Pseudomyrmex ferrugineus (F. Smith, 1877)
Worker diagnosis. HW 0.92-1.15; median clypeal lobe laterally rounded; frontal carinae relatively well separated (FCI > 0.05); head densely punctulate, predominantly opaque or subopaque but weakly shining on upper third of head between ocelli and compound eye; body varying from light reddish- or yellowish-brown to dark brown.
Distribution. Eastern and southern Mexico to El Salvador and Honduras.
Biology. P. ferrugineus is an obligate acacia-ant, whose colonies occupy the swollen stipular thorns of various species of Central American acacias, including Acacia chiapensis, A. collinsii, A. cookii, A. cornigera, A. gentlei, A. globulifera, A. hindsii, A. janzeni, A. mayana and A. sphaerocephala. Most colonies are monogynous. A classic experimental study of the interaction between P. ferrugineus and Acacia cornigera by Janzen (1966, 1967) demonstrated that the ants protect their host plant from herbivores and from competing plants.