Bombadier Beetle
Stenaptinus insignis & relatives

What Kind of Animal is It?

The bombadier beetle is an Invertebrate animal (it has no backbone). Its phylum is the Arthropoda, a group with an external skeleton (exoskeleton) made of a hard substance called chitin. The external skeleton allows these animals to have joints and gives them a much greater range of motion than is found in simpler organisms (compare an earthworm with a butterfly). Arthropods can swim, crawl, run, jump and fly.

Bombadier beetles are in the Insect class because they have antennae, 3 pairs of legs and a body divided into a head, thorax and abdomen. Within the insects, they are in the order Coleoptera, the beetles. In the beetles the forewings (called elytra) are hard and form a protective cover for the posterior wings. It is estimated that there are more than 300,000 kinds of beetles, by far the largest order of the insects.

What is its Lifestyle?

Bombadiers are carabid beetles, adapted for running rapidly along the ground or up trees. They feed mostly on other insects and must avoid being eaten themselves.

Like many insects beetles have a comlex reproduction, involving several developmental stages and metamorphosis from a larval stage to an adult forms.

Chemical Warfare in the Bombadier Beetles

The most notable thing about bombadier beetles is that they engage in chemical warfare in a spectacular way, spraying a hot caustic liquid when disturbed. The chemical reactions have been studied in detail and are fairly well understood:


 At Rest:

  • The bombadier beetle secretes 2 highly reactive substances from 2 different glands into a reservoir:
    • Hydrogen peroxide: you can buy this in a drugstore for sterilizing cuts. There is a difference: the bombadier stores it at a 10X higher concentration than the drugstore variety. At this concentration it would produce a nasty burn if you pit it on your skin
    • Hydroquinone: you can buy this at a photography store. It is a powerful reducing agent, sometimes used in film developers
  • These 2 chemicals are stored in the same reservoir. They do not explode because they react slowly under these conditions
  • A second chamber has the enzymes that can accelerrate the reaction to a rapid rate, but a ring of muscle (sphincter) keeps the reactants from mixing with the enzyme

 When Excited:

  • If the beetle is excited the sphincter relaxes and opens
  • Muscles around the starage chamber contract, forcing the reactants out, into the enzyme chamber
  • This allows the reactants to contact the enzymes
  • The reaction accelerates explosively
    • Solution is heated above the boiling point of water
    • The hydroquinone is converted to toxic benzoquinone
    • Heat causes pressure to build up within the chamber
  • The high pressure causes the hot, toxic liquid to spray from two jets
  • The bombadier has muscles with which to aim the jets toward his enemy

When biologists see a mechanism like this, the first question is: how does it work. The second question is likely to be: how did it come about (evolve)? We have answered the first question fairly well, but the second is much more difficult.

This is certainly a complex defensive mechanism, one that you probably would not expect to find in nature. To biologist Michael Behe mechanisms of this sort are examples of "irreducible complexity", that is they are so complicated that they could not have evolved. He beleives that they are evidence of intelligent design (creation) in nature. However, he admits that there are many possible "evolutionary intermediates" seen in other beetles who have only part of the bombadier's elaborate defense mechanism. For example:

This leads most biologists to beleive that the elaborate bombadier defense mechanism could have evolved step-by-step over a period of time. Somehow it is not very satisfactory to explain every tough biological puzzle by saying, "and then a miracle occured".

Some Things to Think About

More Information

For those of you who like to argue, here are 2 websites with quite different ideas on how the bombadier beetle got here:

Back to Homepage

Back to Creatures Index