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Do Polar BearsGet Lonely?

THE LAST WORD

Eaves droppings

While repairing a gutter on my roof,

I spotted a small patch of what

looked like tiny eggs under the brick

eaves (see photo, right). The patch

was no larger than 2 centimetres

by 1 centimetre. Each egg was the

size of a pinhead. What are they,

what creature is responsible and

are they common in the UK?

■ These are the eggs of the vapourer moth (Orgyia antiqua), a member of the family Lymantriidae. This is a widespread but fascinating moth that is common in the UK. Females of this species are flightless, and as soon as they have emerged from their cocoon they emit a pheromone (hence the name “vapourer”) to attract a male, which has wings. After mating, the females lay their eggs on the cocoon from which they emerged.

Dispersal in this species occurs during the larval – or caterpillar – stage. The young larvae spin a silk thread which is picked up by air currents and carries the larva away, a technique known as

ballooning. Older larvae can move around to look for food plants and pupation sites.

It is not unusual for the vapourer to pupate – and thus emerge and lay eggs – on the sides of houses. The caterpillar

seeks a pupation site that is sheltered but from which the female’s pheromone can easily be broadcast. The house probably acts as a surrogate tree trunk.Martin Harvey

Department of Life Sciences

Open University

Milton Keynes, UK

■ These eggs were probably laid by a vapourer moth or, if not, a related species with a similar life cycle. Female vapourer moths are flightless, so it would be extremely arduous for them to seek out a suitable food plant and then move from plant to plant laying the eggs singly, as winged moths often do. Instead they lay their eggs in a mass as we see here, but more usually on a tree. There are two species of vapourer moths in the UK, and eight in mainland Europe.

The gypsy moth (Lymantria

dispar), recently returned to the UK after a long absence, adopts

a similar strategy, though gypsy moth females can fly if necessary. Their egg masses are at least twice as large and covered with protective hairs from the female’s body, because moth eggs are readily eaten by the larvae of parasitic wasps if the adult wasps can get to them to lay their eggs.

Moths and butterflies generally avoid a situation in which they could produce too many caterpillars for a given plant, as there would then be a danger of them exhausting their food supply prematurely. However, this sometimes happens when there is a local population explosion, which probably explains why these eggs are up in the eaves.

How flightless females travel up to house eaves is an interesting question. Pairs of butterflies can sometimes be seen flying while copulating, and I have seen other moth species with flightless females – in particular the winter

moth, Operophtera brumata – doing so. So it is probable that in this case the male deposited his partner up on the roof after mating. Failing that, she could have walked up there herself from ground level in a matter of minutes.

The high perch enables the caterpillars to disperse by ballooning. Vapourer moths can eat a variety of foods, so it doesn’t matter where they land, as long as it is on or near a deciduous tree or shrub. By searching or beating the lower branches of trees, particularly oak, in the spring one can usually produce a few examples of the distinctive caterpillar, which is covered in long hairs and colourful hair tufts, including a pair of tufts at the head end that look like horns.Terence Hollingworth

Blagnac, France

This week’s question

SPLODGE…

I was having a discussion with my mates about what would happen if you filled a swimming pool with jelly and jumped in. Some of the group believe you would sit happily on the surface. Others, myself included, reckon you would sink, and risk drowning as the jelly collapsed around you. We wouldn’t want anybody to be harmed, so we don’t recommend experimenting to find out. But do any New Scientist readers have a theoretical answer to the question? Ross

Bristol, UK

Last words past and present, plus questions, at www.last-word.com

“Vapourer moth larvae spin a silk thread which is picked up by air currents and carries the larva away”