The small tortoiseshell is one of the most widespread and familiar butterflies in Britain (2). Its beautiful patterning, comprising of black patches, areas of bright yellowish-orange and a fringe of blue spots around both the hind and forewings, makes this species instantly recognisable. The sexes are similar in appearance (1). The caterpillar, which reaches 22 mm in length, has a black head, a largely black body with tiny white spots, and black or yellowish spines on the back and sides. Along each side there are two broken yellow bands, below which the body becomes more purplish-brown in colour (3).
In most areas of Britain, there are typically two generations each year, but in Scotland there is usually just one. Adults that have hibernated through the winter emerge in March or April; the small tortoiseshell is therefore one of the first butterflies to be seen each spring (2). Females lay eggs in batches underneath the leaves of the foodplants (2). The eggs hatch after around 10 days, and the caterpillars live in groups protected by a silk web. After the final moult the caterpillars disperse, and pupate hanging from plant stems or other objects (3). The adults emerge after around two weeks (3); adults of the second generation hibernate in buildings, caves and hollow trees (2), and reproduce the following spring (3).
The caterpillars feed on common nettle (Urtica dioica) and small nettle (U. urens), which thrive in nutrient-enriched soils, and are therefore often associated with human activity. As a result, the small tortoiseshell inhabits a huge range of habitats, wherever the foodplants occur (2).
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.
The process of forming a pupa, the stage in an insect's development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
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