Bugs have been eating plants for over 300 million years. But insects that actually live within the plants they prey on are generally assumed to have emerged about 120 million years ago. Now paleobiologist Conrad Labandeira at the National Museum of Natural History in Washington, D.C., has found signs of an insect larva that apparently resided within tree-fern fronds in the swamp forests of Illinois some 300 million years ago. Modern insects like the sawfly deposit their eggs inside a plant’s leaf or stem so that the larvae will have plant cells to feed on when they hatch. Since the plant responds to injury by surrounding the wound with protective cells that also happen to be highly nutritious, the larvae trick their host into providing them with a choice meal. Labandeira recognized the hallmarks of a similar strategy while studying slices of the 300- million-year-old fossilized fronds. Some of the ferns bore scars left by grubs of some kind that had tunneled through the fronds. The grubs chewed their way out before the ferns fossilized, but they left behind feces (dark dots in photo), which, Labandeira found, contained resin-filled cells identical to surrounding plant cells. Says Labandeira, This is a unique window into the behavior of a very early insect.