Not quite as large as Biston betularia, this is nevertheless one of the largest geometrids in our area. The caterpillar is somewhat crooked in shape. It has convincing "leaf scars" on segments 5 and 8, while its spiracles resemble the lenticels on an alder twig. The third pair of thoracic legs are large and conspicuous and the head seems to bear a pair of powerful forward-projecting "jaws", giving the caterpillar a rather menacing look. The "jaws" are actually antennae and not at all as menacing as the caterpillar doubtless intends.