Tiny mites on a great journey – a review on scutacarid mites as phoronts and inquilines (Heterostigmatina, Pygmephoroidea, Scutacaridae)

The members of the family Scutacaridae (Acari, Heterostigmatina, Pygmephoroidea) are soil-living, fungivorous mites, and some of them are known to be associated with other animals. After reviewing the mites’ behavioural and morphological adaptations to their animal-associated lifestyle, the present publication shows the result of a thorough literature research on scutacarids living in different kinds of associations with other animal taxa. It revealed that within the more than 800 scutacarid species that have been described so far, about the half of them can be found together with various animal taxa. The respective scutacarid species can be phoretic using their hosts for dispersal, they can be inquilines of their hosts benefiting from favourable conditions in the hosts’ nests, or they can be both. The highest number of scutacarid species, by far, (n = 214) is associated with ants. The second highest number can be found on beetles (94; mainly on ground beetles, Carabidae), followed by mammals (52), bees and wasps (35) and other insect taxa (39), and some species can be found together with birds (10) and arachnids (6). The most frequent genera Scutacarus, Imparipes and Archidispus show host preferences: Scutacarus and Imparipes tend to prefer ants, while Archidispus prefers beetles. Usually, scutacarid species are rather specialised on one host genus or one host family, but some seem to be host generalists. The possible influence of scutacarids on their hosts is not known yet, but they could play a sanitary role in their hosts’ nests.


Introduction
Phoresy is a phenomenon that can be found all over the animal kingdom. There are several definitions for this behavioural trait, ranging from strict to relatively loose interpretations. What they all agree about is that it is about a small individual, the phoront, being transported by a larger animal, the host. In contrast to parasitism, the phoront does not feed on its host. In fact, in strict definitions of phoresy like that stated by Farish and Axtell (1971), the phoront does not feed at all while it is attached to its host and does also cease any other activities. According to them, the only aim of phoresy is dispersal in order to reach new habitats which are favourable for reproduction (either for the phoront's own reproduction or that of its progeny). More recent and loose definitions of phoresy like that of Walter and Proctor (1999) don't exclude feeding during the phoretic stage anymore.
As phoresy is about dispersal, it is no wonder that this phenomenon gets more frequent the smaller the studied species are: small animals naturally don't have the ability to move for large distances on their own, instead they have to rely on other dispersal mechanisms like anemochory (transport by air currents), hydrochory (transport by water) or said phoresy. Mites (Acari) are characterized by consistently small body sizes (Krantz and Walter 2009), and several groups of mites have indeed developed different strategies for dispersing through phoresy. In all these mites phoresy is restricted to one stage, either the (gravid) females or deutonymphs (Krantz and Walter 2009). Some mites, like gamasid deutonymphs, do not have any special morphological adaptations to phoresy at all, but freely move on their host's body surface. Almost all other phoretic mites possess adaptations for attaching to their hosts, like ventral sucker plates in astigmatid deutonymphs or anal pedicels secreted by uropodid deutonymphs, both allowing the mites to adhere to smooth body surfaces (Ebermann 2004). Scutacarid mites, which are the main protagonists of the present paper, cling to their host by using enlarged claws on leg I.
The mite family Scutacaridae belongs to the superfamily Pygmephoroidea, Heterostigmatina, and is characterized by remarkably small body sizes of about 200μm, a strong sexual dimorphism and a shortened generation cycle with larvae being the only juvenile stage (Walter and Proctor 1999). All members of the family are fungivorous (Binns 1979;Ebermann 1991a;Ebermann and Goloboff 2002;Jagersbacher-Baumann and Ebermann 2013), and most of them are soil inhabiting mites occurring predominantly in decaying material, often in ephemeral habitats like manure or compost (Ebermann 1991a). Following the current taxonomy, the family Scutacaridae comprises some 24 genera (Zhang et al. 2011). In almost the half of these genera, species associated with other animals can be encountered, and the host taxa range from arachnids to insects and finally to mammals (e.g. Ebermann and Palacios-Vargas 1988, Eickwort 1990, Jagersbacher-Baumann and Ebermann 2012a. Most of the respective species have been encountered phoretic on their hosts, but there are also many species which only have been found as inquilines in the nests of their hosts. These inquilines are also included in the present review.
Although several studies about scutacarids associated with different host taxa have been published, a comprehensive overview about the known associations and all involved taxa was lacking until today and will be given in the present paper. Moreover, the morphological and behavioural adaptations of scutacarids to phoresy will be described.

