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.

The adult sawflies are black and brown with transparent wings and the larvae are yellowish-green and resemble caterpillars.

When disturbed, they characteristically cling to the leaf with the front three pairs of legs and curl their bodies upward in a "S" shape.

There may be several generations of larvae between spring and autumn and the insect overwinters underground in the pupal state.

The hemolymph was found to repel Myrmica rubra ants, however many vertebrate and invertebrate predators feed on the larvae.