Diapause in a Salt-tolerant Desert Caddisfly: The Life Cycle of Limnephilus assimilis (Trichoptera) in Death Valley

Abstract

The life cycle of the desert caddisfly, Limnephilus assimilis (Banks) (Trichoptera: Limnephilidae), in Death Valley, California, includes a shortened aquatic phase (winter) and an extended adult phase (summer). Eggs are laid in aquatic habitats starting in October. Larvae pass rapidly through five larval instars and start to pupate in February. Adult emergence is complete by late April. Upon emergence, adult females have undeveloped ovaries. This desert caddisfly, like many species of temporary aquatic habitats in temperate regions, appears to avoid unfavorable high summer temperatures through adult reproductive diapause. INTRODUCTION Studies of adaptations of insects and crustaceans inhabiting brackish and saline desert waters emphasize physiological and phenological mechanisms for dealing with seasonal desiccation, spatial and temporal variations in chemical composition and concentrations, and high summer temperatures (Bayly and Williams, 1966; Belk and Cole, 1975). While physiological observations have been reported for both salinetolerant insects and crustaceans (Bayly, 1972; Broch, 1969; Colburn, 1980, 1983; Croghan, 1958; Jang and Tullis, 1980; Nemenz, 1960; Tones and Hammer, 1975), life history studies have emphasized crustaceans while neglecting insects (Belk and Cole, 1975; Hutchinson, 1966). Considerable literature exists on adaptations of insect life histories to temporary ponds and intermittent streams (Brittain, 1982; Corbet, 1980; Hynes, 1976; Wiggins, 1973); such studies in desert waters are lacking. Larvae of the desert caddisfly, Limnephilus assimilis, can be found in aquatic habitats of salinities from 0-22?/oo throughout Death Valley, California (Colburn, 1980). I examined the life cycle of this caddisfly as part of a comprehensive study of the effects of salinity and temperature on the distribution and abundance of the species in Death Valley and of the physiological and phenological mechanisms allowing this insect to inhabit aquatic environments in one of the world's most inhospitable deserts (Colburn, 1980). Results of the physiological studies are reported elsewhere (Colburn, 1980, 1983). In this paper I present information on the life cycle of L. assimilis in Death Valley waters. My objectives in performing this study were to document, using standard sampling techniques and cage experiments with larvae in the field, the times of occurrence and duration of the developmental stages of the caddisfly; to relate the phenology to physical-chemical factors in the natural environment and to responses of larvae to these factors in the laboratory; and to look for the presence of a diapause stage. Diapause is common in terrestrial desert arthropods (Cloudsley-Thompson, 1975), in crustaceans from desert waters (Belk and Cole, 1975) and in aquatic insects, including limnephilid caddisflies, from temporary waters (Bouvet, 1971; Flint, 1958; Novak and Sehnal, 1963; Svensson, 1972; Wiggins, 1973), but it has not been confirmed for any desert aquatic insects (Gray, 1981). I hypothesized that upon emergence, adult Limnephilus assimilis, like temperate species of Limnephilus, would be reproductively immature, and that adult life span would be prolonged and would involve reproductive (ovarian) diapause. tPresent address: Massachusetts Audubon Society, Lincoln, Massachusetts 01773.