Early instar larvae of Birch skeletonizer are legless and feed within leaf tissues.
Later instars have fully-functioning legs and feed externally.
Note the presence of silken molting webs. Principal host is white birch, but other native and exotic birches may be attacked. Look for late season browning of foliage and premature leaf drop. By late summer, skeletonizing by the light green larvae causes conspicuous browning. Insects overwinter as pupae in ground litter.