J. HYM. RES. 2(1), 1993 pp.1 17-168 Systematic studies on Pseudomyrmex acacia-ants (Hymenoptera: Formicidae: Pseudomyrmecinae) Philip S. Ward Department of Entomology. University of California. Davis. CA 95616 Abstract. — The obligate acacia-ants ( Pseudomyrmex ferrugineus group) are well known as defensive inhabitants of swollen-thorn acacias in the northern Neotropics. A taxonomic revision of these ants leads to the recognition of ten species: P. ferrugineus (F. Smith), P. flavicornis (F. Smith), P. janzeni, sp. nov., P. mixtecus, sp. nov., P. nigrocinctus (Emery), P. particeps, sp. nov., P. peperi (Forel), P. satanicus (Wheeler), P. spinicola (Emery), and P. veneficus (Wheeler). The following new synonymy is proposed: P. nigrocinctus = P. alfari (Forel) = P. bicinctus (Santschi) = P. peltatus (Menozzi); P. spinicola = P. atrox (Forel) = P. gaigei (Forel) = P. infernalis (Wheeler) = P. scelerosus (Wheeler). Diagnostic descriptions and taxonomic comments are also provided for ten other unrelated species of Pseudomyrmex which have become secondarily associated with swollen-thorn acacias either as obligate and, in at least one case, parasitic occupants (P. nigropilosus (Emery), P. simulans Kempf and P. subtilissimus (Emery); P. reconditus, sp. nov., may also belong in this category ) or as facultative inhabitants ( P. boopis (Roger), P. gracilis (Fabricius), P. hesperius, sp. nov., P. ita (Forel). stat. nov., P. kuenckeli (Emery) and P. opaciceps, sp. nov.). A cladistic analysis of the P. ferrugineus group yields the following result which appears to be fairly robust insofar as there is congruence among the trees derived from worker-, queen-, and male-based character sets: {(nigrocinctus + particeps) + {peperi + ({satanicus + spinicola) + ferrugineus complex))). The "ferrugineus complex" comprises five species whose phylogenetic relationships are not fully clarified. The composite data set (47 characters from all three castes) supports the following partial resolution: {ferrugineus + janzeni + {flavicornis + {mixtecus + veneficus))). The cladogram of the P. ferrugineus group indicates that speciation in the group has occurred primarily as a consequence of geographical isolation, and that the ants and their host acacias have experienced diffuse coevolution rather than strict cospeciation. INTRODUCTION merits have appeared in the ecological literature. In this paper I present a taxonomic revision of the Pseudomyrmex ferrugineus (F. Smith) and re-obligate acacia-ants {Pseudomyrmex ferrugineus lated species of ants form a well-defined mono-group) and an assessment of their phylogenetic phyletic group, the members of which nest exclu-relationships. I also attempt to clarify the identities sively in the hollow, swollen thorns of several New of other, unrelated species of Pseudomyrmex which World Acacia species. Because of their aggressive have become secondarily associated with swollen-behavior and predictable occurrence on the acacias, thorn acacias. these ants have received considerable attention The earlier taxonomic literature on acacia-ants from tropical biologists (Belt 1874; Safford 1922; is scattered in more than a dozen papers containing Skwarra 1934a. 1934b; Wheeler 1942; Janzen 1966, descriptions of various species, subspecies, and 1973). The landmark studies of Janzen (1966, "varieties". Two of the more comprehensive treat-1967b) provided strong experimental evidence of ments are those of Emery (1890) and Wheeler the mutualistic nature of the Pseudomyrmex/ Aca-( 1942). In presenting the results of his ecological cia association, and the relationship between the studies Janzen (1966, 1967b, 1973) summarized two organisms is often cited in discussions of his understanding of acacia-ant taxonomy. Ward coevolved mutualisms (e.g. Gilbert 1983; Beattie (1989) provided a brief diagnosis of the P. 1985; Futuyma 1986). At the same time, the ferrugineus group, together with taxonomic and systematics of the acacia-ants has been neglected, nomenclatural notes on the commoner species, with the result that misidentifications and misstate-
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