Pam Howse of Queenstown asks :-

Whilst tramping in the West Matukituki Valley in Mt Aspiring National Park in January, I saw a very hairy black caterpillar about 4cm long scurrying across a boulder near the river. With each curl-up it made one or two thin lines of bright red/orange visible under its black hair. What can you tell me about this caterpillar?

Ruud Kleinpaste, the bugman, responded.

That very hairy black caterpillar is likely to have been a so-called "woolly bear"; it is the larval stage of that remarkable day-flying moth called the magpie moth (mokarakara), Nyctemera annulata.

A lot of people think that it is a butterfly, but it really belongs to the Family Arctiidae, and is therefore better classified as a diurnal moth. These moths are pretty black critters with white or yellow-ish spots on the wings and orange-ringed abdominal segments. They are mostly active in the mornings and evenings and in the warmer months of the year.

Caterpillars are indeed hairy and black with patches of reddish lines. They feed on Senecio species, both natives and introduced; yes, they are often found devouring the leaves and flowers of Ragwort. A life cycle takes approximately six to eight weeks and there can be a number of generations per year.

Often you can find the caterpillars moving quite fast over bare ground or in conspicuous places, as if they haven't got a care in the world. Arctiid caterpillars tend to have some pretty obvious colours which means that they tend to be rather inedible to birds and other predators; the numerous hairs don't taste too good either, I'd imagine!

But this dare-devil behaviour is not always risk-free. A few months ago I picked up one of these Nyctemera caterpillars to observe it in captivity and rear it to adulthood.

A perfectly healthy and active specimen turned into a lethargic and obviously sick caterpillar in a matter of 36 hours; dozens and dozens of small wasp larvae started to emerge from the caterpillar - they simply tunnelled through the skin! Immediately after emerging these wasp larvae started spinning silken cocoons and pupated next to their hapless and dying host.

Obviously an alert parasitic wasp must have found the caterpillar some weeks ago and deposited some eggs under the skin; the larvae basically chewed their way through the internal fluids and fat reserves of the host, before finishing it all off by totally devouring all internal parts. And you thought that the film "Alien" was all made up?