The size of T. sylvina is very variable and its wingspan ranges from 31 to 49 mm. In this species, especially in the male, the forewings are orange-brown and carry two narrow, fairly straight and uninterrupted dark-edged whitish diagonal lines that form an inverted 'V'. However, these lines do not touch. The wing cilia are not chequered. The female is slightly larger and is less contrasting coloured than the male. As in all other Hepialidae species, the antennae are remarkably short and the moths lack functioning mouth parts so they cannot feed.

The shiny and dirty-white caterpillar has an orange-brown head and somewhat paler thoracal plates. The stigmata are dark brown to black.

Females of this species scatter their eggs over plants as they fly over them in a suitable biotope. The larva lives on the roots of small herbaceous plants and probably also on grasses. This species hibernates twice as a larva and pupates in the ground. The adults come readily to light.

The larva lives in the roots of various small herbaceous plants like: Rumex spp., Taraxacum spp., Pteridium aquilinum and probably also Gramineae.

This species inhabits moorland, waste ground and other wild places.