Morphological adaptations of scutacarid mites to phoresy and phoretomorphism
Phoretic behaviour is presumably always induced by a reduction of the quality of food in the current habitat (Ebermann 2004). As soon as conditions get worse, mites have to be prepared to take action. In order to successfully perform phoresy, every phoretic mite instar, irrespective of the mite taxon, must fulfil two prerequisites: it has to 1) possess the sensory capability to identify suitable hosts and 2) have the mechanical ability to attach to the host (Eickwort 1994). While the sensory equipment responsible for host identification in scutacarids is not really known yet (see below, "Mounting and dismounting of hosts"), the morphological adaptations are well understood.
In Scutacaridae, only adult females perform phoresy, and their habitus is indeed predestined for this mode of dispersal (Figure 1). Their body is covered by curved dorsal tergits, which give them a tortoise-like appearance, and when attached to their hosts, their characteristic habitus offers practically no point of attack and thus it is difficult for the hosts to remove the mites (Ebermann 1991a). In regard of their general body shape, scutacarids are very similar to astigmatid hypopi (Anoetida, Acarida), and that is exactly what they had been mistaken for shortly upon their discovery in the 18 th century (Michael 1884). In fact, a body plan similar to that described above can not only be found in Scutacaridae and astigmatid hypopi, but also in Tarsonemidae, Pyemotidae and some Uropodina (Lindquist 1975, Ebermann 2004. Scutacarids attach to their hosts by using large claws on leg I (Figure 2a) which can grasp setae or soft intersegmental skin (Ebermann 1991a). There are scutacarid species without these large Figure 1 Fronto-dorsal view of a typical scutacarid adult female (Imparipes (Sporichneuthes) dispar Rack, 1964) moving on a carpet of fungi, scanning-electron micrograph. The curved tergites give the mite a tortoise-like habitus.

Figure 2
Differences between the tarsi and the claws on leg I of a) a phoretic and b) a nonphoretic morph of a female scutacarid (Scutacarus acarorum Goeze 1780). claws, and these species are generally supposed not to perform phoresy. As will be seen below, there are also species in which both occur, morphs with and morphs without large claws on leg I. The detailed structure of the claws can also differ between scutacarid genera: for example, the claws of phoretic Lamnacarus females have long, thin tips which are not present in other genera (Ebermann 1991a).
In addition to their characteristic morphology, scutacarid mites can also avoid being removed by their hosts by choosing attachment sites which are difficult to reach for the (grooming) host, like under the elytra of beetles (e.g. . It is a general trend that phoretic mite taxa are not distributed randomly on their host, but gather highly localised in places that are not groomed (Eickwort 1994, Ebermann 2004. However, some scutacarid species on the contrary attach to exposed body parts of their hosts: for example, Thaumatopelvis reticulatus Ebermann, 1980 clings to setae on the head of ants (Ebermann 1980b). The exposed location demonstrates that the claws are powerful attachment structures as the mites can even withstand the grooming of the host.

Phoretomorphism
In the genera Archidispus, Lamnacarus and Scutacarus, dimorphic species characterized by the occurrence of phoretic and non-phoretic females exist. This female dimorphism, which is also referred to as phoretomorphism, had firstly been suggested by Norton (1977) for Scutacaropsis baculitarsus agaricus (Norton and Ide, 1974) and had thereafter been demonstrated in other scutacarid species belonging to the genera Archidispus, Lamnacarus and Scutacarus through laboratory cultures by Ebermann (1991a,b). A similar dimorphism in connection with phoresy is also known from other mite taxa, either in deutonymphs or in adult females (Krantz and Walter 2009). To date, 11 scutacarid species are known to be dimorphic ( Table 1). The main difference between the two morphs is the size of the claw on the tarsus of leg I, which is large in the phoretic morph and small or even absent in the non-phoretic morph (Figure 2). Moreover, the sclerotisation of phoretic females appears to be stronger (Ebermann 1991a). In the genus Archidispus, there are also distinct differences in the shape of particular setae between the two morphs (Ebermann 1991a,b; Figure 3). The large claws on leg I and the stronger sclerotization can easily be interpreted as adaptations to phoretic behaviour because they allow attachment and also protect the mites during their journey. The possible adaptive value of the differently shaped body setae in the genus Archidispus on the other hand remains unclear.
Laboratory cultures showed that females of both morphs can also produce female offspring belonging to both morphs. The mechanisms for morph determination are not totally clear yet, but Ebermann (1991a) convincingly hypothesized that the quality of nutrition of larvae might Table 1 Scutacarid species for which phoretomorphism has been reported.

