Watching the astonishing BBC series Frozen Planet (2011) – for which no superlative is an exaggeration – one is struck again and again by life’s diversity, resilience, and incongruity. Narrated by the inimitable David Attenborough, it is a plum pudding of vistas and facts among which is the astonishing story of the Isabella Tiger Moth.
Named by the English botanist and founder of the Linnean Society Sir James Edward Smith working from a drawing supplied by the American entomologist James Abbot, the Isabella Tiger Moth appeared in The Natural History of the Rarer Lepidopterous Insects of Georgia (1797), the earliest book on American insects.
The moth Pyrrharctia isabella is known by different common names at its two main life stages. The adult is the Isabella Tiger Moth and the larva is called the Banded Woolly Bear. The larvae of many species of Arctiid moths are called “woolly bears” because of their long, thick, fur-like bristles. This particular species is black at both ends with a band of coppery red in the middle. The adult moth is dull yellow to orange with a robust, furry thorax and small head. Its wings have sparse black spotting and the proximal segments on its first pair of legs are bright reddish-orange.
The curious thing about this insect – detailed in one particular episode of Frozen Planet – is that it survives in many cold regions, including the Arctic, which is where Attenborough filmed it. The Banded Woolly Bear larva emerges from the egg in the fall and overwinters in its caterpillar form, when it literally freezes solid. First its heart stops beating, then its gut freezes, then its blood, followed by the rest of the body. It survives being frozen by producing a cryoprotectant (a substance used to protect biological tissue from damage due to ice formation).
In the spring the caterpillar thaws out and eats before hibernating (and freezing) all over again. Caterpillars normally become moths within months of hatching in most temperate climates, but in the Arctic the summer period for vegetative growth and feeding is so short that the Woolly Bear feeds for several summers before it finally pupates. Once it emerges from its pupa as a moth it has only days to find a mate before it dies.
The larvae of this species are the subject of common folklore, which has it that the severity of a winter can be predicted by the amount of black on the caterpillar. This is the most familiar Woolly Bear in North America. In fact, larvae produced in the same clutch of eggs can vary from mostly red to mostly black, even when reared under the same conditions, and this variability invalidates any temperature-related trends that may otherwise be evident. In fact, the orange band will grow towards the ends of the body, with the black bands decreasing in size, as the larva matures.
Frozen Planet is the icing on the cake of a lifetime of self-dedication. It is visually beautiful, spectacularly revealing, and richly informative. David Attenborough is intent upon making people understand and respect the diversity and integrity of the world in which we live. The world will one day be a poorer place without him.