Key to workers of Liometopum from the United States based on workers, modified from Del Toro et al. (2009).
- Long erect hairs (0.30 mm) present on dorsum of gaster mixed with shorter erect hairs (0.12 mm), some hairs nearly as long as those on pronotum, pilosity may or may not cover entire gastral surface, mesosoma evenly convex, not depressed at metanotal suture => 2
- Erect hairs on dorsum of gaster short (0.10mm), or absent and of approximately equal lenth or usually shorter than erect hairs on pronotum; mesosoma convex but strongly depressed at metanotal suture (Washington to west Texas and central Mexico) => Liometopum luctuosum
- Antennal scape surpassing posterior margin of head by at least twice maximum thickness of scape (southwestern United States, west to west Texas, south to southeastern Mexico) => Liometopum apiculatum
- Antennal scape of largest workers surpassing posterior margin of head by amount that does not exceed maximum thickness of scape (Pacific West Coast from Washington south to Baja California) => Liometopum occidentale
- Del Toro, I., Pacheco, J.A. & Mackay, W. 2009. Revision of the ant genus Liometopum. Sociobiology 53: 299-369.