Ecological characterization of two sibling species, IIIL-1 and IS-7, in the Simulium vittatum complex (Diptera: Simuliidae)

Larval polytene chromosomes provided the identification tool that enabled ecological characterization of two black fly sibling species, IIIL-1 and IS-7, in the Simulium vittatum complex. A 4-year study in the eastern United States demonstrated important differences between the two siblings, based primarily on differential occupancy of the stream continuum. Immatures of the IIIL-1 sibling predominated below nonspring-fed reservoirs, downstream of sewage effluent outflows, and in warm downstream areas. Immatures of the IS-7 sibling occurred, with IIIL-1, in cool well-oxygenated flows generally associated with groundwater discharge. These sibling–habitat relations produced sibling gradients along the stream continuum. Over their geographic ranges, the IIIL-1 sibling became increasingly abundant from north to south, relative to IS-7. At mixed-sibling sites, populations were asynchronous; IS-7 generally predominated in early spring and late fall. The major determinants of sibling distributions were those factors affecting respiration: temperature, oxygen, and current. Species such as the black fly Simulium decorum and the midge Chironomus riparius signalled essentially pure populations of IIIL-1. Preimaginal densities of Glossosoma spp. (Trichoptera) also indicated sibling proportions. IIIL-1 and IS-7 shared all pathogens. At mixed-sibling sites, proportionally more larvae of IIIL-1 occurred on rocks than on grasses. Regardless of sibling, larvae on rocks were darker than those on grasses, which suggested differences in the proportion of ingested materials.