Males of this beautiful butterfly have an iridescent purple or bluish sheen to the upper surface of the wings when viewed from a certain angle (3). Females do not have this iridescence, however, and are patterned with dark brown and white markings, as are the males when the iridescence is not visible (1). Both sexes have an eye-spot on the hindwings (1). The plump caterpillar grows to 4.2 centimetres in length, is strongly tapered at each end and sports two long horns, tipped with red. It is bright green in colour, has diagonal yellow lines along the sides and is freckled with tiny yellow spots (4).
This species typically flies high in the tree canopy, and feeds on honeydew produced by aphids, as well as tree sap. Males visit the ground on occasion, in order to obtain salts from dung or from the surfaces of roads with their yellow proboscis. The purple emperor is single-brooded, and adults are on the wing between late June and mid-August (5). Females lay bright green eggs singly on the upper surface of the leaves of the willow foodplants. The eggs hatch after two weeks, and the caterpillars begin to feed (4). They are beautifully camouflaged, and rest on the midribs of leaves on a pad of silk, the font part of the body slightly raised from the leaf (5). During the autumn the caterpillars commence hibernation(4), taking refuge in the forks of branches or willow buds. They start to feed again the following April (5), and towards the end of June they form a leaf-like pupa, which hangs underneath leaves. Adults emerge after around two weeks (4).
The purple emperor once occurred throughout most of England, reaching north to the Humber, and was also found in parts of Wales (5). During the twentieth century it has undergone a severe decline in range (2), and is now restricted to central-southern England. It has also been lost from Wales, and has never occurred in Scotland or Ireland. Recently the range has re-expanded, but it is not yet clear if this represents a temporary change, or a long-term recovery. Elsewhere it is found throughout much of central Europe, and reaches as far east as Korea and China (5).
The decline of this butterfly is thought to have been caused by the widespread and large-scale loss and fragmentation of ancient woodland (2). At present, the species is vulnerable as a result of the small size and increasing isolation of remaining habitat; this isolation also reduces the likelihood of the successful natural recovery of the species, as dispersal is restricted. Even where suitable habitat exists, the relatively short life span of the willows on which the caterpillars depend, means that a continuous supply of willows may not persist. Unsuitable management may also remove willows, and the presence of deer can prevent their regeneration (2).
Butterfly Conservation has produced a Species Action Plan to co-ordinate conservation efforts to conserve the purple emperor (2). In a number of key sites the species is benefiting from active targeted conservation (5), but a major change in forestry management is needed if this magnificent butterfly is to recover (5).
A winter survival strategy characteristic of some mammals in which an animal's metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is 'diapause', a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
A tubular protrusion from the anterior of an animal (e.g. the trunk of an elephant).
Stage in an insect's development when huge changes occur, which reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
Also known as ‘univoltine’. Referring to an organism which has just one brood each year.
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