The life history of Isonychia-sicca ephemeroptera oligoneuriidae in an intermittent stream in North Central Texas USA.

The life history of Isonychia sicca (Walsh) was determined from samples collected in intermittent Clear Creek, October 1976 to June 1978, Denton County, Texas. Fall hatching nymphs formed a small overwintering brood, grew slowly through the winter and emerged in late April. A spring brood hatched in March and emerged late May to early June. Adults from overwintering and spring broods supplied eggs which formed an F2 that emerged late June to early July. Emergence began approximately 20 minutes after sunset and continued for approximately 2 hours. The emergence period was continuous, April to July 1977, with both broods and generations overlapping considerably. Average subimaginal fecundity decreased through the emergence period from 4440-621 eggs/female and was positively correlated with abdominal volume. A decrease in body size was noticeable for nymphs, subimagos and imagos. The sex ratio differed for each stage: 1 male:1.6 females, nymphs; 1.4 males:1 female, subimagos; 1 male:3.4 females, imagos. Nymphs filter particulate matter from water using long foreleg hairs as a seine. I. sicca appears to diapause in the egg stage through dry periods. Upon fall rehydration, eggs slowly break diapause differentially through the fall with those breaking early producing the overwintering brood. Eggs breaking diapause in late fall and winter experience cool water temperatures which probably induce quiescence. Hatching begins again in spring. In a permanent stream, eggs continue to hatch through the summer and fall until cool winter temperatures probably induce quiescence and resume hatching in spring.