The insects, the largest class of the phylum Arthropoda and the largest major grouping of living things, chiefly characterized by flight, great adaptability, vast speciation in terrestrial and freshwater environments, and possession of three pairs of jointed legs and, usually, two pairs of wings. Some are parasitic, others serve as intermediate hosts for parasites, including those that cause many human diseases. Some are wingless; others, such as the Diptera, have only one pair of wings. Respiration is by tracheoles, cuticle-lined air tubes that pass air directly to the tissues. Development in higher forms is holometabolous and passes through distinctive egg, larval, pupal, and adult stages.