Of the major physical factors that influence insect seasonal ecology, moisture is least understood and least appreciated. It is our premise that experimental probing of insects from diverse zones and various habitats would reveal general patterns of insect responses to moisture that are as striking as those for photoperiod and temperature. Using the paradigms of photoperiod and temperature as ecophysiological determinants of insect seasonality, we hypothesize that moisture influences insect life cycles via one or more of three mechanisms-as a token stimulus for diapause, modulator of developmental or reproductive rates, or behavioral cue for vital seasonal events. For heuristic purposes, we offer each of these hypotheses in close association with approaches for testing their validity in insects that undergo dry-season dormancy. The approaches appear appropriate for examining the role of moisture in the life histories of terrestrial invertebrates other than insects, as well as plants and microbes that have a seasonal resting stage. Elucidating moisture's role in insect seasonal cycles is critical to the development of comprehensive phenological models, improved insect management systems, and identification of novel evolutionary mechanisms for adaptation to wet-dry seasons, especially in tropical, subtropical, and Mediterranean regions.KEY WORDS phenology, moisture, soil-inhabiting insects, dormancy, development, reproduction PHOTOPERIOD ANDTEMPERATURE are generally considered prime factors in the seasonal ecology of terrestrial invertebrates (Beck 1980, Tauber et aI.1986). In contrast, moisture is usually consigned a secondary or minor role. This subordinate ranking of moisture as a seasonal cue may be grossly misleading for 2 reasons: First, phenological studies are concentrated in the Temperate Zone where large seasonal changes in photoperiod and temperature typically occur. Consequently, investigators are heavily biased toward using photoperiod and temperature as variables, and data on moisture's function and significance are lacking. Second, the natural histories of a large proportion of earth's insects (e.g., those from tropical, Mediterranean, and other regions with conspicuous wet and dry seasons) are highly correlated with changes in moisture conditions (Essig 1926, Edney 1977, Wolda 1978, Shapiro 1979, Mooney et al. 1980, Labeyrie 1981, King 1984, Denlinger 1986, Paarmann 1986, Powell 1986, Brakefield 1987, Dobkin et a1.1987, Janzen 1987, Bradshaw and Holzapfel 1988, Clouds ley-Thompson 1991, Hadley 1994, Somme 1995. Indeed, it appears that cycles of rainfall and moisture in wet-dry regions may rival those of temperature in temperate regions, in both their seasonal predictability and their importance for survival and development of insects. Thus, we contend that moisture constitutes a greatly un-

For those arthropod species adapted for living below the soil surface, the soil is a refuge from the biotic and abiotic perturbations existing above ground. Convergent morphological, physiological, and behavioral adaptations of epedaphic, euedaphic, and hemiedaphic arthropods to selective aspects of subterranean existence are examined in light of overlapping ecological niches. The abiotic impact of the soil environment and its relationship to arthropod evolution, radiation, and ecology are discussed as well. Specific areas addressed include the invasion of land by marine arthropods, the impact of morphology on arthropod mobility, osmoregulatory/respiratory systems, and defensive strategies.

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