Early Thorn

The wingspan is 28–40 mm. The wings are scalloped and there are 3 crosslines on the forewings. There is a dark stain at the apex of the forewing. Adults are extremely variable in colour. Distinguished by the less deep excision between the radials of the hindwing, less rich colouring, enlarged costal spots at the origin of the lines, a median area not differentiated in colour from the distal area, transparent discal marks not well developed. The spring form is large, strongly irrorated with fuscous, sometimes more olivaceous.the median and postmedian lines darkened into a band. -ab. illunaria Esp. is the summer generation, much smaller and much lees irrorated. Sometimes pale, sometimes warmer brown. - ab. minima Strand is a dwarfed form, sometimes sharply marked, frequent in Arctic Norway.

The larva is orange-brown or reddish brown, sometimes inclining to purplish; mepo- and metathorax enlarged laterally, 4th—5th abdominal swollen, with small dorsal humps; anterior segments marked with bright orange.