If you want to drive someone away, then throwing up on them is probably going to do the trick. But the caterpillars of the small mottled willow moth (aka the beet armyworm; Spodoptera exigua) take defensive vomiting to a whole new level. Their puke is both detergent and chemical weapon; its goal is not to cause revulsion but to break through the waterproof layer that its predators find so essential.
Willow moths are attacked by a variety of predatory ants. To study their defences, Rostas and Blassmann reared several caterpillars and exposed them to the European fire ant (Myrmica rubra). After mere seconds, the ants would attempt to bite and sting the caterpillar, which, in response, would regurgitate droplets of fluid at its attackers. If the ants came into contact with the fluid, their assaults would immediately stop and instead, they started to furiously clean themselves. All the caterpillars survived.
The regurgitated fluid wasn't toxic in itself; if some sugar was added to it, it served as a perfectly pleasant cocktail that the ants would happily drink from. So why was it such an effective deterrent?
Michael Rostas from the University of Wurzburg and Katrin Blassmann from the University of Basel discovered the answer by rearing several willow moth caterpillars and 'milking' them for their "oral secretions", in much the same way that venomous snakes are milked for their venom. They found that the fluids are loaded with surfactants, chemicals that are used in detergents. They lower the surface tension of a liquid and allow it to spread more easily over a surface. On a water-repellent surface, a drop of water will sit in an almost spherical bead. But load that water up with surfactants, and it will start to spread out into a flatter disc.
This ability to 'wet' a surface is the key to the caterpillar's defence, which Rostas and Blassmann demonstrated by applying small drops of fluid to the heads of individual ants. If the drops were pure water, they just rolled straight off for ants are covered in a waxy layer that effectively repels water. But if the drops contained concentrations of surfactants that matched those of the caterpillar spit, they spread over the ant's head and enveloped it. To avoid drowning, the ant was forced to clean itself for several minutes.
The tactic doesn't work well against species that attack in large numbers. Another ant, Solenopsis invicta, managed to overwhelm the caterpillars' defences by rapidly recruiting large numbers of other attackers. But Myrmica rubra relies on small numbers of scouts to find food and against them, or other solitary hunters like spiders, the caterpillar's surfactants provide a good baseline level of defence.
It's also possible that the fluid works in other ways. In some cases, the ants' antennae were glued together even after they had cleaned themselves, suggesting that the spit could contain proteins that affects the ants' ability to sense their surroundings.
Throwing up is a fairly common defensive tactic among the larvae of plant-eating insects. Plants use a variety of toxic chemicals to ward off hungry mandibles but habitual vegetarians adapt to these defences. Predators, however, do not and giving them a faceful of unfamiliar plant poisons is a good way of deterring them.
But the willow moth's chemical protection doesn't come from any plant; Rostas and Blassmann raised caterpillars on celery, tomatoes or an artificial diet and found that in all cases, their vomit still acted as a surfactant. Manufacturing the chemicals itself means that the caterpillar's security isn't tied to any specific food source. Indeed, the willow moth has become an important agricultural pest because of its ability to eat more than 50 different species of plant.
The use of surfactants for defence may be new to science, but it may also be a widespread tactic. Rostas and Blassmann tested the larval spit of five other moth species and one species of beetle and all six acted as surfactants.
Scientists have been trying to isolate such chemicals from biological sources as they have a variety of industrial uses and are thought to be generally less toxic than their synthetic petroleum-based counterparts. Bacteria have provided most of these "biosurfactants" but caterpillars could be a rich new source. Ironically, surfactants are also being considered as cheaper alternatives to conventional insecticides and the caterpillar's own secretions may one day provide a way for controlling other agricultural pests. It's yet another case of natural selection beating us to it by several million years.
Reference: Michael Rostás, Katrin Blassmann (2008). Insects had it first: surfactants as a defence against predators Proceedings of the Royal Society B: Biological Sciences, -1 (-1), -1--1 DOI: 10.1098/rspb.2008.1281
Image: Ant by Rostas and Blassmann; Caterpillar from PLoS