Melanic magpie moth caterpillar

More photos from the 1990s found in the back of a cupboard.

Every few years our garden gooseberry bushes are infested with magpie moth caterpillars. They're hard to miss, with this gaudy black, white and red colour scheme which is a form of warning colouration (aposematism), warning birds that they are foul tasting: the theory is that any bird eating one would remember not to eat these colourful but distasteful caterpillars again.

One infestation included this all-black melanic mutant magpie moth caterpillar. 

Industrial melanism, where mutant black forms of the pale grey peppered moth Biston betularia appeared and rapidly increased in frequency, is one of the classic textbook examples of natural selection in action, based in crypsis. In that case the melanic form provided better camouflage against sooty tree trunks in industrial cites, was less frequently predated by birds than the pale grey form and so rapidly increased in frequency. When air quality improved in cities and the soot disappeared from tree trunks the melanic form became more conspicuous and vulnerable and the better camouflaged pale form increased in frequency again.

With this melanic magpie moth caterpillar the selection pressure is quite different because this insect larva benefits from being conspicuous and warning birds of its unpalatability. Losing its bright colours ought to be a distinct disadvantage. 

Maybe that's why, in the 20 years since I took this photograph, I've never seem another melanic magpie moth caterpillar on our gooseberry bushes.