Taxonomical consequences of phoretomorphism
The detection of phoretomorphism had important consequences for scutacarid taxonomy as some taxa, which had been treated as separated species before, had to be merged into one single species. Accordingly, Ebermann (1990) demonstrated that Archidispus soosi  is the non-phoretic morph of A. armatus , Lamnacarus coprophilus Mahunka, 1968 is the non-phoretic morph of L. ornatus Balogh &Scutacarus subfimetarius Momen &Bagoury, 1989 is the non-phoretic morph of S. longitarsus (Berlese, 1905). In the same work, Ebermann also discouraged from using Variatipes, a subgenus of the genus Scutacarus that is defined by the absence of claws on leg I. This absence of claws could indeed be a "good" character evolved by a gradual reduction of the claws, but specimens without claws could also be non-phoretic morphs of other species.
Most taxonomic problems caused by phoretomorphism arise in the genus Archidispus as the two female morphs differ extremely in this taxon. For several Archidispus species, only the non-phoretic or the phoretic morph have been described so far (Ebermann 1990), and further analyses may reveal that many of these species in reality represent the two morphs of one single species. Until now, cumbersome rearing experiments have been the only way to unequivocally identify the two morphs of a phoretomorphic species (Ebermann 1991a). Nowadays, molecular genetic methods are also available for investigating the status of different morphs, but so far no studies on the topic have been performed.

Mounting and dismounting of host Mounting
The detailed act of identifying and mounting hosts is unclear for most phoretic scutacarids. Most species that could be observed alive didn't show any recognisable searching or appetence behaviour, but simply "adhered" to host individuals when being put together with them in plastic boxes in behavioural experiments (Schousboe 1986, Ebermann 1991a. Some scutacarids, however, display appetence behaviour: In order to locate potential new hosts, females of these species move to exposed positions and put themselves in an upright position, the "perching stance": The first legs are held out with or without questing movements similar to what can be observed in ticks (Binns 1979). The mites stand on their extended legs IV which are normally trailed during locomotion, and they are apparently backed by their dorsal setae. The appetence behaviour described above has been reported for Scutacaropsis baculitarsus (Mahunka, 1968) by Binns (1979), for Archidispus amarae (Kurosa, 1970), A. armatus, A. bembidii (Karafiat, 1959, A. magnificus A. minor (Karafiat, 1959) by Ebermann (1991a,b), for Imparipes (Sporichneuthes) dispar Rack, 1964by Ebermann (1995 and for the genus Lophodispus by Schlagbauer (1997) and , and it has moreover been observed in Imparipes (I.) histricinus Berlese, 1903, Scutacarus longipes Rack, 1975and S. longitarsus (Berlese, 1905 (Ebermann pers. comm.). In the genus Archidispus, Ebermann (1991a) noted that only phoretomorph females performed appetence behaviour, while non-phoretomorph females always fled whenever potential hosts approached.
Imparipes (S.) dispar, Lophodispus bulgaricus  and L. irregularis (Mahunka, 1971) are the only scutacarid species which are known to mount its hosts by jumping, as has been reported by Ebermann (1995) for Imparipes and by Schlagbauer (1997) in his master thesis for Lophodispus. The jumping behaviour of I. dispar is provoked by tactile stimuli of the mite's dorsal setae or even by air currents, and three types of jumping have been demonstrated: the aimed jump, the sideward jump and the high-and-long jump. While the latter two are interpreted as escape mechanisms for evading predators, the aimed jump is part of the mite's phoretic behaviour as it permits I. dispar to jump onto fast-moving hosts. The two Lophodispus species show the same jumping types as I. dispar and additionally are able to jump backwards, but it is more difficult to provoke the jumping behaviour in them. Although observations indicate that leg IV in all cases plays the most important role in jumping, the detailed mechanisms of the movement are unknown: neither I. dispar nor the two Lophodispus species show clear (external) morphological differences compared to non-jumping scutacarids.
Generally speaking, for successful host finding and identification, both mechanical as well as chemical stimuli are necessary. Mechanosensitive setae are located on the mites' dorsal side as well as on their legs (Jagersbacher-Baumann 2007), and chemosensitive setae (solenidia) can be found on the legs, with legs I possessing the highest number of solenidia. The perching stance of some scutacarids, where legs I are outstretched, strongly points to a great importance of chemical cues, whereas experiments with I. dispar showed positive reactions to tactile stimuli (Ebermann 1995), suggesting that mechanical stimuli are more important here (Ebermann 2004). Different scutacarid species presumably follow different strategies in the act of host finding and identification, but detailed studies on this issue are lacking to date.

