Organisms are built from distinct modules, which are internally coherent but flexible in their relationships among one another. We examined morphological variation within and between two candidate modules: the fore- and hindwings of bumblebees (Hymenoptera: Apidae: Bombus empatiens). We used the techniques of geometric morphometrics (Procrustes superimposition) to analyze the variation of landmark configurations in fore- and hindwings. Regression was used to correct for size-related shape variation (allometry). Principal component analysis revealed patterns of variation that were remarkably similar for individual variation and fluctuating asymmetry (FA). Because covariation of FA among parts must be due to direct transmission of the developmental perturbations causing FA, this agreement of patterns suggests that much of individual variation is also due to direct developmental interactions within each developing wing. Moreover, partial least squares analysis indicated that the patterns of shape covariation between fore- and hindwings were nearly the same as the patterns of within-wing variation. Shape covariation of FA was only found in bees that had been reared under elevated CO(2) concentration but not in bees from the control treatment, suggesting that the mechanisms of developmental interactions between fore- and hindwings are related to gas exchange. We conclude that the fore- and hindwings are developmental modules that maintain internal coherence through direct developmental interactions and are connected to each other only by relatively few links that use the system of interactions within modules.

Where the distribution ranges of the waterstriders Limnoporus notabilis and L. dissortis meet in western Canada, extensive hybridization and introgression occurs. Multivariate ordination analyses of genetic and morphometric data by principal component analysis revealed that a single axis separating the two parent species could account for nearly all the variation in both data sets. Maps of principal component scores for both data sets revealed geographical patterns of variation reflecting specific topographic features in the region. Comparisons of morphometric data from some of the samples collected in the 1980s and from the same sites revisited in the 1990s revealed substantial changes. An ‘island’ of dissortis‐like populations inside the range of L. notabilis in interior British Columbia expanded, and a marked local protrusion of notabilis‐like phenotypes into the range of L. dissortis on the east slope of the Rocky Mountains diminished during the decade between collections. We conclude that introgressive hybridization between these two species of waterstriders is a spatially complex and highly dynamic process.