The researchers isolated two genes involved with leg development. Under a microscope, they were able to see the two genes were activated in the legs while the spider was still an embryo. Next, using RNA interference, they were able to block the activation of the two genes in several samples. The spiders grew up with short legs, and the short legs became pedipalps, limbs typically used for handling food. The researchers also identified and disabled another gene linked to leg development, which led to spiders with short legs that did not become pedipalps but who did lose their tarsomeres—joints that...