The bombardier beetle is slow. Like many other beetles, its wings are held underneath a hard outer shell. In order to fly, it must first unfurl its wings from beneath that shell. This takes time, and that provides a window for other creatures to attack. Some insects get around this problem with camouflage. The bombardier beetle responds with chemical warfare: that is, by squirting acid out of its abdomen. Boiling acid.

The greatest danger to the bombardier beetle comes from ants, which attack in swarms. Since ants can come from any direction, or all directions at once, the beetle needs a mechanism which can kill multiple attackers approaching from any side. Accordingly, the beetle can aim its corrosive spray with great accuracy. If an ant were to attack the lower segment of the beetle's hindmost leg, the beetle could aim its chemical spray at exactly that part of that specific leg. In fact, it can target any one of its legs, or the top of its body. It can't twist the end of its abdomen abdomen far enough to shoot straight over its back, but it doesn't need to. It has a pair of reflector plates attached to its abdomen which it uses to direct the stream of acid precisely at its target.

The spray is produced within a pair of chambers in the beetle's abdomen. One of these chambers contains hydrogen peroxide and hydroquinone. The other chamber contains two enzymes called catalase and peroxidase. If another insect threatens the bombardier beetle, the beetle will respond by mixing the two chemicals. The enzymes cause the hydrogen peroxide to decompose into water and oxygen. This chemical reaction releases heat, which raises the fluid and gas inside the chamber to 100 degrees Celsius - in other words, boiling. The boiling temperature causes the oxygen gas to expand. The discharge mechanism works in much the same way as a pellet gun or air rifle. The expanding gas forces the fluid out at the offending predator. Interestingly, this is not a one-shot defense like a bee's sting. The beetle carries enough chemicals in its reserves to discharge 20 times.

For all its firepower, the bombardier beetle appears innocuous. For something a bit more frightening, try my article on the anglerfish.

References:

National Academy of Sciences - Spray Aiming in the Bombardier Beetle

Dallas Zoo - Bombardier Beetle

Radioactive symbol from publicdomainpictures.net