From the 2 samples, the researchers were able to determine the haploid chromosome number to be N=9 but could not find other differences.

Similar to most ant species, the caste system consists of workers, male, and a queen.

Compared to other species in the Linepithema genus, males of L. fuscum have a smaller, more rounded propodeum with a straight to the convex posterior face.

Furthermore, the wing venation pattern of the males is similar to that of the queens and consists of two elongate submarginal cells.

Some of the key characteristics useful for diagnosis of L. fuscum workers include sparse to absent metapleural pubescence, variable propodeal shape, 0–9 standing setae, and well-developed pre-sutural clypeal groove.

Some of the key characteristics useful for diagnosis of the queen include mesoscutum with more than 10 standing setae, medium brown body color with lighter colored antennal scapes, legs, and mandibles.