The fossil record of land isopods is of little help in tracing the evolution of their terrestrial adaptations; however, we can usefully correlate the traits of various extant species with their various environments. Loss of cuticular water varies greatly between species, generally in relation to habitat, although the mechanism restricting loss is unclear. Both endocuticular and epicuticular barriers have been reported, the latter in Venizillo arizonicus. Pseudotracheae are advantageous for respiration in dry air, but again the mechanism is not clear. Excess nitrogen is excreted mainly as ammonia gas. Wide osmotic variation in the hemolymph resulting from dehydration is tolerated. Lethal temperatures vary in general according to habitat, and are affected by size, humidity, permeability of the cuticle, and previous temperature history. Evaporative cooling is sometimes of survival value for short periods. Metabolic rate is affected by season, and both long photoperiod and high temperatures are necessary for gonadal development and breeding. Responses to light, temperature, and humidity are more labile than previously thought. A peripheral hygro-receptor has tentatively been identified. The mechanism of bunching is largely thigmotactic, but olfaction may play a part, as it does in shelter-seeking. More information is now available about the micro-climates of habitats. In Porcellio scaber, nightly activity on trees in some situations permits elimination of excess water by transpiration. In Armadillidium vulgare nightly activity is greater in moister air, and drowning is a density-dependent mechanism for population control during heavy rains. Isopods are successful as land animals. Further study of population and distribution limits by combined ecological and physiological methods would be fruitful.