Phoretic Adaptations

Phoresy might be facultative for a mite species or obligate. If it is obligate, a particular species cannot develop to adulthood or reproduce without prior engaging in phoretic transport. Examples for obligate phoresy at immature or juvenile stages include Histiostoma brevimanus (Histiostomatidae, Astigmata), Parasitus coleoptratorum (Parasitidae, Mesostigmata), for obligate phoresy as imagoes (adults), Macrocheles muscaedomesticae. Carrier specificity spans the whole range from specific for a host family to a particular body part of a single species. Occasionally, phoretic mites might have an alternative carrier. The phoretic deutonymphs of the uropodine mite, Fuscuropoda marginata (Urodinychidae, Mesostigmata) usually attach to dung beetles but can switch to ground skinks, Scincella lateralis. This mite and other acarine taxa may be preadapted to shift between arthropod hosts covered with chitinous scler-ites and vertebrate hosts covered with keratinous scales (Mertins and Hartdegen 2003). Necrophagous mites might shift carrier as carrion beetle species pass in and out of their reproductive seasons (Brown and Wilson 1992).

Athias-Binche gives a comprehensive overview of the many, often inimitable life history traits that have been realised in mites to accommodate phoresy (Athias-Binche 1994). There are five basic types of phoretic adaptations in the acari: regular nymphs and females, two specialised nymphs and specialised females. A population of mites can be monomorphic or facultative dimorphic at a specific stage of development or gender. A phoront often needs to be modified to resist desiccation during transportation. The adaptations for phoresy often have such an impact on the morphology that phoretic stages have been described as different species even belonging to different families.

The most basic forms are the regular adult females or regular deutonymphs, which have not evolved elaborate morphological modifications but exhibit physiological changes for transportation. In the Mesostigmata, suborder Dermanyssina, and in the Prostigmata, superfamily Cheyletoidea, it is the adult female that makes the move. Most Macrocheles mites provide good examples for females adapted for transportation (Fig. 5.1); they use their chelicerae to hold on to the hairs of their beetle or fly carriers. Some Parasitus species (Parasitidae, Mesostigmata) start their journey when they reach the stage of deutonymph. The claws of their front legs seem to be used to grasp the setae of their host, however, some of these species are

Fig. 5.1 Forensically important phoretic mite. Scanning electronmicrograph of dorsal view of an adult female, phoretic stage, of Macrocheles sp. Scale bar = 500 ^m

not fixed to a certain spot but likely to be seen moving around the dorsal side of their host during transport.

Drastic modifications in both the morphology and the physiology of adults and nymphs are documented in the orders Mesostigmata, Astigmata and Prostigmata. They include inactive or quiescent stages in which the gut, for example, might not develop completely. These mites might be monomorphic for a given population or engage in a dimorphic population where phoretic and non-phoretic forms can be seen side by side. The term 'hypopus' (hypopi, hypopodes), originally a specific epithet, refers to the altered phoretic deutonymphs existing in several species of Astigmata. Interestingly it was Pierre Mégnin, one of the fathers of modern forensic entomology who introduced the term.

... une mue bien extraordinaire: dans l'enveloppe des nymphes (jeunes sujets octopods non encore sexués), - et des nymphes seulement, - se forme, non pas comme dans les circonstances ordinaries un individu semblable à elles, plus avancé en développement, mais un Acarien cuirassé totalement différent: ses organs buccaux sont atrophiés et il est muni d'un groupe de ventouses sous et post abdominals qui lui permettront de s'attacher à tout être animé qui passera à sa portée et à fuir un lieu de désolation où il mourrait de faim comme cela arrive forcément à ses parents adultes et aux jeunes larves hexapodes qui n'ont, ni les uns ni les autres, le pouvoir de subir cette métamorphose. Lorsque les hasards du voyage ont fait arriver le nouvel Acarien cuirassé dans un lieu d'abondance, il descend de son omnibus improvisé, se dépouille de son costume de voyage, reprend sa forme ancestrale et se met en devoir de constituer une nouvelle colonie. A la suite de la découverte de ce fair curieux nous avons nommé cette mue particulière, mue hypopiale, parce que ces Acariens cuirassés, masqués, en habit de voyage, avaient été pris par les Acariologues pour des espèces définies qu'ils avaient nommées Hypopus, ... etc., ces noms ont disparu pour faire place à l'adjectif hypopial. (Mégnin 1892).

a quite extraordinary moult: under the cuticle of the nymphs (young octapod individuals that are not yet sexual) - and nymphs only, - is formed, not as is the case under normal circumstances an individual similar to them, just more advanced in development, but a completely different phoretic mite: its mouth parts are atrophied and it is provided with a group of suction cups under its abdomen and at its end which will enable it to stick to all lively creatures which will pass within its range and to flee a place of desolation where it would die of hunger like the ones that arrived inevitably as its adult parents did and young hexapod larvae, neither of which have the capacity to undergo this metamorphosis. When luck during the voyage brought the new phoretic mite to arrive at a place of abundance, it disembarks from its improvised carrier, shed its travel suit, regains its ancestral shape, and is put in a state to start a new colony. Following the discovery of this curious fact, we named this particular moult, a hypopal moult, because these phoretic mites, masked by their travel suits, were mistaken by acarologists for new species, which they had named Hypopus, ... and so on; these names have disappeared to make place for the adjective hypopal.

Mégnin even described two types of hypopi for the species Falculifer rostratus (Falculiferidae, Astigmata), the nymph adventive or hypopiale première form for large, male hypopal nymphs, which have been depicted in many textbooks, and the nymph adventive or hypopiale deuxième form for small, female hypopal nymphs giving the term heteromorphic a total different meaning (Robin and Mégnin 1877). However, Mégnin might himself have fallen victim of the difficulties of acarology. Graf Vitzthum contests that the large form represents both sexes of the hypopi and the small form is actually the hypopus of another species of another family, Megninia columbae (Analidae, Astigmata) (Fain and Laurence 1974; Graf Vitzthum 1933). At present there is still some controversy about the use of the term 'hypopus' for the morphologically modified phoretic deutonymphs of the Astigmata; some specialists prefer to call these nymphs heteromorphic deutonymphs (Houck and OConnor 1991; Walter and Proctor 1999).

Heteromorphic phoretic deutonymphs of Uropodidae (Mesostigmata) and the hypopi of Histiostomatidae (Astigmata) have evolved into aerodynamic flattened and sclerotised shapes with reduced legs normally folded inside lateral cavities during flight. They can even fold up their rudimentary, unfunctional gnatosoma or they might just have a reduced gnatosoma or have lost the chelicera. The anal region of the phoretic deutonymphs of these distantly related families is characteristically modified. The deutonymphs of uropodid species, such as Uroseius spp, secret a cement to build a pedicel that adheres to the exotegument of the host. The phoretic hypopi of Histiostomatidae on the other hand present strong anal suckers that fix the mites to the carrier.

Perhaps the phoretomorphic females are the most recently studied. The type was described in the Prostigmata family Tarsonemidea by Moser and Cross as 'a female specialised for riding insects' (Moser and Cross 1975). Again, these females can arise facultatively, which allows dimorphism in some species, with phoretic and non-phoretic females produced at the same time depending on the need for transportation to a suitable habitat.

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