It differs from the poplar kitten (Furcula bifida) in its generally smaller size, but more especially in the shape of the black line forming the outer margin of the central band; this is always more or less angled or dentate towards the front margin of the wings, whereas, in the poplar kitten, this portion of the line forms a clean curve.

The blackish grey median band is sharply edged with black, its distal sinus usually extends only to the apex of the cell; beyond the middle 3 parallel dentate lines.

In borealis Borkh., from Lapland and Northern Russia, thorax and wings are blackish smoke-colour; specimens from Amurland also are somewhat darker than those from Central and Southern Europe.

[1] The dark purple, hemispherical eggs are laid in batches of two or three on the upper surface of leaves in May or June and again in August.

The pupa is dark purple-brown and pupation takes place in a tough cocoon constructed from a mixture of chewed wood-pulp and silk.