Chemical warfare is nothing new to insects--or the researchers who study them. But now it turns out that the pupae of the squash beetle can concoct an arsenal of chemical deterrents with a technique human chemists thought they had a monopoly on: combinatorial chemistry, in which hundreds of different compounds can be assembled from the same set of basic chemical building blocks. The finding  is reported in today's issue of Science by a group led by Cornell University organic chemist Jerrold Meinwald.
The pupae deploy their hundreds of defensive chemicals in droplets secreted from glandular hairs. The team discovered that the deterrent polyamine compounds were formed from simpler subunits called (w-1)-(2-hydroxyethylamino)alkanoic acids. The pupae seem to have linked the subunits head to tail, in random order and varying proportions, to form scads of molecular rings.
Because the large ring compounds are too heavy to evaporate, they collect in the defensive droplets, where the chemical cacophony continues. By analyzing secretions of different ages, the researchers found that over time, the rings isomerize (flip bonds) to form compounds with the same molecular formulas but different structures. When combined with newer rings pumped out by the pupae, these isomers add to the potent cocktail that deters predators.
"It's really pretty nifty" for evolution to have come up with this way of upping chemical diversity, says Cornell chemical ecologist Thomas Eisner, a co-author. The mix-and-match approach might also have a practical payoff for humans, adds organic chemist Gordon Gribble of Dartmouth College in Hanover, New Hampshire. It could be used against pests someday, Gribble notes, "much like we use DEET to repel mosquitoes."