Several species of fishflies (Megaloptera: Corydalidae: Chauliodinae) have been reported from intermittent streams in western North America, but the life histories and distributions of these species are poorly understood. We studied the life history of Neohermes filicornis (Banks 1903) for 2 years in Donner Creek (Contra Costa County, California), a small intermittent stream that flows for 5–7 months per year. Neohermes filicornis had a 3–4-year life span and larval growth was asynchronous. Analysis of gut contents showed that larvae were generalist predators of aquatic insect larvae including Diptera, Ephemeroptera, and Plecoptera. Final instars dug pupal chambers in the streambed and began pupation within several days of stream drying. Dry conditions in 2007 resulted in stream drying 50 days earlier than in 2006, which was reflected in earlier pupation and adult emergence in 2007 than in 2006. Pupal and adult stages each lasted for 1–2 weeks. Although active burrowing into the dry streambed was not observed in the field, N. filicornis larvae initiated burrowing in response to simulated stream drying in laboratory experiments, while Orohermes crepusculus (Chauliodinae), a fishfly species common in perennial streams, did not exhibit burrowing behavior. We present new and historic collection records for Neohermes spp. in western North America. Neohermes filicornis is distributed in California, Arizona, western New Mexico, Baja California, and Sonora. A sister taxon, Neohermes californicus, has been collected in California, Oregon, and Idaho. In California, N. filicornis is primarily distributed in the Coast Range and mountains of Southern California, whereas N. californicus occurs in the foothills of the Sierra Nevada, with possible range overlap at the northern and southern ends of the Central Valley. As a long-lived aquatic insect, N. filicornis exhibits considerable plasticity in the phenology of its life cycle and undergoes multiple episodes of larval dormancy in intermittent streams of western North America.