On the host
On their host, the mites usually are motionless pers. obs.). There is only one observation by Travis (1941) describing scutacarid mites ("Disparipedidae") riding fire ants while standing "rather erect, bobbing up and down and tapping the ant with the first pair of legs". Travis further describes that the mites feed on liquids on the mouthparts and occasionally also on the anal opening of the ants. The described behaviour rather resembles the appetence behaviour described above and does not fit to what is known about the feeding habits (fungivory) of scutacarids at all. As his descriptions are extraordinary, it is possible that Travis either misinterpreted the observations he made in the artificial environment of a laboratory or in fact saw mites of the family Antennophoridae (Mesostigmata) and mistook them for Scutacaridae. Another remarkable report is that of Schousboe (1986) who noted that Scutacarus acarorum (Goeze, 1780) specimens on overwintered Bombus queens discharged dark fecal pellets, something that has never been observed by other researchers afterwards.
Scutacarid mites are no parasites of their hosts, although this assumption had been stated at least in the case of the bumble bee associated S. acarorum (Chmielewski 1971) and since then has been circulating like an urban legend (Chmielewski and Baker 2008, Kontschán 2015, Kontschán et al. 2016. Indeed, Chmielewski never really observed parasitic behaviour (pers. comm. between Chmielewski and Ebermann). Experiments by  and Schousboe (1986) proofed that scutacarids are not parasitic, and the morphology of their gnathosoma with its delicate, stylet-like chelicerae also shows that they hardly could be parasites (Schousboe 1986).
The number of scutacarids phoretic on one host specimen usually is low and ranges around 10 mites per host individual (eg. Ebermann and Hall 2003, Hall and Ebermann 2005, Kurosa 2005. Some species, however, can also be found in higher numbers. Mites belonging to the genus Archidispus associated with different carabid beetles for example frequently occur in abundances of around 40 mite individuals per host specimen (e.g. Kurosa 1983Kurosa , 1991Kurosa , 2003. The highest numbers of scutacarids are known for A. bembidii: more than 100 individuals have been reported from under the elytra of different carabid species , Ebermann 1991a. Generally, reports of more than 50 scutacarids on one host specimen are rather rare (e.g. 115 Scutacarus acarorum on Bombus terrestris queen (Schousboe 1986), 71 Imparipes apicola (Banks, 1914) on Andrena flavipes (Ebermann and Hall 2005)).

Dismounting
Dismounting of hosts has rarely been observed.  reported that Archidispus bembidii dismounted from beetles if the host was dead or if the mite was disturbed, and Binns (1979) noted that Scutacaropsis baculitarsus detached from anesthetized, decapitated flies. On the other hand, Schousboe (1986) was only in one exceptional case able to observe detachment of S. acarorum from bumble bees. He reported that apart from this case, the mites never dismounted, even if the host was dead. It is not clear which factors induce dismounting, but most probably the mites detect new habitats by chemical stimuli. "Negative stimuli" like disturbance or even death of the host seem to play a minor role in inducing dismounting. This assumption is supported by the fact that phoretic scutacarid mites can often be encountered on pinned, dried insects and also on animal material stored in ethanol (Ebermann pers. comm., pers. obs.).
Apparently the act of phoresy is risky for the mites because suitable environments cannot always be reached. Indeed, it is not too uncommon to find dead scutacarids on their host which obviously never arrived at their desired destination , Ebermann 1991a).

Sporothecae
As mentioned above, presumably all scutacarids are fungivorous. Only species of the subgenus Sporichneuthes of the genus Imparipes are known to feed on spores, all others suck the content of fungal hyphe (Ebermann 1998). While some species apparently are food generalists, others seem to be rather selective, feeding on only a few or even only on one fungal species (Ebermann et al. 2013). Some of the scutacarids with a constricted food spectrum developed a very rare phenomenon: they possess sporothecae. Sporothecae are defined as repositories inside the mites' body which are used for storage and transport of fungal spores (Suski 1973, Ebermann andHall 2003), and Jagersbacher-Baumann and Ebermann (2012a) moreover suggested that these repositories have to be filled actively by the mites. By transporting the spores of their preferred fungal species, the mites can secure food for their offspring: the spores are released and fungal hyphae start to grow as soon as the mites reach suitable habitats. Using this strategy, the mites avoid the risk of not finding their nutritional fungi when colonizing new environments (Ebermann et al. 2013).
Presumed sporothecae have been identified in four species belonging to the genera Heterodispus and Imparipes, and the two genera use different anatomical structures for transporting fungal spores (Figure 4): The African mite Heterodispus foveatus Jagersbacher-Baumann & Ebermann, 2012, associated with coleoptera and small mammals, possesses characteristic cavities in its posterior sternal plate which frequently are filled with spores and thus are interpreted as sporothecae (Jagersbacher-Baumann and Ebermann 2012a). Imparipes apicola, I. breganti Hall, 2004 andI. haeseleri Ebermann &Hall, 2003, all three phoretic on sphecids and wild bees, on the other hand use their genital atrium for fungal spore transfer Hall 2003, 2004). So far it has not been possible to identify the fungal species transported in the sporothecae Hall 2003, Ebermann et al. 2013).
Transport of fungal spores by scutacarids can also happen occasionally as the spores tend to adhere on the mites' surface (Jagersbacher-Baumann and Ebermann 2012a), which can frequently be observed in microscopic slides. Of course, this mode of transport does neither include specialised anatomical structures nor active collection of spores on the part of the mites, so this random occurrence of spores on the mites should not be confused with real sporothecae.

