Roy writes:

> Unlike Denys, I have no trouble with the eye; there are still creatures
>  out there with the first primitive nerve pits, and ones with the six or
>  seven different goes that nature had at inventing the eye in different
>  ways and different places.

Photoreceptive eyes have been estimated to have evolved 39 times 
independently, with complex, fully image-forming eyes having evolved independently three 
times, once each in the molluscs, the arthropods and in the vertebrates.


  
>  But the bombardier beetle does indeed seem unlikely; I wonder if there
>  are creatures out there with protean versions of its capabilities?
>  Though I can see why we might never discover the shattered remains of
>  the failed mutations that didn't go in quite the right direction :-)

If you go to Google and type:

     bombardier beetle evolution

you will be overwhelmed with creationist nonsense.  Apparently they have 
taken these animals up as their most recent proof that evolution could not work. 
The argument is that the system of chemical defense is too complex to have 
evolved on its on, nor could it have worked at any intermediate stage, piecemeal 
and incomplete. Given these few statements, the only logical conclusion is that 
only God could have made the beetle.

Of course, none of that is true. Bombardier beetles are in the family 
Carabidae (ground beetles), a very large family of insects, and the carabids are just 
one family in the suborder Adephaga, containing about 40,000 species. All 
ground beetles and other adephagan beetles have paired pygidial glands that 
produce a wide variety of chemical compounds used in chemical defense.

Chemical defenses are not rare in insects, as anyone who has been stung by a 
bee, ant or wasp would know. But unfortunately, hardly anyone puts beetles in 
their mouth, so they don't have first-hand experience with the exceedingly 
distasteful quality of most ground-dwelling insects' defenses. Most insects, 
including most adephagan beetles, do not propel their defensive chemicals. Rather, 
they only ooze their defensive chemicals from their skin, and of those 
defenses, quinones are among the most common of chemicals. From this little bit of 
knowledge, it's not hard to imagine that the bombardier beetles are only the 
most exaggerated form of an exceedingly common defensive structure.

For more than you might want to know on this group of animals, please see:

http://www.colostate.edu/Depts/Entomology/courses/en570/papers_1996/miller.htm
l

The person most well-known for his work on the chemical ecology of insects is 
Thomas Eisner of Cornell. One of his recent papers on bombardier beetles is:

http://www.pnas.org/cgi/reprint/96/17/9705.pdf

He outlines the ecological impetus for the evolution of the beetles' 
extraordinary talents in the first few paragraphs:

"Beetles have a problem. Unlike flies, butterflies, dragonflies, and many 
other insects, they cannot as a rule take instantly to the air. To activate the 
wings they must first unfurl these from beneath the wing covers (the elytra), 
and this takes time. Delays are unaffordable in emergencies, and it should come 
as no surprise that many beetles have evolved means for ‘‘buying time’’ 
when under attack. The species of the family Carabidae, the so-called ground 
beetles, are a case in point. Living at soil level, carabids are in constant 
danger from ants, against which they are protected by their dischargeable defensive 
glands (1–3). Paired devices, these glands take the form of more or less 
capacious sacs, lying side by side in the abdomen and opening on the abdominal tip 
(4, 5). Diverse toxicants are produced by these glands, often at high 
concentrations, including acids, aldehydes, phenols, and quinones (2, 3). Most 
carabids are able to eject these fluids forcibly, in the form of
sprays (5–9).

"Ants can attack from virtually any direction and, for maximal effectiveness, 
need to be targeted to be repelled. Not surprisingly, many carabids have the 
capacity to aim their spray in different directions (5–9). None are perhaps 
better marksmen than the so-called bombardier beetles, as we document here 
photographically for one species, the African Stenaptinus insignis.

"The spray of bombardier beetles contains p-benzoquinones (10), compounds 
well known for their irritant properties (11). A single bombardier beetle can 
discharge upward of 20 times before depleting its glands (6). The discharges are 
accompanied by audible detonations, and they have been shown to be potently 
deterrent to a number of predators, including ants (6, 12–15)."

Wirt Atmar

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