General Parasitology - Nomenclature of the Parasitic Organism
Organisms in these associations may either be on the outer surface of the host organism, (in which case the prefix Ecto- is used), or inside the host organism, (in which case the prefix Endo- is used). These prefixes may be used with any of the animal associations listed above. For example the flagellate protozoa in the termite guts are Endosymbionts, whilst the anemone can act as an Ectocommensal with the crab. Parasites may act as both ecto- and endoparasites. Parasites may also be classified according to the closeness of the relationship. For example Facultative Parasites (such as many bacteria) are those where the parasitic lifestyle is only taken up opportunistically, whereas Obligate Parasites (such as all viruses, and most of the helminth parasites described below) are those where the organism must parasitise another organism. These parasites may often cause disease, in which case they are referred to as Pathogenic Parasites.
In a somewhat wider interpretation of the term parasitism some organisms exhibit parasitic behaviour only early in their lifecycle, these being referred to as Brood Parasites. Examples of these include caterpillars of the Large Blue butterfly, which chemically mimic other caterpillars with mutualistic associations with ants (see above), but both fail to produce honeydew as compensation and consume ant grubs, and may in fact destroy the nest, (thereby acting as a pathogenic parasite for the ants). In this case the parasitic lifestyle probably evolved from the mutualistic lifestyle of the other, related, butterflies, again illustrating how one form of association may change into another. Another more well known example of a brood parasite is a bird, the cuckoo.