The adults grow up to 7–10 millimetres (0.28–0.39 in) long and can be encountered from April to May. The head and thorax are black, while the abdomen and wings are reddish orange.
The larvae are yellow and bristly with black markings.

This species is very similar to Athalia rosae (Tenthredinidae), but in Arge ochropus the antennae are composed of three sections, the third of which is greatly elongated, while in Athalia rosae the antennae are composed of 10–11 items.
Arge pagana is mainly black above.
The adults grow up to 7–10 millimetres (0.28–0.39 in) long and can be encountered from April to May. The head and thorax are black, while the abdomen and wings are reddish orange. The larvae are yellow and bristly with black markings.
This species is very similar to Athalia rosae (Tenthredinidae), but in Arge ochropus the antennae are composed of three sections, the third of which is greatly elongated, while in Athalia rosae the antennae are composed of 10–11 items. Arge pagana is mainly black above.
Arge ochropus