Phoretic Mites Separated by Carrier and Time

In the historic case detailed by Megnin and the more recent case reported by Goff from Hawai'i, mites have been used to estimate the post mortem interval (Goff 1991; Megnin 1894). Present cases often reveal large infestations with mites, e.g., case 7 in (Arnaldos et al. 2005), however the tools and expertise to identify these mites have still to be recruited and basic knowledge about the biology, especially the succession of these mites is still lacking. Population dynamics of mites in decomposing human or animal remains will prove to be of value in estimation of post-mortem intervals in cases of homicide, suicide, accidental death or unattended death due to natural causes where the post-mortem interval is greater than 17 days (Goff 1989). Perhaps the greatest value of phoretic mites might lie in increasing the resolution of entomological data during later stages of decomposition.

Already a good body of information about phoretic mites of necrophagous and necrophilous insects is scattered in the acarological literature.

One of the best studied systems is the association between phoretic mites with carrion beetles, Poecilochirus spp. and Nicrophorus spp. (Schwarz and Koulianos 1998; Wilson 1983). Due to their close interaction and high specificity, these mites might become as valuable as the beetles for forensic investigations.

The small scuttle or hump-backed flies (Phoridae) may be useful indicators of time of death when blow flies have disappeared (Disney 2005; Leclercq 1999). However, in some forensic cases these flies have presented problems of misidenti-fication and variable pre-oviposition times, both of which can lead to wrong estimates of the time of death (Disney 2005). Phorids might visit carcasses to feed but they might not lay eggs (Centeno et al. 2002). Alex Fain alone has described a new genus and seven new species of phoretic mites (Prostigmata and Mesostigmata) on Phoridae and Ephydridae from around the world (Fain 1998a, b). Phorid flies are now known to carry six species of Mesostigmata, five species of Prostigmata and one species of Astigmata (Fain and Greenwood 1991). The flies could easily be linked to the occurrence of their specific phoretics and it would help to precise the identification. Some phoronts will land when the fly oviposits, some others when the fly is only feeding on the corpse.

Dermestes or skin beetles are usually present at later stages of decomposition or dry decay. These beetles transport Lardoglyphus mites (Lardoglyphidae, Astigmata). The mites are much specialised to their carriers and compete for food, cleaning the left soft tissue attached to the bones. In laboratory cultures the mites can grow out of control and the prevalence of the phoretic nymphs on the carrier insect can be deleterious (Iverson et al. 1996). These mites have also been isolated form the gut of human mummies excavated in Chile and the United States (Baker 1990).

Although associations between muscoid flies (Muscidae, Diptera) and phoretic mites have been widely studied and data of life history traits of several species are well known (e.g., Ciccolani 1979; Ciccolani et al. 1977; Rodrigueiro and do Prado 2004; Singh et al. 1967), the phoronts of blow flies have been poorly characterised. In a preliminary survey in Argentina, phoretic mites were examined in relation with the stages of decomposition and arrival of insects to a pig carcass (Centeno and Perotti 1999). Parasitus fimetorum phoretic deutonymphs were found at the very fresh stage of decomposition together with the first blow flies. Macrocheles mamifer females arrived 2 days after P. fimetorum. M. mamifer is a very widespread macrochelid in the Argentinean pampas that has not (yet) established a specific association with carrion beetles although uses them for transportation (Perotti 1998). M. mamifer is likely to be found traveling on filth, carrion flies and mammals (Krantz and Whitaker 1988). M. muscaedomesticae collected under carcasses of cats were correlated with the presence of Musca domestica. Both, the occurrence of mites and flies were synchronised at the beginning of decomposition, on day 3 and following day 21 (Goff 1989). P. fimetorum will also arrive during a second wave. Carrion beetles (Silphidae) will carry P. fimetorum to a carcass (Hyatt 1980). The influence of phoretic mites of burying beetles on the competition between Nicrophorous quadripunctus and the blow fly Chrysomya pinguis (Diptera) for mice carcasses revealed that the phoretic mites contributed significantly to the success of this beetle species in colonising carcasses by killing blow fly eggs and larvae (Satou et al. 2000).

The best life history stage to found a new colony is a non-gravid meaning non-mated female (Binns 1982; Southwood 1962). A haplodiploid genetic system also called arrhenotokous reproduction perfectly matches with such requirements and phoresy. The phoront for Macrochelidae is the virgin female. Females are diploids and males are the outcome of asexual reproduction, they develop from unfertilised eggs resulting in haploids. When the virgin female arrives at a new and suitable breeding site carried by its fly or beetle, she detaches and shortly after feeding commences laying asexually a very few haploid eggs. The handful sons will be enough to fertilise their own mother and later on their sisters, which will bring then daughters to the site. In the first wave only males will be produced, then only females, which will overlap with the few males of the first wave. In a third wave the second generation daughters will then produce a less sex ratio biased and stable but likely spanandrous (scarceness of males) population depending to the environmental conditions. The short life cycle and the succession of sex ratio biases of the macrochelid and other haplodiploid mites might provide a highly increased resolution of the time line.

Similarly, representatives of the families Histiostomatidae and Glycyphagidae (Astigmata) are known to disperse by heteromorphic deutonymphs and hypopus types. Mites of the genus Pelzneria (Acaridae, Astigmata) are found in large numbers under the elytra of Nicrophorus beetles and synchronise with the life cycle of their carrier. Due to the haplodiploid genetic system common to Histiostomidae, the specialised deutonymphs must be females and must disembark from the parent beetles. The newly developed females will give birth asexually to sons who will fertilise their mothers. The next generation of female phoronts need to be ready only at the moment of the emergence of the next generation of adult beetles.

A different scenario is observed for movile phoretic deutonymphs of Poecilochirus mites during the landing of carrion beetles. Most Parasitidae mites are diplodiploid species. Because they need to have sex to reproduce, the phoretic deutonymphs hurry to moult into the adult stage to mate just after disembarking. The beetle has to carry phoretic deutonymphs of both genders. They disperse always in mixed groups having always the chance to find a mate at the destination point. Again, looking at the succession in situ, in a matter of hours the populations of landing deutonymphs will be no longer be detectable and adults with an equal, unbiased sex ratio will have replaced them building subsequent generations with a stable and even sex ratio.

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