Comparative morphology of the mandibles of female polistine social wasps (Hymenoptera, Vespidae, Polistinae) Orlando Tobias Silveira1 & José Nazareno Araújo dos Santos Jr.1 1Coordenação de Zoologia, Museu Paraense Emílio Goeldi, Av. Perimetral 1901, 66077–830 Belém-PA, Brazil. [email protected] ABSTRACT. Comparative morphology of the mandibles of female polistine social wasps (Hymenoptera, Vespidae, Polistinae). Diversity of mandibular forms in female polistines is explored and compared among 116 species of all polistine genera. Inferences about function are made and discussed based on observed form differences. Mandible length and width measurements are analyzed for a subset of polistine species plus two vespines and two eumenines. A variable expressing the ratio between these variables is also considered in morphometric analyses. The following mechanical interactions among mandible structural elements are highlighted: opposition and crossing of mandibles’ apical teeth at the middle of the closing trajectory; shearing action of the apical teeth against mesial denticles of the opposite mandible; shearing action of the mandible anterior margin against the ventral margin of the clypeus. In the genera Agelaia and Angiopolybia, exceptionally developed mesial mandibular structures may be related to necrophagy. In some epiponine genera, poorly developed mesial denticles and strong torsion of mandibles may be partially related to use of short wood fibers in nest construction as advanced by Sarmiento (2004). The considerable morphological variation found across the subfamily Polistinae is certain to be important in taxonomic and phylogenetic studies at the genus and species levels. KEYWORDS. Adaptation; form-function analysis; morphometrics; phylogenetic methods; systematics. RESUMO. Morfologia comparada da mandíbula da fêmea em vespas sociais da subfamília Polistinae (Hymenoptera, Vespidae). A diversidade morfológica da mandíbula da fêmea é explorada e comparada entre 116 espécies de todos os gêneros de Polistinae. Inferências sobre aspectos funcionais são feitas baseadas nas diferenças de forma observadas. Medidas do comprimento e largura da mandíbula, incluindo a razão entre essas variáveis, são analisadas para um subconjunto das espécies de Polistinae, mais duas de Vespinae e duas de Eumeninae. Os seguintes padrões de interação mecânica entre elementos estruturais são ressaltados: oposição e intercruzamento dos dentes apicais ao meio da trajetória de fechamento das mandíbulas; ação dilacerante dos dentes apicais contra os dentículos mesiais da mandíbula oposta; ação dilacerante da margem anterior da mandíbula contra a margem ventral do clípeo. Nos gêneros Agelaia e Angiopolybia, estruturas mesiais excepcionalmente desenvolvidas podem ser relacionadas ao hábito necrófago. Em alguns gêneros de Epiponini, o escasso desenvolvimento dos elementos mesiais e a forte torção do plano principal da mandíbula podem ser relacionados, pelo menos parcialmente, ao uso de fibras vegetais curtas na construção dos ninhos, como postulado por Sarmiento (2004). A considerável variação morfológica encontrada na subfamília Polistinae deverá prover informa- ções importantes para análise em estudos taxonômicos e filogenéticos nos níveis de gênero e espécie. PALAVRAS-CHAVE. Adaptação; análise de forma-função; métodos filogenéticos; morfometria; sistemática. Adult female social wasps use their mandibles in various In spite of great importance in wasp biology and behavior, important contexts (Duncan 1939), such as for getting food mandible morphology has received little attention either in stud- (mostly arthropod prey) and feeding the larvae, in the col- ies of form-function correlation, or as source of characters for lecting and application of materials used in nest construc- phylogenetic reconstruction. Morphological work has been tion (mainly wood fibers), in self and colonial defense, in done attempting to establish possible functional connections social aggressive interactions, and in other more subtle and between the mandibular gland and “mandibular grooves” in less known ways (see for example O’Donnell 2003). Some Polistes species (Hermann et al. 1971; Singh & Singh 1982), of these uses have been highlighted as traits of higher “sa- on the morphology of mandibular gland in Polistes fuscatus, lience” in the evolution of social behavior in vespid wasps with special attention to location of the gland opening and its (Hunt 1999), whose first appearance in a phylogeny is in functional consequences (Downing & Jeanne 1982). Hansell close proximity to appearance of eusociality. Hunt mentions (1987) studied morphometric aspects of female mandibles in two such roles of mandibles in vespids: mandibular capture some major vespid lineages as related