Tetramorium atratulum is a rare workerless socially parasitic ant from the Palaearctic region, which has even been introduced together with its host in North America. This extreme inquiline is represented only by female and pupoid type male individuals, whose morphology and anatomy indicate a highly specialized level of parasitism. The body of males is depigmentated, the cuticle is thin, the petiole and postpetiole are widely connected, and degenerate mandibles, palps, and antennae are observed. Female wing venation is reduced and the occipital region is narrowed. Mature females are typically physogastric and found in queenless host nests.