Phoresy, sometimes called phoresis, describes the transport of one animal through another one. The movement or dispersal is directional and has been compared, especially for mites, to assisted migration (Binns 1982). Many desperate taxa have developed it as a live history trait. Its frequency in a taxon is inversely correlated to the size of the animal. For vertebrate species, it is a rare trait. A recent example is the phoresy of remoras, fish of the family Echeneidae, on marine turtles (Sazima and Grossman 2006). For invertebrate taxa, phoresy becomes a more common strategy such as a sea anemone transported on a predator black snail, pseudoscorpions on flying insects or bromeliad annelids and ostracods using frogs, lizards and snakes for transport (Aguiar and Buhrnheim 1998; Lopez et al. 2005; Luzzatto and Pastorino 2006). Many insect taxa display far reaching adaptations for phoresy. Examples include species of ischnoceran lice (Phthiraptera) phoretic on louseflies (Diptera), simuliid black flies (Diptera) with heptageniid mayflies (Ephemeroptera), mealybugs (Hemiptera) on ant queens (Hymenoptera) or midge larvae (Diptera) on the shells of water snails (de Moor 1999; Gaume et al. 2000; Prat et al. 2004). Eggs of torsalo, the human bot fly Dermatobia hominis (Diptera), are transported by mosquitoes and muscoid flies to their human host (Disney 1997). The bot fly itself also carries the phoretic mite, Macrocheles muscaedomesticae (Macrochelidae, Mesostigmata) (Moya Borja 1981). Depending on the agricultural or medical importance of the phoretic or phoront, the phoretic might be classified as parasite and the transport host as vector. The sphaerocerid fly Norrbomia frigipennis (Diptera) uses the time of transport on scarabid dung beetles to mate (Petersson and Sivinski 2003). Some pseudoscorpions also mate during carriage on their harlequin beetles. Larvae of the blister beetle Meloe franciscanus (Coleoptera) attach themselves to males of the solitary bee Habropoda pailida (Hymenoptera) for transportation, then transfer to a female bee during mating of the bees to continue transportation to the bees' nests (Saul-Gershenz and Millar 2006). Scarab beetles of the genera Canthon, Sylvicanthon, Parahyboma and Caththidium use arboreal mammals to reach carcass-baited traps in suspended at a hight of 10 m in semi-deciduous rainforest in the state of Minas Gerais, Brazil (Vaz-De-Mello and Louzada 1997).

In microarthropods and microinvertebrates like mites and nematodes dispersal by active locomotion becomes of minor importance compared with dispersal by wind (anemochory), by water or by animal carriers (phoresy) (Siepel 1994). The many species of nematodes involved in the decomposition of animal excrements rely for a significant part on phoresy by beetles and flies to reach their feeding grounds. The phoretic nematodes establish a clear succession of species (Sudhaus 1981; Sudhaus et al. 1988). Such a succession of phoretic mites has not yet been shown for animal and human remains.

The major carriers for mites are insects and most insect species carry phoretic mites. Phoretic association of the Astigmata have been reviewed by Houck and OConnor and of the mesostigmata by Hunter and Rosario (Houck and OConnor 1991; Hunter and Rosario 1988; OConnor 1982). While phoretic mites have been well described for fleas (Siphonaptera), they are not known for lice (Phthiraptera) (Fain and Beaucournu 1993; Schwan 1993). Other arthropods such as woodlice (Isopoda), centipedes (Chilopoda) or sand hoppers (Amphipoda) might serve as carriers equally well (Bloszyk et al. 2006; Colloff and Hopkin 1986; Pugh et al. 1997). However, phoretic mites have not (yet) been described for spiders (Araneae). Also vertebrates such as lizards, hummingbirds, small mammals and bats may function as transport hosts (Athias-Binche 1984; Colwell 2000; Domrow 1981; Krantz and Whitaker 1988; Tschapka and Cunningham 2004).

