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| A Winter House Guest |
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Page 4 of 4 Many people are familiar with the little silken tents that the caterpillars spin around the top of a nettle plant. In the early stages, when they are small and have not developed their spines, they collect in these tents for protection from predators and adverse weather. In the final phase of development they disperse and go it alone, feeding avidly before evolving into the copper pink or brown chrysalis that hangs suspended from vegetation or a hard structure like a wall. Inside the hard exterior of this beautifully sculpted structure, the contents degenerate to a soupy consistency. This material takes about twelve days to reassemble and the adult butterfly emerges in late June/July – the whole process little less than a miracle.  When they are small, the caterpillars cluster together for protection. In most years a second generation is produced from these adults, flying in late August/September. They will either enter hibernation or, in unusually warm conditions, as may happen in Cornwall, some may breed yet again and produce a third generation. Research has shown that temperature determines the speed of development and hence the number of generations produced, but whether a caterpillar develops into an adult that breeds or one that hibernates is dependent on day length. Our August butterfly would have probably been an individual from the second generation of flying adults, but the situation is more complicated than it seems because Britain is also host to migrants from continental Europe, and Cornwall is well placed to receive them. They arrive in varying numbers and at different times of year, even as early as March, before our residents have come out of hibernation. Hundreds of thousands have been recorded coming to the coast of Britain in exceptional years, many alighting on ships or light vessels on the way. These butterflies will also breed, or attempt to, and their breeding cycles are not in synchrony with our residents. Personally, I have not seen many Small Tortoiseshells this year, but their populations are very prone to annual fluctuations due to various causes; for example droughts, resulting in low nectar supplies, can prompt a low survival rate of hibernating adults, or it may be that high rainfall in May and June produces an abundance of good quality nettles, allowing the fast development of caterpillars and high numbers of pupae forming. Butterflies dying during hibernation may mean that they did not build up sufficient fat reserves to sustain them or that a sudden rise in temperature roused them too early. Such is the tenuous life of a butterfly. Spare a thought for this small, attractive butterfly that still has the power to delight a child, inspire an artist or make a garden more beautiful. Leave a nettle patch when clearing your ground or perhaps create one on spare ground in a sunny corner. |
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