Each egg contains a number of tiny funnel-shaped openings at one end, called micropyles; the purpose of these holes is to allow sperm to enter and fertilize the egg. Butterfly and moth eggs vary greatly in size between species, but they are all either spherical or ovate.[citation needed] Butterfly eggs are fixed to a leaf with a special glue which hardens rapidly. As it hardens it contracts, deforming the shape of the egg. This glue is easily seen surrounding the base of every egg forming a meniscus. The nature of the glue is unknown and is a suitable subject for research. The same glue is produced by a pupa to secure the setae of the cremaster. This glue is so hard that the silk pad, to which the setae are glued, cannot be separated.[citation needed] Eggs are almost invariably laid on plants. Each species of butterfly has its own hostplant range and while some species of butterfly are restricted to just one species of plant, others use a range of plant species, often including members of a common family.[citation needed] The egg stage lasts a few weeks in most butterflies but eggs laid close to winter, especially in temperate regions, go through a diapause (resting) stage, and the hatching may take place only in spring. Other butterflies may lay their eggs in the spring and have them hatch in the summer. These butterflies are usually northern species, such as the Mourning Cloak (Camberwell Beauty) and the Large and S all Tortoiseshell butterflies. The Blackleg Tortoiseshell or 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 found in Europe, northern Africa, and western Asia.[1] It is 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 late February or 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.), Sallow (Salix caprea and Salix viminalis), Pear (Pyrus spp.), and Prunus spp. trees.[1] 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.