This is the only species of this widespread genus to be found naturally in Britain; some other tramp species may occur sporadically. The workers are small (2.5-4 mm) and dark brown or black with a robust morphology typical of myrmecine ants. All castes have a two-segmented 'waist' and pectinate spurs on all tibiae, along with propodeal spines and a sting in the females. Despite its size, T. caespitum is an aggressive ant and uses its sting freely; it is often the dominant ant species where abundant. There is a marked dimorphism between the castes with males and gynes being much larger than the workers at 5.5-8 mm. There is normally only one queen per colony.
This species occurs predominantly along the south coast of England and up the west coast as far as North Wales, the Isle of Man, Ailsa Craig, Canna and West Sutherland. On the east coast there are records as far as north Norfolk, with a notable gap until the Berwickshire coast. The species is also recorded from coastal sites along southern and south-eastern Ireland. Inland, T. caespitum is found on some lowland heaths and south-facing hillsides in southern and eastern England.