The damselfly genus Argia is found throughout the New World where some species are common and abundant members of lotic freshwater and adjacent ecosystems. Argia species are not only important predators of aquatic and terrestrial invertebrates but are themselves an important prey item to a variety of other insects and vertebrates. The distribution of species is highly variable within the genus and some species are locally threatened or endangered due to range limitation and habitat loss. Damselflies and dragonflies (Odonata) may be useful indicators of aquatic ecosystem health as well as indicators of climate change.
There are approximately 120 species described with at least twenty suspected undescribed species. The taxonomy of the North American species is well known, but the Central and South American species are in need of revision. The phylogeny of the genus has never been studied using modern, repeatable methods. Therefore the evolutionary history of the genus has never been thoroughly explored.
The reproductive biology of Odonata is unique among insects and provides a model system for testing hypotheses related to character evolution by sexual selection and other mechanisms of evolution. Argia species have unique morphologies of male and female secondary sexual characters, the modified cerci and paraprocts of males and the corresponding plates of the female pro- and meso-nota that are grasped by males during copulation and oviposition. The patterns of variation in these structures, both within and among species, may reveal the extent to which sexual and natural selection help shape the current diversity of the group.
This dissertation presents phylogenetic hypotheses for the genus Argia using data from external morphology and ribosomal DNA. Maximum parsimony and maximum likelihood analyses were performed on the data, resulting in topologies that are mostly congruent, well-resolved, and moderately to highly supported. The variation in male cercus morphology is examined using three dimensional morphometrics where shape is quantified from computer tomography models. The phylogenetic hypotheses are used to examine patterns of cercus variation across the genus. The same methods are applied to populations of the widespread species Argia moesta in an attempt to test whether intrasexual selection applies to these important reproductive structures.