to the mechanics in- of larval provisions (“frees the sting from selection on provi- volved in nest building and food preparation, regarded as fa- sion anesthetization, and it opens a gateway to foraging effi- cilitating factors in the evolution of eusociality. More recently, ciencies” [p. 231]), and malaxation of larval provisions O’Donnell (1995) postulated the adaptive nature of mandibu- (“provides the basic proteinaceous nourishment to support lar features in the context of necrophagy observed in some adult longevity accompanied by sustained synovigenic oo- epiponine species. Sarmiento (2004) extended the investiga- genesis at levels greater than those that can be achieved by tion of adaptive hypotheses in a study of correlations between solitary Aculeata” [p. 231]). mandibular traits and nest construction materials, and feeding Revista Brasileira de Entomologia 55(4): 479–500, dezembro, 2011 480 Silveira & Santos Jr. habits, using well-formalized methods within a phylogenetic hypognathous insects, i.e. structures commonly said to be framework. With respect to studies on vespid phylogeny, Car- dorsal or ventral (as referring to drawings or images of dis- penter (1982) dismissed the value of mandible length as a char- sected specimens; e.g. in O’Donnell 1995; or Sarmiento 2004) acter that could be used to recognize Eumeninae sensu Richards are here considered to be anterior or posterior, respectively. (1962) as a monophyletic group. More recently, Carpenter et This is important when considering relations of mandibles al. (1996) discussed the relevance of mandible shape as evi- to other structural elements of the head. dence for a sister-group relationship between the genera Morphometric data and analysis. Distance measurements Asteloeca and Metapolybia. of the mandible-length (from the posterior condyle to apex of In this paper, diversity of mandibular forms in female the posteriormost apical “tooth 1”; Lmd) and basal mandible- polistines is explored and compared among the genera and width (Wmd) were taken on specimens under the stereomi- species so as to produce a detailed survey of basic form ele- croscope with an ocular micrometer. Head-width as estimated ments, with a corresponding nomenclature. Inferences about by interocular distance at the level of ocular sinus (Hdw), and function are also made and discussed based on observed form height of the mesopleuron (from articulation of mid coxa to differences. The morphological data presented show that man- spiracular entrance; Mph) were also measured for controlling dible form varies to some extent among polistine genera and of size or scale effects. One single set of measurements was species, in both qualitative and quantitative aspects. Some taken for each of 94 species of 24 genera of Polistinae plus features may prove to be useful as characters in phylogenetic two vespines and two eumenines. Distance variables were log- studies, and it is also expected that a morphologically well- transformed for conversion of exponential trends into linear founded baseline may be of great value to comparative be- ones, and to permit evaluation of allometry in bivariate rela- havioral studies. tionships. A variable expressing the ratio (R) between length and width of mandible was also considered in analyses. MATERIAL AND METHODS Size correction was accomplished by using the residuals of ordinary least squares regressions (OLS) of variables on One hundred and sixteen (116) species representing all the mesopleuron-height (Mph), performed with the program polistine genera had their mandibles studied, in most cases Past 1.79 (Hammer et al. 2001). For the sake of comparison, being also figured and measured (see Appendix 1). Speci- “phylogenetic residuals” were also obtained with the phylo- mens were either collected for the study in the proximities of genetic generalized least squares (PGLS) procedure described Belém (PA, Brazil) and Rio Claro (SP, Brazil), or selected by Revell (2009), with a code for the program MatLab ® from the collection of the Museu Goeldi. The species included (The MathWorks Inc. 2006). For this analysis, a species phy- in this study are mostly common and easily identifiable Neo- logeny (Fig. 11) was assembled based on the genus level tree tropical social wasps. Ropalidiines were partly donated and published by Wenzel & Carpenter (1994), corrected for syn- identified by Dr. James Carpenter (AMNH) and Dr. Junichi onymies and extended to species level after Carpenter et al. Kojima (Ibaraki University). A few vespines and ropalidiines (1996), Carpenter et al. (2000), Carpenter (2004), Pickett et from the “Ducke Collection” at the Museu Goeldi have most al. (2006), Pickett & Wenzel (2007), and Silveira (2008). probably been identified by R. Du Buysson.