Scutacarid species and their hosts
Scutacarid mites occur in associations with other animal taxa because it is beneficial for them. Either the mites find favourable environmental conditions in the nests of their hosts (most likely, these conditions simply mean "availability of appropriate nutritional fungi"), or they mount their hosts in order to take advantage of their dispersal abilities so that they can reach new habitats, or both factors are true.
The following chapter will give an overview on (to my knowledge) all reports of associations between scutacarids and other animal taxa published until May 2017. In total, 414 species and subspecies in 17 genera have been reported from some kind of associations with other animals (reports of species listed as "near" or "cf" have been counted as separate species and are thus included in this number). Most species (n = 162) belonged to the genus Scutacarus, followed by Imparipes (125) and Archidispus (76; Figure 5). In the last published comprehensive summary of the taxonomic richness within the order Trombidiformes by Zhang et al. (2011), the family Scutacaridae is indicated to contain about 800 species in 24 genera, so it seems safe to estimate that approximately the half of all scutacarid species live in some kind of association with other animal taxa.
The following subchapters are divided into the reported host taxa, starting with Arachnida, moving to various taxa within the large group of Hexapoda, then to Aves and finally to Mammalia. Due to clarification reasons, the groups of hosts that are used in the graphs do not represent consistent taxonomic taxa, they rather are groups which combine hosts of different taxonomic levels using their common names (e.g., "ants" only describes members of one family while "mammals" covers the members of one whole class). Anticipatory it can be stated that by far most scutacarid species can be encountered in association with ants (Formicidae) (Figure 6) and that the prevailing types of associations vary between the different host groups (Figure 7). The subchapters give information on the number of associated scutacarid species of the host taxa and, if available, details on the associations and possible biological backgrounds. The included annex is ordered according to the hosts, giving information on the associated scutacarid species, the type of association (inquiline, phoront, or both), the geographic range and occasionally other interesting remarks (for example, other known habitats of the mites, like soil or moss). In another table available in the online supplementary material, the same information is organized the other way round, listing the scutacarid species first and adding the reported host taxa for each species afterwards.

Arachnida
Associations between mites and other arachnids in general are rare (Ebermann 2004), thus it is no surprise that scutacarid mites also can only rarely be found in association with other arachnids (Annex I.). So far, two Scutacarus species have been reported phoretic on spiders, Araneae, of the family Nemesiidae from Argentina (Ebermann and Goloboff 2002). Another species, the usually soil inhabiting Imparipes tocatlphilus Ebermann & Palacios-Vargas, 1988 from Central and South America, has also been found phoretic on Ricinulei (Ebermann and Palacios-Vargas 1988). On spiders and ricinuleids, the scutacarids attach to the soft integument of the legs Palacios-Vargas 1988, Ebermann andGoloboff 2002).
Even other mites, Acari, have been reported as phoresy hosts for scutacarids. Scutacarus talpae (Oudemans, 1913) was found by Oudemans (1913) inside of a mole's nest, and all five encountered individuals were attached to deutonymphs of the mesostigmatid Haemogamasus ambulans. In the respective publication, Oudemans expressed his surprise as the scutacarids could neither be found on protonymphs or adults of H. ambulans nor on any other gamasid genus, all of which were available in enormous numbers. Other scutacarid species using mites as phoresy host are S. acarorum and S. deserticolus Mahunka, 1969. Both are common Figure 6 Number of scutacarid species and subspecies, genera marked by colour, associated with different host groups. The group "insects" comprises Entognatha, Dermaptera, Diptera, Heteroptera, Isoptera and Orthoptera as well as hosts that have not been defined further in literature. phoronts on gamasid Parasitellus deutonymphs (mostly P. fucorum), which are in turn phoretic on various bumble bee (Bombus) species (Rack 1964, Chmielewski 1971, Schousboe 1986, Ebermann 1991a, Schwarz and Huck 1997). On the Parasitellus deutonymphs, the scutacarids cling to setae on body and legs (e.g. Schousboe 1986). The association between S. acarorum/S. deserticolus and Parasitellus sp. is the only example of frequent hyperphoresy in Scutacaridae. Hyperphoresy generally is an uncommon phenomenon that has apart from this example been demonstrated in mesostigmatid mites Błoszyk 2003, Szymkowiak et al. 2007) and accidentally in astigmatid hypopi (Athias-Binche 1994). Ebermann (2004) described one exceptional finding of hyper-hyperphoresy from a bumble bee: he found a scutacarid phoretic on a Parasitellus deutonymph, and the scutacarid in turn also had a hypopus of Anoetidae (Astigmata) attached.

Hexapoda Entognatha
Only one scutacarid species, Imparipes intentatus Khaustov, 2008, has been reported from Entognatha, all other hexapod hosts belong to Ectognatha. The respective host is a member of the Diplura, genus Campodea ; Annex I). As diplurans occur frequently in soil and don't have strong dispersal abilities due to their small size, the found association between mite and diplurans may have been by chance. When extracted through Berlese-Tullgren funnels and kept alive in boxes filled with plaster of Paris, some scutacarids appear to be stressed and for short periods of time tend to attach to whatever host available (pers. obs.). In these artificial environments scutacarid individuals have even been observed to "desperately" cling to other conspecifics. Bearing this behaviour in mind, unusual reports of phoresy like that on diplurans should be taken with a grain of salt.

