Developmentally, arthropods proceed from an egg, through larval and/or nymphal stages, to the adult. Generally, the term "larva" applies to stages in which major morphological changes occur, and these stages are often fixed in number. The term "nymph" is applied to stages which change in little other than size between molts, and are usually indeterminate in number. Of course, given the diversity of the arthropods, there has developed a diversity of nomenclature for the different larval or nymphal stages that rivals that of the Platyhelminthes.
Arthropods are usually ectoparasites, and may require 1 or more hosts to complete their life cycle. Many species move freely from one host to another, but others become attached permanently. Some species complete the entire life cycle on the host, but others attach only to feed and mate.
Aquatic hosts usually harbor crustacean parasites, whereas terrestrial hosts harbor insects and arachnids.