Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae)

Stenamma is a cryptic "leaf-litter" ant genus that occurs in mesic forest habitats throughout the Holarctic region, Central America, and part of northwestern South America (Colombia and Ecuador). The genus was thought to be restricted primarily to the temperate zone, but recent collecting efforts have uncovered a large radiation of Neotropical forms, which rival the Holarctic species in terms of morphological and behavioral diversity. By inferring a broad-scale molecular phylogeny of Stenamma, Branstetter (2012) showed that all Neotropical species belong to a diverse Middle American clade (MAC), and that this clade is sister to an almost completely geographically separated Holarctic clade (HOC). Here, the Middle American clade of Stenamma is revised to recognize 40 species, of which 33 are described as new. Included in the revision are a key to species based on the worker caste, and for each species where possible, descriptions and images of workers and queens, images of males, information on geographic distribution, descriptions of intraspecific variation, and notes on natural history. Several species groups are defined, but the majority of species remain unassigned due to a lack of diagnostic morphological character states for most molecular clades. The following species are redescribed: S. alas Longino, S. diversum Mann, S. expolitum Smith, S. felixi Mann, S. huachucanum Smith, S. manni Wheeler, and S. schmidti Menozzi. The following are described as new: S. andersoni sp. n., S. atribellum sp. n., S. brujita sp. n., S. callipygium sp. n., S. catracho sp. n., S. connectum sp. n., S. crypticum sp. n., S. cusuco sp. n., S. excisum sp. n., S. expolitico sp. n., S. hojarasca sp. n., S. ignotum sp. n., S. lagunum sp. n., S. llama sp. n., S. leptospinum sp. n., S. lobinodus sp. n., S. longinoi sp. n., S. maximon sp. n., S. megamanni sp. n., S. monstrosum sp. n., S. muralla sp. n., S. nanozoi sp. n., S. nonotch sp. n., S. ochrocnemis sp. n., S. pelophilum sp. n., S. picopicucha sp. n., S. saenzae sp. n., S. sandinista sp. n., S. stictosomum sp. n., S. tiburon sp. n., S. tico sp. n., S. vexator sp. n., and S. zelum sp. n. Although many of the newly defined species consist of challenging species complexes, this study establishes a robust baseline that will guide future work on the systematics of MAC Stenamma. The total global diversity of Stenamma now includes 84 extant species.