Figure 7
Percentages of differently associated scutacarid species and subspecies according to their different host groups. Types of associations: p, phoretic; i, inquilines; ip, inquilines and phoretic, n.k., not known (not given in literature). The group "insects" comprises Entognatha, Dermaptera, Diptera, Heteroptera, Isoptera and Orthoptera as well as hosts that have not been defined further in literature.

Coleoptera
In the taxon Coleoptera, by far the most scutacarid species (62 spp.) can be detected on ground beetles, Carabidae (Figures 8-9; Annex II). All of the mites associated with Carabidae belong to the genus Archidispus, no other scutacarid genera have been found so far (e.g. , Kurosa 1977, 1989, 2005; for other references see Annex II). Moreover, Archidispus can only rarely be seen on beetles belonging to families other than Carabidae. Only two species, A. armatus and A. bembidii, can apart from Carabidae also be encountered on Heteroceridae, Hydrophilidae and Staphylinidae , and A. irregularis Katlav & Hajinqanbar, 2016 was found only on Staphylinidae. Generally speaking, different scutacarid genera can be found on other beetle families, but among them, the genus Scutacarus is the most prominent (Figure 10).
In the following, the beetle families reported to be hosts for scutacarids will be discussed.
Within the family Carabidae, most associated Archidispus species are rather opportunistic regarding their host choice: the majority of the reported species have been found on different species of one carabid genus, and about the half of the scutacarids occurred on several different carabid genera. The scutacarids have only been reported from imagines, they have never been found on the beetles' soil living larvae or pupae (Karafiat 1959). On their carabid hosts, the mites can be found on four typical attachment locations: the cervical membrane between head and prothorax, the intersegmental membrane between pro-and mesothorax, between thorax and abdomen, and finally under the beetle's elytra (e.g. , Kurosa 1970, 1972a. Some Archidispus species show no preference for special attachment sites (Kurosa 1984), while others prefer certain regions. For example, A. magnificus preferably attaches on the anterior part of the prothorax of its carabid hosts and A. sugiyamai Kurosa, 1991 on the posterior part of the prothorax ). These two species can also simultaneously be found on the same host, each one on its preferred attachment site. It is common to encounter more than one Archidispus species on one host (e.g. Kurosa 2009). Only five Archidispus species found on Carabidae have also been reported from soil samples, and all of them display phoretomorphism ( Table 1). The carabid beetles used for phoresy usually Number of scutacarid species belonging to different genera associated with beetles (Coleoptera). "Other genera" combine Diversipes, Lamnacarus, Symbolacrasis and Thaumatopelvis.

