Spectember 24: Halloween Butterfly

The Lycaenidae, or gossamer-winged butterflies, are a large family that includes many familiar butterfly groups, such as the coppers, the blues, and the hairstreaks. Many of them live underground as caterpillars, protected and and fed by ants in exchange for a sweet liquid they give off. Not all gossamer-winged butterflies, however, need such protection. The Halloween butterfly (Pyropapilio ekrixis) of Australia is another member of this family, but one with a deadly weapon. 

The Halloween butterfly is remarkable for being the only animal known to generate fire as a defense mechanism. While a far cry from the fire-breathing dragons of legend, it is nevertheless unique in the history of evolution. During its caterpillar phase, the Halloween butterfly is capable of emitting a small but potent burst of flame from its abdomen. This is usually sufficient to, if not badly injure, then at least dissuade the attentions of any would-be predators, such as birds, lizards, and larger insects. When threatened, the caterpillar arches itself into a "handstand" pose, and raises its abdomen into the air. If the predator is not put off by this display, it blasts a jet of fire at its attacker. 

The fire is generated through the production of diethyl zinc, an organozinc compound that combusts upon exposure to oxygen. The Halloween butterfly's ancestors seemingly evolved the ability to concentrate this compound as a secondary byproduct of their metabolism. As the compound is flammable upon contact with air, it would have had to be vented periodically, and this in turn led to the evolution of an air-tight chamber in the abdomen that could hold the diethyl zinc until it needed to be released. The resulting jet of fire, which blasts out of the caterpillar's abdomen, can extend for up to eight inches. 

The adult butterfly lacks the fire-producing ability of the caterpillar, and has a lifestyle that is for all intents and purposes similar to that of any other butterfly. However, it is less likely to visit flowers, and tends to favor feeding on tree sap and aphid honeydew. The caterpillar, on the other hand, feeds on the leaves of Eucalyptus trees, making it one of the few caterpillars to do so. The Halloween butterfly is yet another example of the deadly and bizarre wildlife that has earned Australia its reputation around the world.