Science: Beetle Artillery

The bombardier beetle is an inconspicuous insect that ranges much of the world with calm and self-assurance—and for jood reason. When attacked by a ferocious ant, its natural enemy, the bombardier beetle (Brachinus) merely stands its ground, pushes a flexible tube from its rear end and points it at the enemy. With a small but audible bang, a cloud of acrid vapor envelops the ant, reducing it to paralysis or trembling confusion. Until recently, the bombardier beetle’s efficient defensive weapon was pretty much of a mystery. Entomologists thought that it simply squirted out a liquid that exploded on hitting the air. But in West Germany’s Angewandte Chemie, Dr. Hermann Schildknecht of Erlanger University’s Institute for Organic Chemistry has revealed the bombardier’s secret: it is armed with two genuine cannons, each with a strong chamber for real internal explosions.

Dr. Schildknecht, a famed expert on delicate chemical analyses, some years ago devised a set of instruments so perfect that he yearned to try them on organic matter that had long defied other chemists. He remembered the bombardier beetles, which he had known when his father took him to hear their small artillery on Sunday afternoons. Enlisting his wife and 17 students, Dr. Schildknecht searched a limestone region near Bayreuth and collected a good supply of the beetles. After training himself in the art of insect surgery, he learned how to extract intact the complicated plumbing in their behinds.

Each of the paired cannons, he found, has glands that discharge a fluid into a saclike reservoir. Using his best microtechniques, Dr. Schildknecht next analyzed the fluid and found to his amazement that it was about 10% hydroquinone and toluhydroquinone (acrid compounds related to carbolic acid) and 23% hydrogen peroxide. When mixed in a test tube these chemicals reacted spontaneously, giving off copious gas, but something still unknown keeps them from reacting as long as they lie undisturbed in the beetle’s ammunition sacs.

After it pokes out its cannon to meet threatening danger, the bombardier simply opens an internal valve and forces some of the stored fluid into a small, strong-walled combustion chamber, where it is “ignited” by enzymes from glands lining the chamber. The peroxide quickly decomposes, giving off oxygen gas at considerable pressure—and shooting out of the cannon a loud, offensive discharge that makes the bombardier the insect kingdom’s biggest gun.