This is a great, clear video showing the defense mechanism of a wonderful, little creature. The bombardier beetle is a catch-all name for 500-odd related species of beetle that have a nifty, chemical-warfare defense mechanism built into their rear ends. Basically, the beetles can make their own hydroquinone and hydrogen peroxide, store it in their abdomens and, when threatened, mix the two chemicals to create a potent, heat-generating reaction that forces a boiling, vile-smelling liquid out of the beetle —and into the face of whatever was bothering it.
I was first introduced to the bombardier beetle in high school biology class. See, I went to fundamentalist Baptist high school, and the bombardier beetle is often trotted out as an example of something so complex, that it couldn't have possibly evolved. (I was also given the impression that this was just one, single type of beetle, rather than an array of varying, related beetles that produce and expel chemicals in slightly different ways. But that's not really the worst of the misleading and inaccurate things I learned in that biology class. Textbooks from Bob Jones University Press. I wish now that I still had the thing to scan pages.)
Anyway, I was told that "evolutionists" had no answer for how the bombardier beetle could have evolved. Naturally, that turns out to be bunk. As does much of what I was taught about how the beetle works. TalkOrigins explains the reality of both pretty well. With references! More fascinating, to me anyway, is the way the bombardier beetle actually fits with the predictions made by evolutionary theory:
Creationism, on the subject of design, says little except that similar forms were created for similar functions and different forms were created for different funtions, [Morris, 1985, p. 70] or, briefly, that form follows function. However, that does not describe the pattern we see in nature.
The same function often takes different forms. Many ground beetles have habits and habitats quite similar to centipedes, but the two groups look nothing alike. One group of bombardier beetles (the paussines) uses the same chemical mechanism to shoot their defensive spray as other bombardier beetles, but they have a totally different method of aiming. Brachinine bombardier beetles have their gland openings at the tip of their abdomen and simply bend their abdomen to aim; paussines have their gland openings more to the side, shoot from only the chamber on the desired side, and if they want to shoot forward, move their abdomen slightly so that the opening is adjacent to a flange on their elytra that deflects the ejecta forward. [Eisner and Aneshansley, 1982] Pygidial glands are used for defense not just by bombardier beetles but by virtually all beetles in the suborder Adephaga, but the structure of the glands and the chemicals they secrete vary significantly among different families and genera of beetles. [Forsyth, 1970; Kanehisa & Murase, 1977; Moore, 1979; Eisner et al., 1977]
The same form is sometimes used for different functions. I know of no good examples among bombardier beetles, but rove beetles show an example. Many species exude defensive chemicals from the tip of their abdomen. Beetles of the genus Stenus have another use for those chemicals. When threatened while foraging on water, they touch their abdominal glands to the surface of the water. The chemicals disrupt the surface tension, which rapidly propels the beetle up to several meters. [Eisner, 1970, p. 200]
Finally, some forms have no function. Some species of bombardier beetles (and many other insects, for that matter) cannot fly but still have vestigal flight wings. [Erwin, 1970, pp. 46, 55, 91, 114-115, 119] Some may argue that the wing stubs have an as yet unknown function, but even in the remote chance that functions can be found for all vestigal wings, the situation merely changes to the previous case of different functions for the same form.
Bombardier beetles on Wikipedia
Short piece on the beetles from the Ecological Society of America
The in-depth explanation/refutation of Creationist thought on the beetles by the good folks at TalkOrigins