Image: Courtesy of William McGinnis
 LOTSALEGS: The six-limbed fruit fly, Drosophila melanogaster

As school children are commonly taught, adult insects have six legs. No more, no less. But there's more to this story. The class Insecta is descended from multilimb ancestors, and most other living arthropods, including Insecta's closest living relative, the Crustacea, usually have at least five pairs of legs or leg-like appendages.

How insects lost their limbs has interested those in the burgeoning field of "evo-devo," the study of developmental evolution. In particular, biologists have suspected that the insects' progression to six legs, and the evolution and modification of arthropod appendages in general, are driven by changes in Hox gene activity. Hox genes are clusters of regulatory genes that control development by turning other genes on and off. Biologists had attributed changes in the arthropod body plan to changes in Hox gene expression, not changes in the Hox...

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