Craesus septentrionalis, the flat-legged tenthred or birch sawfly, is a species of insect in the order Hymenoptera, the suborder Symphyta and the family Tenthredinidae. It was first described by Carl Linnaeus in 1758. The adult sawflies are black and brown with transparent wings and the larvae are yellowish-green and resemble caterpillars. The larvae feed on the leaves of various species of deciduous tree.