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HOW did insects come to have only six legs? American developmental biologists
have now answered this question. By inactivating two genes, they persuaded
beetle larvae to sprout 16 extra legs, recreating limbs their ancestors lost
around 400 million years ago.

Insects and other arthropods evolved from animals resembling
modern millipedes and centipedes, which have many separate, unspecialised body
segments, each with their own pair of legs. In insects, six segments fused to
make the head, three with two legs each formed the thorax, while between eight
and eleven combined to form the legless abdomen.

With only six legs insects were far more agile than their
multi-legged predecessors, and researchers suspected that insects must have
turned off some leg-making genes as they evolved. Now, Randy Bennett of Brigham
Young University in Provo, Utah and his colleagues have shown the key lies in
two genes, Ultrabithorax(Ubx) and abdominal-A (
abd-A). Switching off these genes in relatively primitive flour beetle
larvae made them sprout 22 legs, 16 on the abdomen as well as six on the
thorax.

Many insect larvae develop stick-like “prolegs” on their abdomens, which help
them move their long bodies around. These prolegs then disappear in the adult.
“But when we take out both the genes, the insect has real legs on all its
abdominal segments,” Bennett says.

He believes the two genes have distinct functions in insects, with the
abd-A gene affecting whether or not the limb grows at all, and the
Ubx gene modifying limb growth. While other animals have the genes, their
function is unclear.