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Bagworms

Ambika Kamath | Updated on October 18, 2014

Hanging by a silk thread: A bagworm waits to ‘balloon’ with the wind. Photo: Ambika Kamath

The pivotal role of the wind and of ‘ballooning’ in the life of bagworms

Those of us whose livelihoods do not depend directly on nature sometimes forget how powerful the weather can be. It takes something as catastrophic as a flood, cyclone or drought to remind us that our survival is contingent on unpredictable and uncontrollable physical processes. A far less destructive way to appreciate the dependence of all living creatures on the weather is to understand the unusual life of the bagworm.

A bagworm begins its life by spinning a little silk cocoon around itself. It then proceeds to stick little pieces of vegetation on this cocoon, forming an odd agglomeration of sticks and leaves, with a hole at either end to facilitate the fundamental bodily processes of eating leaves and expelling waste. Male bagworms eventually metamorphose into winged moths. But adult females don’t grow wings even after metamorphosis and cannot fly to locate a mate. Instead, they release a strong scent, a pheromone, which helps the males navigate to them.

However, since they can’t fly, the females can’t choose where to lay their eggs. And when the next generation of bagworms is born, they can’t choose where to live either. Walking around encased in a jumpsuit of plant matter is hardly an efficient way to move; so young bagworms can’t search for a home among multiple trees. Instead, they harness the power of physical forces to disperse. They suspend themselves from a silk thread and get carried to another tree by the breeze, behaving like tiny balloons. ‘Ballooning’ lets bagworms travel much farther than they could by walking.

But in exchange for convenient travel, they surrender to the unpredictability of the wind the choice of the tree on which they will feed and on which their children will be born. Similarly, bagworm reproduction, which depends on the spread of pheromones, is subject to the whims of the wind — a big gust can scramble the signal sent out by females that enables males to find them. To counter the odds and regain some control over their lives, they have evolved a few measures.

Even if they can’t pick where they balloon to, bagworms can pick when to balloon. Females, who can’t travel far except by ballooning, are more likely to balloon than males. And both male and female bagworms make fairly sophisticated decisions about when to balloon. Individuals are more likely to balloon when sharing a tree with many other bagworms, choosing to move from trees that lack leaves to eat; the risks of reaching an unsuitable tree are outweighed by the costs of competing for food. And some species of bagworm have evolved a radical way to avoid the vagaries of wind-assisted reproduction — the females do away with mating altogether, producing babies that are little clones of themselves instead of babies that are genetic mashups of mother and father. None of these measures eliminate the role of wind in their life; rather, they represent a compromise between capriciousness and control. It’s a bit like closing all the windows of your house during a dust storm — you know the dust will find its way in, but leaving the windows open would signal defeat at the hands of the weather that you aren’t quite ready to admit.

( Ambika Kamath studies organismic and evolutionary biology at Harvard University)

Published on October 17, 2014

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