The parasitoid has a noticeable constriction between the head and the thorax, and the thorax and the abdomen. The abdomen tapers to a sharp tip. Antennae are elbowed and the tips are dark and swollen. Females are reddish-brown with black eyes. Males are black with long antennae approximately the same length as the body.
Females insert eggs into fungus gnat larvae.
They can develop in each of the three larval instars. S. pauperi is a solitary parasitoid, with only one parasitic wasp emerging from each fungus gnat pupa. The parasitized larvae live until pupation, then die, after which the wasp pupates. The parasitoid has been demonstrated to be a useful biological control agent of fungus gnats in Swedish greenhouses.