|The divine comedy|
Bombardier beetles are over 500 species of ground beetles (the tribes Brachinini, Paussini, Ozaenini, or Metriini within the family Carabidae) that are most notable for their defense mechanism: when threatened, these beetles eject a noxious chemical spray (consisting of hydrogen peroxide and hydroquinone) in a rapid, pulsating burst from special glands in the abdomen.
 The science
The proven science behind the beetle's defenses is that it uses a mixture of two enzymes, hydroquinone and hydrogen peroxide that it mixes together (a catalase to decompose the hydrogen peroxide, and a peroxidase to oxidize the hydroquinones and thereby break them down into the simpler quinones), causing the mixture to heat to nearly 100°C. Inside the insect's defense system, there are two chambers: the larger inner chamber (called the "reservoir" or "collection bladder") empties into the smaller outer one (called the "vestibule" or "explosion chamber"), which in turn empties into the outside world through an opening near the anus. There are two sets of these organs, one on each side of the anus. The collection bladder collects hydrogen peroxide and hydroquinone. The explosion chamber collects a brown gooey mixture of the two enzymes and has a thick chitin wall with numerous little holes in it through which single-celled glands secrete and deposit the enzymes into the chamber. When the insect's defenses become stimulated, a muscle opens up and through this opening, the two chemicals are forced into the explosion chamber, where the enzymes cause them to explode out of the insect's derrière as oxygen, quinone, and water.
From an evolutionary standpoint, hydrogen peroxide is a normal metabolic byproduct found in insects and various quinones are used to harden (or "sclerotinize") insect cuticles. As a byproduct, hydroquinone tastes bad to predators and, for example, is the chemical that makes stink bugs (family Pentatomidae) stink. So, if an insect's cuticle became indented, forming little sacs to store some of this hydroquinone, it would have an evolutionary advantage over its fellows. Furthermore, it is a well-known trait of the carabid family (of which the bombardier beetles are members) to have bladders for the secretion of various poisons. Therefore, from an evolutionary standpoint, it was only a minor evolutionary step for the bombardier beetle to hold both of these chemicals and, all they would need to do, is evolve a ready supply of the mixture enzymes.
 Creationist nonsense
While there is ample research into the beetle's evolutionary background and ample proof to back it up, creationists have held up the beetle as a prime example of irreducible complexity, citing the false belief that, having both chemicals inside its body at the same time would cause the beetle to burst had it not been by the grace of God that He created it with two sacs in which to hold the chemicals. The Institute for Creation Research has even written a children's book called Bomby the Bombardier Beetle which attempts to teach this nonsense to children.