The bombardier beetle, a small ground beetle found across most continents, possesses a unique reputation in the insect world due to its remarkable chemical defense. When threatened, these beetles eject a hot, noxious spray from their abdomen with an audible popping sound. This distinctive ability raises a question: what animals can successfully prey on an insect armed with such an explosive, boiling-hot deterrent?
The Bombardier Beetle’s Defense System
The beetle’s defense mechanism relies on a precise chemical reaction occurring within its body. It stores two separate chemical compounds, hydroquinone and hydrogen peroxide, in distinct internal reservoirs located in its abdomen. When the beetle senses a threat, these chemicals are rapidly mixed in a hardened reaction chamber. Here, catalytic enzymes instantly trigger an exothermic chemical reaction.
This reaction causes the mixture to heat rapidly, reaching temperatures near 100°C. The reaction also generates gas, which builds pressure and forcefully expels the scalding liquid as a pulsating jet. Some species, like the African bombardier beetle, can even rotate the tip of their abdomen to direct this irritating spray with considerable accuracy, firing it in almost any direction. A single beetle can discharge its chemical spray multiple times, sometimes up to 20 consecutive times, before depleting its reserves.
Vertebrate Predators
Despite the bombardier beetle’s potent spray, some vertebrate predators have developed ways to overcome this defense. Toads and frogs, opportunistic feeders, are examples. Studies showed that after swallowing a bombardier beetle, toads often vomit it back out, sometimes hours later, due to the beetle releasing its chemical spray internally. The vomited beetles frequently survived the ordeal, with some living for an extended period after being expelled.
Larger beetles tended to escape more often from toads, and smaller toads were more likely to vomit their prey. Beetle survival may involve a high tolerance to gastric juices or the chemicals affecting the toad’s digestion. Birds, such as jays, also encounter bombardier beetles, and some have learned behavioral tactics like “anting” or “sand-wiping” to disarm the beetle by inducing it to spray into their feathers or the ground before consumption. Lizards employ quick, precise strikes to incapacitate the beetle before it can deploy its spray.
Invertebrate Predators
Within the insect’s own world, certain invertebrate predators have evolved specialized strategies to deal with the bombardier beetle’s defense. Orb-weaving spiders demonstrate different approaches to capturing these beetles. Some spiders may directly grasp the beetle and attempt to bite it, but they are often repelled by the defensive spray, allowing the beetle to escape from the web.
Other orb-weavers employ a more indirect method by first wrapping the beetle in silk. This strategy immobilizes the beetle from a safe distance, preventing it from spraying effectively. The silk wrapping also provides a protective barrier for the spider when it eventually moves in to deliver a bite, trapping much of the discharged fluid.
Praying mantises, known for their ambush hunting, can quickly seize bombardier beetles with their raptorial forelegs. Mantises immediately release the beetle after being sprayed, often grooming affected body parts, indicating the spray’s effectiveness. Experiments show that if a bombardier beetle cannot spray, it is readily consumed by a praying mantis, confirming the spray’s role in its defense against these agile predators.