A caterpillar is an eating machine - a mobile set of mandibles, whose sole mission is to survive long enough to munch its way to adulthood. Standing in their way are spiders, birds and predatory insects that want to eat them, and parasitic wasps that want to convert them into living incubators for their own larvae.
With so many enemies, defence is paramount for caterpillars and the various species have evolved a dazzling array of countermeasures. Some camouflage themselves, others use bright colours to advertise their toxic chemical weapons, which in at least one species is powerful enough to kill a human. They are coated with irritating hairs, throw up their digestive juices, emit foul odours, hang from silken safety lines and recruit ants as bodyguards.
But one group of caterpillars - those of the metalmark moths (Brenthia) - lack any of these. They feed on the topsides of leaves, sheltering only under a flimsy sheet of silk that they themselves spin. Out in the open, they are among the most conspicuous of caterpillars and surely would make easy target for enterprising predators or parasites. But not so; Jadranka Rota and David Wagner from the University of Connecticut found that the metalmark caterpillars use a defensive measure all their own - a wormhole.
The metalmarks chew a small hole in the leaf they feed on, directly under their silken shelter. It's an escape tunnel that allows them to flee to safety of the leaf's underside if danger threatens. The caterpillar senses the arrival of danger with extremely long hairs that protrude from its sides. Those on its rear end are so unfeasibly long that they always touch the silk that surrounds the caterpillar and often form part of the silk web itself. These hairs convert the entire silken tent into a giant sensory organ.
The very hairy caterpillar
Rota and Wagner collected 18 Brenthia monolychna caterpillars from Costa Rica and watched how they reacted when prodded with pencils. Touching the silk web had the same effect as touching the caterpillar itself - it sent the animal scurrying down its "wormhole" within a quarter of a second. Tapping the leaf, or removing the silk and touching the area where it had once been, didn't faze the caterpillar.
The hairs also provide the caterpillar with a way of knowing when danger has gone. After it passes through its escape hatch, it leaves its bottom near the hole so that the long hairs are still on the other side and still touch the silk. These allow the caterpillar to effectively be on both sides of the leaf at once, and monitor for danger while in total safety. If the hairs don't pick up any untoward vibrations for a few seconds, the cautious caterpillar reverses back through the hole.
Only one group of hunters seems to be able to foil the metalmark's defences - parasitic wasps from the family Microgastrinae. They attack the caterpillars at their earliest stage of development before they gain the ability to produce silk at quantity. Without the early warning system provided by these self-spun fibres, the caterpillars are easily ambushed.
Those that survive soon develop the ability to defend themselves and their extraordinary adaptations don't end once they become adult moths. Two years ago, Rota and Wagner found that adult metalmarks mimic jumping spiders. When confronted, they raise their wing markings which are patterned to bear an uncanny resemblance to the spider that hunts it.
Reference: J ROTA, D WAGNER (2008). Wormholes, sensory nets and hypertrophied tactile setae: the extraordinary defence strategies of Brenthia caterpillars Animal Behaviour DOI: 10.1016/j.anbehav.2008.06.024