Beetles With a Deadly Aim
Bombardier beetles are the gunslingers of the insect world. For centuries, these insects have been known for the explosive, boiling-hot discharges they release when harassed. Now it turns out that the beetles not only wield a powerful weapon but are formidable marksmen as well.
Bombardier beetles have two glands in their rear ends, one filled with hydrogen peroxide--used in rocket fuel--and phenol, the other with enzymes. In an emergency, the insect allows the enzymes to mix with the peroxide, releasing oxygen that transforms phenol into a noxious quinon compound and heats the liquid to boiling point. At that moment, the beetle will aim its backside--and shoot. This is unpleasant to human skin and certainly a good defense against biting ants, says entomologist Daniel Aneshansley of Cornell University in Ithaca, New York. "I've seen cases where the animal is walking through an ants' nest and it will clear an area around it with a single spray," he says.
To find out how accurate the insects are at hitting their targets, Thomas Eisner and Aneshansley tethered specimens of an African species, Stenaptinus insignis, from wire hooks. They then set up a camera with high-speed flash units and proceeded to bother the beetles: Using a forceps, they pinched various parts of the insects' bodies. The photos, published in this week's Proceedings of the National Academy of Sciences, reveal that each time, the beetle retaliated with an accurately aimed hot quinonoid spray.
By curling the tip of its abdomen, a beetle managed to strike not just the leg where it was being held, but also the exact segment of that leg, regardless of whether the leg was up or down. The researchers discovered that it was even able to shoot around a corner: Using two disk-shaped "reflectors" on either side of the nozzle, the beetle could deflect its spray to hit hard-to-reach spots, like the top of its own head.
William Agosta, a chemical ecologist at The Rockefeller University in New York City, is impressed. "The photography is beautiful!" he says. But he points out that the pictures can't solve one old riddle: how the beetle itself remains unscathed by its horrific chemical warfare.