Crustaceans are a wide and diverse group of arthropods that include decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods, and mantis shrimp. Crustaceans are classified as a subphylum within the clade Mandibulata. Most crustaceans are aquatic, living in either marine or freshwater habitats, but a few taxa, such as terrestrial crabs, terrestrial hermit crabs, and woodlice, have adapted to life on land. Crustaceans are as ubiquitous as insects on land in the waters. The antennule is an important chemosensory organ in crustaceans, but it is also extremely sensitive to mechanical stimuli such as touch and water-borne vibrations, providing information that crustaceans use in a variety of behaviours. Each somite, or body segment, can have two pairs of appendages: two pairs of antennae, mandibles and maxillae on the head segments, and legs on the thoracic segments, which can be specialised as pereiopods. An arthropod's mandible is a pair of mouthparts used for biting, cutting, and holding food. Crustacean appendages are often biramous, which means they are divided into two sections; this includes the second pair of antennae, but not the first, which is typically uniramous, with the exception of the Class Malacostraca, where the antennules may be generally biramous or even triramous.