Key to US Monomorium species

Key to workers of Monomorium known from the United States and Canada. Monomorium inquilinum, Monomorium pergandei and Monomorium talbotae are not included as they lack the worker caste.

1

 * Dorsal and posterior faces of propodeum equal in length => 2
 * Dorsal face of propodeum shorter than posterior face => 3
 * Dorsal face of propodeum longer than posterior face => 4

2

 * Mesopleuron punctate; fewer than 8 setae projecting above dorsum of mesosoma => Monomorium cyaneum
 * Mesopleuron not punctate; more than 10 (rarely 8 or 9) setae projecting above dorsum of mesosoma => Monomorium ebeninum

3

 * Fewer than 6 setae projecting above dorsum of mesosoma; occurs in northeastern U. S. => Monomorium emarginatum
 * More than 8 setae projecting above dorsum of mesosoma; occurs in northern Florida => Monomorium trageri

4

 * Mesopleuron smooth or nearly so, never punctate => Monomorium minimum
 * Mesopleuron punctate or otherwise sculptured => 5

5

 * Anterior clypeal margin weakly concave medially, clypeus weakly bicarinate => 6
 * Anterior clypeal margin strongly concave medially, clypeus strongly bicarinate => 7

6

 * Body uniform yellow with head, mesosoma and anterior section of gaster essentially the same color (posterior section of gaster often dark); petiolar node higher, node narrower, anterior and posterior faces converging only slightly dorsally => Monomorium pharaonis
 * Body strongly bicolored with head and gaster much darker than mesosoma, or occasionally body dark brown and essentially uniformly colored (gaster always uniformly dark); petiolar node lower, node broader, anterior and posterior faces converging more strongly dorsally => Monomorium floricola

7

 * Occurs from New Jersey to Florida => Monomorium viridum
 * Occurs along California coast => Monomorium ergatogyna