Most terrestrial animals crawl. Crawling is the characteristic mode of locomotion for most reptiles, amphibians and mammals. Insects generally crawl when they aren't flying, and terrestrial molluscs, such as snails and slugs, crawl on a single, specialized foot.
Newts and salamanders crawl, as do frogs and toads when they aren't hopping. Lizards crawl with a characteristic side-to-side motion that is replicated by their close relatives the snakes. Birds generally do not crawl, though some penguins slide on their bellies for high-speed and long-distance travel. Most terrestrial invertebrates crawl, and even flying species, such as flies, moths and bees, usually revert to a stepwise, six-legged locomotion to move efficiently across surfaces.
Some animals crawl at various stages of life, but not at others. Humans, for example, typically crawl in infancy and can revert to all fours when the situation calls for it, but this is slow and inefficient relative to the adults' normal upright posture.
Many terrestrial animals have ways of moving apart from crawling, such as the horse's gallop or trot and the adult human's bipedal walk, but crawling on four, six or eight legs is common. Some animals can also form a ball and roll, such as the pangolin.