Arthropods excrete waste through special structures known as Malpighian tubules, which are extensions of the digestive tract. Terrestrial arthropods excrete their waste as solid uric acid, just as birds do, which requires no water and thus preserves precious moisture.
While arthropods all share a number of features, including the Malpighian tubules, their defining features are the jointed exoskeleton and limbs that these animals possess. They all have well-defined heads along with complex nervous systems. Every functional appendage, from antennae to jaws, is constructed from modified legs. Their bodies are segmented, which is best shown in species such as millipedes that closely resemble the ancestral form of arthropods. More forms, however, have fused various segments together to form body sections such as a head, thorax and abdomen. Most arthropods have eyes, which can be either simple or multi-faceted compound eyes. Breathing structures vary between terrestrial and aquatic forms, with terrestrial forms using tiny tunnels called trachea and aquatic forms using gills.
Despite these common features, arthropods are incredibly diverse. There are over three times as many species of arthropods as there are species of all the other animal groups put together. There are arthropods in the air, on the ground, and in both salt and fresh water.