Specificity of Phoretic and Carrier

Carrion as a microenvironment or microhabitat undergoes constant changes. Necrophilous insects and the many occasional visitors of carcasses may attract and associate with phoretic mites less specialised to their carriers. Necrophagous insects, specialists for fabric or hair and specific predators and parasitoids will often exhibit a much higher degree of carrier specificity. For many of the dipterans that arrive after death, the carcass is just one of the many different habitats that they are able to colonise, often having the choice of another suitable rotten food source. On the other hand, there is often a high degree of specificity for carrion beetles and the mites carried.

Euryxene mites will use different arthropod taxa while stenoxene ones will travel on different species of a same genus. Mites that are transported by only one species of host are oioxene mites (Athias-Binche 1994). And, according to Lindquist it is possible to discriminate up to four types of life cycle synchronization between phoront and carrier (Lindquist 1975).

Macrocheles muscaedomesticae is a wide spread phoretic mite of forensic interest. Although it was early classified as a eurexene phoretic (Athias-Binche 1984) it seems to be specialised to muscoid flies. The behaviour of macrochelid mites is particular unusual when it combines food with transportation. They feed on the progeny of their own carriers, Muscidae or Fanniidae flies (Perotti and Brasesco 1997). Its name refers to its close association as phoretic with Musca domestica (Diptera). It has evolved into a complex interaction of phoresy and predation at the same time. The life of this mite is one of the shortest known, properly adapted to fast-changing microhabitats and shortage of food, it only feeds on the eggs and first instar larva of the muscoid fly, never on older stages. In optimal environmental conditions it takes the species to develop from egg to egg only 3 days, pasing through larva, protonymph, deutonymph and adult (Rodriguez and Wade 1961; Wade and Rodriguez 1961). M. muscaedomes-ticae arrives at a decomposition site as a virgin female attached to the body of only female Musca spp. (Muscidae, Diptera) or Fannia spp. (Fanniidae, Diptera) (Perotti 1998). Coincidentally, Musca and Fannia commonly overlap in their breeding habitats. It has been proved that the mite is not host-density dependent, what already determines its selectivity, becoming a stenoxene or semi-specific phoront, attracted by a chemical gradient produced by the female carriers (Farish and Axtell 1971; Niogret et al. 2006; Perotti and Brasesco 1996).

For some species, the phoretic mites aggregate before the carrier develops to adulthood. Myanoethus muscarum (Histiostigmatidae, Astigmata) hypopi cluster on the anterior end of the pupa of the host fly, which is the location from which the imago will emerge (Greenberg and Carpenter 1960). Less than 1 h after pupation of the fly, Muscina stabulans (Diptera), the hypopi heavily aggregate on the anterior third of the pupa. Different experiments conducted by Greenberg have demonstrated that the mite nymphs are attracted to a volatile substance secreted or emitted from the anterior end of the pupa of several fly species including Musca domestica, Muscina stabulans and Stomoxys calcitrans. The grouped hypopi remain on the pupal case until eclosion, at which time they transfer to the emerging fly. After a brief excursion on the carrier the mites occupy characteristic locations for attachment. They rather prefer areas bearing setae and avoid smooth cuticle. Horizontal rows of mites build outward and toward the rear. They uniformly orient themselves toward the rear (Greenberg 1961). After dismounting, a minimum of 2 days at 24°C is required for the moulting into the next stage, the tritonymph, which may moult to adult in 1 day.

The relation between carrion beetles and their phoretic mites have been studied in detail (Schwarz and Koulianos 1998). Nicrophorus beetles are used by mites of the Poecilochirus carabi (Poecilochiridae, Acari incertae sedis) species complex, which are specialists that depend on the food provided by the carrier and its nests. The mites are not able to search for new carrion on their own. They have synchronised their life cycle with that of their beetle hosts. They arrive at a new carrion on the parental hosts, reproduce to such a strict time table that their offspring is ready to leave as soon as the parental male decides it is time to abandon the brood chamber in the search for a new breeding site. The female beetle follows the male with the remaining mites attached to her, and in cases where some passengers didn't get a seat, they have to wait until the larvae of the next generation has fully developed. Poecilochirus mites also switch between burying beetle species, they discriminate between adult beetles olfactorily and they choose those reproductively active. Almost every adult male carrion beetle arrives at a body transporting phoretic mobile deutonymphs of Poecilochirus and this phenomenon is expected worldwide. Within hours after arrival, the deutonymphs disembark, moult into adults and reproduce immediately. The non-phoronts live off microfauna associated with the carcass in the brood chamber. The favourite association of the deutonymphs are always the parent beetles. 100-250 deutonymphs can be found on adult males compared to less than five commonly observed on the larvae. In central Europe, one might expect three to four different species of carrion beetles at a big carcass at a time, which would contribute up to seven mesostigmatic and three astigmatic mites species to the carcass' microfauna, probably originated from diverse allopatric populations.

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