Phoresy in an evolutionary perspective is a transitional stage (Athias-Binche 1990; Athias-Binche and Morand 1993; Houck and OConnor 1991; Sivinski et al. 1999). This means that phoresy spans the whole spectrum from beneficial associations to parasitism. Definitions for phoresy in acarology can therefore be controversial (Walter and Proctor 1999). Some might limit phoresy to the middle ground where the phoretic or phoront has no interaction with its carrier other than transport, especially not feeding on its carrier. Phoresy implies that normal behaviour and physiological processes change. In many species it will lead to vast morphological alterations. In cases such as bless or betsy beetles (Passalidae, Coleoptera) where feeding off the host of numerous mesostigmatic mites has been observed, it does not seem overly detrimental to the carrier (Walter and Proctor 1999). In fly species, the phoretic mites might actually be predators of the newly oviposited eggs of the carrier (Jalil and Rodriguez 1970; Polak 2003). The mite, Proctolaelaps sp. (Ascidae, Mesostigmata), transported by the economically important flower weevil of the African oil palm feeds during transfer on phoretic nematodes that share the same carrier (Krantz and Poinar 2004). Thread-footed mites (Tarsonemidae, Prostigmata) phoretic on southern pine beetles carry a fungal species that can outcompete the symbiotic mycangial fungus carried by the beetle itself (Lombardero et al. 2003). Phoretic mites might become the food of phoretic pseudoscorpions riding on the same beetles (Zeh and Zeh 1992).

Phoretic associations of mites can lead to unexpected manifestations. Imagine a forensic investigator approaching a corpse that just has been discovered; he/she might be using protective clothing including disposable boot covers, not only for protection but also to avoid any contamination of the crime scene. Ants searching for a corpse behave as curious and perhaps careful as a forensic scientist, however they do not wear boot covers. At least two species of Macrocheles mites have become the boot covers or 'healing foot pads' of the larger workers of army ants (Ecitoninae, Hymenoptera). Females of Macrocheles cling to the pulvilli of leg III of the ant attaching themselves with their chelicerae for transportation. The mites have enlarged legs IV, which are now used as the new 'tarsal claws' by the ant and support all the weight. The diversity of associations between army ants and mites is remarkable. A collection on 1,600 army ant colonies yielded over 45,000 mites. Only 3% have been studied but resulted in three new families and 149 new species of mites (Elzinga et al. 2006). Thus, Macrocheles species do not travel alone. Phoronts belonging to different families attach to the most disparate areas of the body of ants. Trichocylliba species (Uropodidae, Mesostigmata) fasten themselves symmetrically to the insect abdomen; Planodiscus species (Uropodoidea incertae sedis, Mesostigmata) probably ride on the underside of the middle or hind tarsi of workers; Circocylliba crinita (Circocyllibamidae, Mesostigmata) is not only restricted to the mandibles of soldiers of Eciton dulcium but has also been found riding on the inner curve of the mandibles; Antennophorus species (Antennophoridae, Mesostigmata) clasp the venter of the head of the worker ant with the forelegs directed towards the mouthparts where they obtain food by trophallaxis from the ants; Messoracarus (Messoracaridae Mesostigmata) species sit below the head of the ant, there they are palpated by the ant's antennae and may steal provisions like cleptoparasites; Urodiscella species (Oplitidae, Mesostigmata) cling to the antenna cleaners on the forelegs of ants and apparently scavenge on debris combed by the ants; and others, like Laelapidae mites (Mesostigmata) occur on the body and are equally ignored by the insect during their journey (Eickwort 1990; Elzinga et al. 2006).

Looking at a single species of a medically and veterinary important fly, an incredible diversity of mite taxa were discovered. Adult stable flies, Stomoxys calcitrans (Diptera), collected in the United Kingdom, carried 12 species of mites from 10 families and three orders (McGarry and Baker 1997). A single insect depending on species can carry anything from a single mite to several hundred individuals of several species.

That phoresy is indeed the most important mechanism or in this case the only mechanism, by which forensically important mite species arrive at a carcass has been shown by Goff in an experiment in which he studied the mites on carcasses of adult cats and in the seapage zone beneath the carcasses (Goff 1989). He found 22 different species belonging to 15 different families. Mites of four of these families, Macrochelidae, Parasitidae, Uropodidae and Pachylaelapidae (all Mesostigmata) might be potenial indicators of post mortem interval. Mites of all these four gamasid families use phoresy to arrive at the carcass.

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