Arthropods live everywhere on Earth, including on land, in water and in the air. There are well over a million species of arthropods, and they have adapted to live in numerous habitats that include mountains, deep seas and tropical rain forests.
Scientists believe that even a million species are just the tip of the iceberg, and there are millions more species of arthropods not yet discovered.
Arthropods such as spiders, ticks, centipedes and millipedes are terrestrial, which means they mostly live on land. Crustaceans such as shrimp, crabs, lobster and krill live mostly in the water. However, there are exceptions. The roly-poly bug is considered a crustacean and lives on land. The horseshoe crab isn't a crustacean, but lives in the ocean. It's also one of the oldest animals alive.
Insects are largely terrestrial as well, and many of them are strong flyers. Interestingly, some insects, such as dragonflies, begin life in the water.
Though the size and habitats of different arthropods can vary widely, nearly all have exoskeletons made out of chitin, a hard material that needs to be shed from time to time to allow the animal to grow. They get the name arthropod from jointed appendages. These appendages not only form legs and feet but antennae, reproductive organs and mouth parts. Some arthropods, such as the millipedes, can have hundreds of pairs of legs.