Phoresy is a phenomenon in which an individual is transported by a larger individual of another species. This relationship benefits the carried and does not directly affect the carrier, although in some cases its progeny may be disadvantaged (as we shall see below). Phoresy provides a means of finding a new host or food source. An often observed example involves ischnoceran lice (Phthiraptera) transported by the winged adults of Ornithomyia (Diptera: Hippoboscidae). Hippoboscidae are blood-sucking ectoparasitic flies and Ornithomyia occurs on many avian hosts. When a host bird dies, lice can reach a new host by attaching themselves by their mandibles to a hippoboscid, which may fly to a new host. However, lice are highly host-specific but hippoboscids are much less so, and the chances of any hitchhiking louse arriving at an appropriate host may not be great. In some other associations, such as a biting midge (Forcipomyia) found on the thorax of various adult dragonflies in Borneo, it is difficult to determine whether the hitchhiker is actually parasitic or merely phoretic.
Amongst the egg-parasitizing hymenopterans (notably the Scelionidae, Trichogrammatidae, and Torymidae), some attach themselves to adult females of the host species, thereby gaining immediate access to the eggs at oviposition. Matibaria manticida (Scelionidae), an egg parasitoid of the European praying mantid (Mantis religiosa), is phoretic, predominantly on female hosts. The adult wasp sheds its wings and may feed on the mantid, and therefore can be an ectoparasite. It moves to the wing bases and amputates the female mantid’s wings and then oviposits into the mantid’s egg mass whilst it is frothy, before the ootheca hardens. Individuals of M. manticida that are phoretic on male mantids may transfer to the female during mating. Certain chalcid hymenopterans (including species of Eucharitidae) have mobile planidium larvae that actively seek worker ants, on which they attach, thereby gaining transport to the ant nest. Here the remainder of the immature life cycle comprises typical sedentary grubs that develop within ant larvae or pupae.
The human bot fly, Dermatobia hominis (Diptera: Cuterebridae) of the neotropical region (Central and South America), which causes myiasis (section 15.3) of humans and cattle, shows an extreme example of phoresy. The female fly does not find the vertebrate host herself, but uses the services of blood-sucking flies, particularly mosquitoes and muscoid flies. The female bot fly, which produces up to 1000 eggs in her lifetime, captures a phoretic intermediary and glues around 30 eggs to its body in such a way that flight is not impaired. When the intermediary finds a vertebrate host on which it feeds, an elevation of temperature induces the eggs to hatch rapidly and the larvae transfer to the host where they penetrate the skin via hair follicles and develop within the resultant pus-filled boil.