Previous authors have called attention to nest form variation between and within species of the Neotropical swarm-founding polistine wasp genus Parachartergus. One author characterized nests oriented horizontally and having the nest comb petiole central and in-line with nest cells as anomalous, and he suggested that anomalous nests constrain colonies from successful reproduction. We report two distinct nest forms of Parachartergus amazonensis(Ducke), one of which was oriented primarily horizontally and one only vertically, from a single locality in Peru. Horizontal and vertical nests, with corresponding difference in orientation of nest comb petioles, have additionally been observed in P. fulgidipennis (de Saussure) in Peru and Brazil andP. griseus (Fox) in Brazil and French Guiana. Nest orientation and comb petiole placement in these Parachartergus species may reflect a labile trait that is variably expressed as a function of availability of suitable nest sites. ''Anomalous'' ''Comparison of the gross features of the nesting behavior of various wasps teaches us much about the evolution of wasps ... .'' H. E. Evans and M. J. West-Eberhard (1970: 113) Studies of insect nest architecture have a distinguished history and have contributed significantly to works on taxonomy, phylogenetics, behavior, ecology, and evolution. Starr (1991) highlighted a milestone in the study of social evolution in Hymenoptera: ''Since Evans's landmark paper (1958, expanded in Evans and West-Eberhard (1970)), it is generally regarded as a precondition for the evolution of aculeate sociality that the female have a fixed nest to which she repeatedly returns.'' For social wasps of family Vespidae, Wenzel (1991) noted that ''the early taxonomy ... relied on architecture nearly as much as on morphology,'' citing the major contributions to classification and description of social wasps' nests by de Saussure (1853-1858), Ducke (1914), Richards and Richards (1951), and Richards (1978). Ducke (1905) was explicit about the primary importance of nest architecture in phylogenetic interpretation and generic classification. Jeanne (1975) assessed the adaptiveness of social wasp nest architecture. Wenzel (1991) reviewed nest architectural diversity for social wasps in the three subfamilies of social Vespidae: Stenogastrinae, Polistinae, and Vespinae. Much of the literature just cited reflects a phenomenon well known to vespologists— that nest architecture can be used to identify taxa in the field, often even at the species level. Wenzel (1998) notes that ''indigenous people who are alert to insects can identify the majority of local species by nest structure alone.'' Experienced vespologists can do the same (pers. obs.). Wenzel (1998) cautions, however, that ''not ... every species has a distinctive nest architecture'', and he continues by noting that ''intra-specific variation between habitats or populations can be great.'' These caveats do not explicitly encompass