Axarus

Abstract: Connecticut River midges in the genus Axarus are constrained to semi-isolated populations associated with submerged clay exposures, and gene flow between populations is restricted. I sampled larval midges from fifteen spatially separated sites in the Connecticut River and determined karyotypes for 3111 larvae in two closely related species. In one of these species, an undescribed species referred to as Axarus species varvestris, 5chromosomal rearrangements were present as polymorphisms. Four of these are paracentric inversions, and the fifth is a terminal insertion or deletion. I used chromosomal rearrangement frequencies to calculate Nei’s genetic distance measures for all pairwise population comparisons and I constructed a neighbor-joining phenetic tree for the populations using these distances. Genetic distance between populations of A. sp. varvestris increases with geographic separation, and tree structure indicates that restricted gene flow only occurs in a downstream direction. The effect of unidirectional (downstream)gene flow on population genetic structure in riverine midges is described here for the first time. A genetic break is apparent between populations associated with 12 000-year-ol sediments originating in different ancient lakes. The geological break between lakes also coincides with a break in the species composition of the Axarus populations

Abstract: Larval midges in the genus Axarus are common macroinvertebrates in areas of the Connecticut River where varved clay substrates are found. Using SCUBA to make collections, we investigated the habitat use, species composition, and life history of the two Axarus species found in the river. Larvae were only found in clay or decaying wood, with a population density of 13,345 (± 4881) larvae/meter2 at one site. Two species were found co-existing in clays in northern sites, while only one species was found in southern collections. The species are temporally isolated, with adult emergence for the southern species preceding that of the northern species. Life history data are presented for the southern species, an undescribed taxon designated Axarus species varvestris. This species is univoltine with adult emergence taking place in late June in southern locations. The sex ratio at emergence was 1:1, and protandry was evident.