Monday, May 13, 2013

Bombardiers, True and False


One advantage of country living is that you don't always have to go out to find nature - sometimes it comes in to find you.  Last Friday I saw this ground beetle crossing the living room floor, intent on finding safety under the couch.  It wasn't too happy to end up in the box, but at least it didn't let fire its defenses at me.

This is a false bombardier beetle, most likely Galaretia bicolor or possibly G. janus.  The two species are both common and similar in appearance.  The head and pronotum shapes are somewhat different, but I don't have the ability to separate them without a key and a lot more knowledge.  My guess is G. bicolor but go to bugguide.net and make your choice.


Galaretia are said to be found in forested areas, in or under decaying or rotting timber.  This didn't give me much confidence in our cabin flooring until I found a number of sites recording finding them in houses and basements.  They are carnivorous predators, eating primarily caterpillars and other insects.  I imagine they think they are in a cafeteria in our creek house which is seemingly porous to all invertebrates (and a few vertebrates of mouse-like inclinations). 

False bombardier beetles demonstrate that chemical warfare is nothing unique to humans.  It has glands on the abdomen which produce formic acid with a little acetic acid thrown in for good measure.  This produces a noxious odor, discouraging predators and can cause a burning sensation on the skin.  They have two side-by-side glands and nozzles and can selectively spray the side that is being attacked, such as holding a leg with tweezers.  They hold the other side's gland in reserve and can administer 6 doses before running dry.

With armament like that, you might ask "then what does a true bombardier beetle have up its abdomen?"  The true bombardier beetles are smaller but don't let that fool you.  Their weapons are much more powerful, with separate chambers holding hydroquinones and hydrogen peroxide, suddenly mixed together with oxidative enzymes just as they are fired.  The chemical reaction heats the spray to near boiling temperature and the gas powers the spray.  To quote the Bug Lady, "drop for drop, the chemical is more potent than skunk spray, and a toad that is sprayed in the mouth gags, sticks out its tongue and rubs it against the ground."

"Just chillin' out"
Like many other insects, these beetles don't like having their picture taken.  Many species will fly away just before you finishing focusing on them.  In this case, Galaretia seldom fly but scamper about quickly.  After an hour in the refrigerator slows them down for several minutes there is enough time for taking their portrait.  The insects aren't harmed, escape once they warm up and any goose bumps they have don't show at this magnification.

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