The nature of things
Small tortoiseshell butterfly
THE case of the disappearing tortoiseshell: it’s a conundrum worthy of Holmes or Poirot. Formerly abundant and seen everywhere, Aglais urticae has been missing, in recent years, from many of its formerly familiar haunts. Various grounds for its retreat have been suggested, from too-tidy gardeners to the modern default reason for all ills—climate change. Another thought is that the flourishing of the parasitic fly Sturmia bella in recent years has had a knock-on effect on butterfly numbers. It’s the stuff of horror stories: the fly lays its eggs close to the butterfly’s feeding caterpillars; the latter eat the fly eggs whole, whereupon the swallowed eggs hatch out grubs, which slowly eat the developing butterfly larvae from the inside.
Male and female small torties look similar. The upper surface, when wings are open, reveals a background of deep tangerine, against which smudges and bars of black and yellow are symmetrically arranged, beaded with little dots of blue along the wing margins. The folded wings, however, are the acme of smudgy-brown discretion, cautiously imitating unappetising, rotting leaves to fend off potential predators.