Figure 10
Number of scutacarid species belonging to different genera associated with bees and wasps (Apoidea and Vespoidea excl. Formicidae). "Other genera" combine Diversipes, Lamnacarus, Lophodispus, Nasutiscutacarus, Parascutacarus, Reductacarus, Rettenmeyerella, Scutacaropsis, Symbolacrasis and Thaumatopelvis. occur in moist environments, which are often inhabited by scutacarid species as they favour the growth of fungi. The moist habitats don't offer stable environmental conditions as they are prone to frequent flooding, and thus the respective scutacarids strongly depend on phoresy in order to escape from flooded regions (Ebermann 1991a).
Additional to Carabidae, scutacarids in moist habitats also use ripicolous Heteroceridae and Hydrophilidae as hosts (Annex III). On these two beetle families, species belonging to the genera Archidispus, Imparipes, Pygmodispus and Scutacarus can be found , Kurosa 1980, Messner 2001, Ebermann et al. 2016. The respective mites have been reported from under the elytra or attached randomly on the beetles' surface .
Members of Staphylinidae also often occur in moist habitats, moreover on dung and on carcasses (Zahradník 1985). Mites belonging to five scutacarid genera have been found phoretic on staphylinid beetles from the Holarctic and from Sudan: Archidispus armatus, A. irregularis, Imparipes dispar, I. histricinus, Lamnacarus ornatus and 3 Scutacarus species (e.g. , Ebermann 1991a, Truck 2012, Ebermann et al. 2016; for other references see Annex III). Most of these scutacarid species have also been recorded from other habitats like soil, moss, manure or Fragaria plants. On their host, the mites can be found rather randomly on different attachment sites like the legs, the lateral abdomen or between caput and prothorax , Ebermann 1991a.
Scutacarids can also be found on beetle families which occur in other than moist habitats which may be suitable for the mites as well: fungus-infested wood, manure, carcasses and different types of decaying material in general.
Beetle species with an affinity to fungus-infected wood can be found in the families Curculionidae and Cupepidae (Annex III). In Curculionidae, Scutacarus scolyti Mahunka &Moser, 1980 andS. cf. culmusophilus Sevastianov, 1975 have been reported from four species of wood borer and bark beetles from Eurasia (Mahunka and Moser 1980, Knapp 2008. The mite occurs in the galleries made by the beetles  and also phoretic on setae on the base of the coxae (Mahunka and Moser 1980). As the galleries made by these beetles are known to contain fungi (e.g. Vega & Blackwell 2005, Okabe 2013, it is probable that the scutacarids feed on them. Moreover, dead wood and microorganisms are available as possible food sources (Okabe 2013). The red palm weevil Rhynchophorus ferrugineus is another curculionid beetle which has been reported as host for a not nearer identified species of the genus Scutacarus in Egypt (El-Sharabasy 2010). The beetle is an important pest of palms in Asia and Europe which is known to be a host to several mite species (Al-Deeb et al. 2011), but associated scutacarids had never been reported before.
One species of Cupepidae, the North American Tenomerga cinerea, has been given as host of the scutacarid Imparipes cupes Delfinado &. Larvae of T. cinerea are known to thrive in fungus-infested wood (Hörnschemeyer 2005), which probably is a suitable habitat for the scutacarids as well.
Scarabaeidae, which often are coprophilous and as such visit habitats that are commonly used by scutacarid species, have also been reported as phoresy hosts for several scutacarids: Archidispus sacculiger (Mahunka, 1968), Heterodispus near elongatus (Trägardh, 1904), two Pygmodispus species and three Scutacarus species have been found phoretic on Scarabaeidae from Brazil, Canada, Chile, Iran, Java and the USA , Norton 1973, Ebermann and Rodrigues 2001, Ebermann et al. 2003; Annex III). Not all of the scarabaeid species used as hosts are dung beetles: imagines of Osmoderma eremicola feed on sap and damaged fruits, but the juvenile stages thrive in decaying wood (Norton 1973), and the juveniles of Bothynus striatellus are crop pests in South America (SiNAViMo 2015). As attachment sites on the beetles, the underside of the elytra, hairs on the ventral side and the suture between forelegs and head have been reported , Ebermann and Rodrigues 2001, Ebermann et al. 2003.
Another family of beetles occurring in different types of decaying material is the family Ptiliidae. These mycophagous beetles are characterized by very small body sizes (Zahradník 1985), still three different Scutacarus species have been reported from one single specimen of Ptiliidae which has not been identified to genus-or species level Zaki 1992, Truck 2012; Annex III). The mites attach to thick hairs on the abdomen or legs of the beetles (Truck 2012).
Members of the family Tenebrionidae also are used as hosts by scutacarids (Annex III). Beetles of this family are generalists (Zahradník 1985), and thus it is very likely that they often visit habitats that are suitable for scutacarids, too. Two Heterodispus species from Ukraine and Africa have been reported phoretic on tenebrionid beetles, whereas the concrete attachment sites are unknown (Khaustov 2008, Jagersbacher-Baumann andEbermann 2012a).
The last family of Coleoptera which has been reported as host for Scutacaridae is the Pselaphidae. Only one scutacarid species phoretic on Pselaphidae is known to date, namely Imparipes pselaphidorum Ebermann, 1988 from individuals of the genus Centrophthalmus in Tanzania (Ebermann 1988; Annex III). Many Pselaphidae are known to be myrmecophilous (Zahradník 1985), which could be an explanation for why the scutacarids can be found on with them: the mites could be primarily commensals of ants which also can move to the syntopically occurring beetles. However, the pselaphids found to bear I. pselaphidorum are not myrmecophilous and thus the reasons for the associations between mites and beetles remain unclear (Ebermann 1988).

Hymenoptera: bees and wasps
In addition to beetles, different Hymenoptera belonging to the suborder Apocrita are important hosts for Scutacaridae. Apocrita divide into the monophyletic subclade Aculeata (containing ants, bees and wasps) and the artificial, paraphyletic group Terebrantia/ "Parasitica". Members of Terebrantia have only been reported as hosts of Scutacaridae in one case: Imparipes dispar has been found phoretic on specimens of Eucoilidae, Proctotrupoidea and Pteromalidae (Ebermann et al. 2016; Annex I). On the other hand, a larger number of scutacarids (35 spp.) is associated with different social, but also with solitary Aculeata, as will be described below.

