Small Tortoiseshell Caterpillars

From Ian B:

“I photographed this small tortoiseshell nest this morning in Simpsons Field; there is a nettle bed on the right, about half way up the hard path from the entrance.”

The female small tortoiseshell butterfly lays her eggs on nettles. They hatch after about 12 days and immediately after hatching, the caterpillars eat the empty egg shells, and then spin a communal web around the topmost leaves of the plant. They shelter in the web at night or when the weather is bad. and feed whenever the sun shines.

When young, they can easily be mistaken for the caterpillars of the peacock butterfly, but small tortoiseshell larvae are paler, and even when they are quite small it is possible to see the yellow lines along their backs.

If disturbed the caterpillars wriggle and twitch in unison. This is their defence against  parasitoid wasps or flies, most often Sturmia bella, which lays its eggs on the nettle leaves close to where the caterpillars are feeding. The tiny eggs are eaten whole by the caterpillars and then hatch inside their host; the fly larvae feed on the caterpillar but avoid the vital organs. The larva eventually kills the host caterpillar, either when it is fully grown or after it has pupated, and then emerges in order to pupate itself . Although the fly attacks other butterfly species, such as the peacock and red admiral, it is believed that the life cycle of the small tortoiseshell is better matched to that of the fly.

As they grow, the caterpillars split up into progressively smaller groups, spinning a new web after each moult. The final moult sees a change in behaviour: the caterpillars abandon their webs to live solitarily. By this time they are a dull blackish colour, spiky, with broad yellow lines running along their backs and sides.

The chrysalis is variable in colour, ranging from grey to olive or buff, often with a pinkish or golden metallic sheen. It can be found suspended from woody stems, fence posts, walls, or beneath the stems or leaves of nettles. The adults emerge at dawn, about 12 days later.

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