Resumen
Emergence of Diamesa cheimatophila was determined for a five year period (1981?1986) using a permanently operated walk-in emergence trap over a spring-fed tributary of Fourmile Creek in Erie, northeast Pennsylvania. Annual emergence started in September/October, continued sporadically through winter until April/May and occurred over a span of 185?236 days. Averaged over the five years of our study, > 80% of annual emergence occurred during winter when long-term average air temperatures are less than 10 degrees C. Emergence of D. cheimatophila shows patterns similar to emergence of D. mendotae Muttkowski at groundwater-influenced stream sites in Minnesota, where it is known to produce 2?3 cohorts per year during winter. Well-defined peaks in winter emergence of D. cheimatophila indicates at least two, but possibly three or four cohorts emerging per winter. The year-to-year variability in emergence of D. cheimatophila strongly suggests this species also conforms to the Labile Life Cycle Hypothesis proposed for D. mendotae, (but refined in this paper) with the actual number of cohorts produced per winter depending on the onset and duration of cold air temperatures.