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If there’s one defining trait of daddy longlegs, it has to be their legs. Now, scientists have shortened the legs of one species—and turned them into food-handling limbs—by tweaking the arachnids’ DNA expression.

To figure out which genes cause these spider relatives to develop long legs, researchers assembled the first draft genome of Phalangium opilio and looked at three genes that act as a blueprint for where various body parts should go. When they traced the activity of two of those genes, they found they were turned on in the legs of embryos under a microscope. Next, they used RNA interference—a technique that reduces gene expression—to knock them down in hundreds of developing P. opilio embryos. When they did so, they saw that in the surviving hatchlings, six of eight legs were about half their normal size, they report today in the Proceedings of the Royal Society B: Biological Sciences. What’s more, they had transformed into short pedipalps, a type of limb for food handling.

When the researchers knocked down a third developmental gene, thought to help build embryonic legs, the limbs didn’t turn into pedipalps, but they did get shorter. They also lost their tarsomeres, a set of about 100 knuckle-like joints that can wrap around sticks like a monkey’s tail. Similar transformations have been seen in fruit flies, which may give scientists a clue to figuring out how P. opilio’s legs may have evolved.