Caste dimorphism in the wasp Epipona guerini (Hymenoptera : Vespidae ; Polistinae, Epiponini) : further evidence for larval determination

Queens of the social, swarm-founding wasp Epipona guerini are larger than workers in each of 13 morphometric variables. Multivariate analyses revealed that 6 of the variables contribute significantly to queen-worker differentiation. Relationships among tho rax length, wing length, and width of the second gastral tergite differ between queens and workers in ways that can only occur through difference in developmental pathways during larval development. Pre-imaginal caste determination has been documented in six genera of Epiponini; Epipona is the second of these that now has been subjected to detailed analysis that revealed caste differences that are not explained by size-based allometry. Difference in morphology between queen and worker castes and among worker subcastes of social insects has been a focal problem in evolution since Darwin (1859) specified such differences (but not the origin of sterility and, so, not the origin of sociality) as "one special difficulty, which at first appeared to me in superable, and actually fatal to my whole theory. " Limited interchange of ideas between evolutionary theorists on one hand and empiricists exploring caste de termination on the other has perhaps delayed progress on the problem (Hunt and Nalepa 1994; Hunt 1994), and our understanding of caste evolution remains in complete. Queen-worker dimorphism has long been known to reflect strong in fluence of environmental factors, especially nutrition, in both bees (Michener, 1974) and ants (Holldobler and Wilson, 1990). Wheeler (1986) suggests that an increase in the range of body sizes may have been a first step in queen-worker dimorphism among social Hymenoptera generally. Among social wasps of the Vespidae, queen-worker dimorphism is apparently not found at all in the Stenogastrinae, whereas queen-worker dimorphism is typ ically pronounced in the Vespinae (but see Vecht, 1957; Matsuura and Yamane, 1984). Intermediate between these subfamilies is the Polistinae (Carpenter 1982), in which a range of conditions apparently exists from no caste dimorphism to discrete queen-worker dimorphism (Jeanne et al., 1995 and references therein). Among Polistinae, Epiponini is of interest as a taxon in which all species exhibit perennial coloniality and swarm-founding behavior (reviewed in Jeanne, 1991), arguably manifestations of social complexity, but queen-worker dimorphism has been reported as both present (e.g., Protopolybia exigua: Simoes, 1977 in Jeanne et al., 1995) and absent (e.g., Parachartergus colobopterus: Strassmann et al., 1991). Jeanne et al. (1995) recently reported a detailed morphometric analysis of Apoica pallens, which is in the sister genus to the 21 other genera of Epiponini. They document a pattern of non-size-based queen-worker dimorphism that can 1 This paper is dedicated to the memory of George C. Eickwort, colleague and friend. This content downloaded from 157.55. 39.92 on Wed, 22 Jun 2016 06:57:30 UTC All use subject to http://about.jstor. org/terms VOLUME 69, NUMBER 4 363 only be explained by reprogramming of growth parameters (Wheeler, 1991) dur ing the larval period. Occurrence of non-size-based caste difference in the basal genus of Epiponini calls for careful examination of other genera of Epiponini to ascertain the extent to which such caste difference may characterize the tribe. The work to be reported here was undertaken with this goal in mind. Materials and Methods Epipona is among the more derived of the 22 genera of Epiponini, the Neotrop ical swarm-founding polistine wasps (Wenzel and Carpenter, 1994). Epipona guer ini (Saussure), one of 3 species in the genus, ranges from Mexico to Peru (Saussure, 1854; see J. M. Carpenter in prep, for clarification of species-level nomenclature of Epipona). A complete colony of E. guerini was collected at nightfall at Volcan Cacao Biological Station, Guanacaste Province, Costa Rica on 27 January, 1991. All adult wasps of the colony, freshly killed, were placed into 70% ethanol. Voucher specimens have been placed at the University of Missouri-St. Louis, INBIO (He redia, Costa Rica), and the American Museum of Natural History. Caste of females was determined by examination of the ovaries; queens were those with developed ova, and workers were those without. Fifty queens were prepared for morphometric analysis, as were fifty workers chosen hapazardly from more than twice that number dissected. Preparation consisted of pinning, removing the legs and wings from the left side of the body, and taping the removed forewing to a microscope slide. Measurements were then taken on 13 morphometric variables (Fig. 1) as defined and used by Jeanne et al. (1995): HH, head height; HW, head width; HL, head length; TH, thorax height; TW, thorax width; TL, thorax length; WL, wing length; G1BW, basal width of the first gastral tergite; G1AW, apical width of the first gastral tergite; G1H, first gastral tergite height; GIL, first gastral tergite length; G2W, second gastral tergite width; and G2L, second gastral tergite length. Measurements were taken using a zoom stereomicroscope equipped with a video camera linked to a computer programmed with DIAS, Digital Image Analyzer System (copyright 1993 The C. Squared Corporation). The system permitted con tinuous (vs. interval) measurement of all variables. Data were common log transformed; all statistical analyses were performed using SAS (SAS version 6.02; SAS Institute, Cary, NC). Queen-worker difference for each variable was tested by independent Mests. Stepwise discriminant analysis was used to identify the most significant contributors to caste difference. Stepwise regression analysis was then used to identify those variables that differed signif icantly in intercept between castes when regressed against thorax length. Analysis of covariance (ANCOVA) was used to test for homogeneity of slopes in queen vs. worker contrasts for those variables found to differ significantly by the step wise regression.