About one-third of gastropod species are terrestrial. In terrestrial habitats they are subjected to daily and seasonal variation in temperature and water availability. Their success in colonizing different habitats is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance. They are adapted to most of the habitats on Earth. The shell of a snail is constructed of calcium carbonate, but even in acidic soils one can find various species of shell-less slugs. Land-snails, such as Xerocrassa seetzeni and Sphincterochila boissieri, also live in deserts, where they must contend with heat and aridity. Terrestrial gastropods are primarily herbivores and only a few groups are carnivorous. Carnivorous gastropods usually feed on other gastropod species or on weak individuals of the same species; some feed on insect larvae or earthworms.