Bees
In the superfamily Apoidea, scutacarid mites occur together with Apidae, Andrenidae, Colletidae, Crabronidae, Halictidae and Sphecidae (Annex IV). In fact, Scutacaridae are amongst the most frequent and diverse associates of bees (Eickwort 1994). Most of the associated scutacarids belong to the genera Imparipes, followed by Scutacarus (Figure 10).
In Apidae, scutacarid mites play no large role as inquilines of the economically important honey bee Apis mellifera: only two species, Imparipes apicola and Scutacarus acarorum, have been reported from nests of honey bees, and neither of them is a frequent associate (Banks 1914, Schousboe 1986). More scutacarids can be found as common inquilines and phoronts of bumble bees (genus Bombus). The scutacarid acarofauna of bumble bees consists of 6 Scutacarus species, Parascutacarus indicus and Imparipes degenerans Berlese, 1904(e.g. Karafiat 1959, Cross and Bohart 1969, Larsson 2007, Jagersbacher-Baumann 2015 for other references see Annex IV). Imparipes degenerans, however, has only been reported once from Bombus and can normally be found on ants and in their nests and also in rodents' nests. Among the Scutacarus species associated with bumble bees, S. acarorum is particularly worth mentioning as it is one of the most common "bumble bee mites" (Chmielewski 1971), as it performs hyperphoresy on phoretic mesostigmatid deutonymphs (e.g. Schousboe 1986) and as it was one of the first mite species to be described in history by Goeze in 1780 (Goeze 1780). On bumble bees, scutacarid mites can be found on the base of the forewings, on the thorax or between thorax and abdomen, and they are most frequent on hibernated queens (e.g. Chmielewski 1971, Schousboe 1986). In the nests, the scutacarids can mostly be found in the outer portions, and they are much less frequent in artificial hives than in natural nests (Chmielewski 1971). As workers emerge constantly in bumble bee nests, the life cycles of most associated mites are expected not to be well synchronized with that of their hosts (Okabe 2013).
Indeed, the developmental cycle of S. acarorum takes around 9 days (Jagersbacher-Baumann and Ebermann 2013), being much shorter than that of its host.
In nests of Apidae, flower nectar, pollen, microorganisms and nest debris are present, although microorganisms and nest debris lack in honey bees (Okabe 2013). Dead brood, provisions and parts of the debris can become moldy and then offer suitable food for saprophagous mites like Scutacaridae (Eickwort 1994). As there is not much substrate which can become moldy in the nests of honey bees (Okabe 2013), this may explain the scarceness of associated scutacarids.
Solitary bees belonging to the families Andrenidae and Colletidae generally have few associated mite genera (Eickwort 1994). However, they serve as hosts for four scutacarid species of the genus Imparipes (e.g. Eickwort 1979, Ebermann and Hall 2003, Ebermann et al. 2013; for other references see Annex IV). The two bee families offer different habitat conditions: members of Andrenidae are ground-nesting, while Colletidae nest in dead wood. Imparipes breganti and I. burgeri, which use bees of both families as hosts, are thus considered to be pronounced generalists Hall 2004, Ebermann et al. 2013).
Imparipes apicola is associated with different species of Andrenidae, but also with Halictidae, which are another family of soil-nesting and rather primitive bees that exhibit a great diversity of mite associates (Eickwort 1979(Eickwort , 1994. Eickwort (1979) gives a thorough description of the life history of I. apicola inside the cells of a laboratory reared Lasioglossum host, showing that, in contrast to species associated with bumble bees (see above), the mite's life cycle follows that of its host. Halictid bees are also hosts for 15 other Imparipes species, moreover for two Nasutiscutacarus and two Scutacarus species (e.g. Beer and Cross 1960, Ebermann and Hall 2005; for other references see Annex IV). Most of these scutacarids have been found phoretic on the bees, but some have also been reported from their nests. Like in Apidae, nests of Halictidae provide flower nectar, pollen, microorganisms and nest debris as potential food sources for different mites (Okabe 2013). Accordingly, Ordway (1964) reported mites of the genus Imparipes from halictid nest cells containing pupae, near or on the fecal deposit, and Eickwort and Eickwort (1971) also found larvae of I. eickworti Mahunka in cell contents of its halictid host.
Sphecoid wasps of the families Crabronidae and Sphecidae have been identified as phoresy hosts for five scutacarids, one Archidispus (A. sphecis  and four Imparipes species (e.g. Kuhlmann 1998, Ebermann and Hall 2003, Ebermann et al. 2013; for other references see Annex IV). Among these mites, only A. sphecis (Lang and Mahunka 1977) is exclusively known from sphecids. The reported host species build nests in different substrates like sand, mud, dead wood or stalks. After deposition of the eggs, the nests are supplied with different paralyzed insects or spiders which serve as food for the offspring (Witt 1998). The nests probably are suitable habitats for mites as they contain enough material which could become moldy. However, no scutacarids have been reported from nests so far, all of the reported species were phoretic on the sphecoid wasps. Lang and Mahunka (1977) described the coxal region as favored attachment site on the hosts.

Wasps
Associations between Vespoidea (exclusive Formicidae!) and Scutacaridae are extremely rare. Species of Mutilidae and Pompilidae have been found to carry phoretic specimens of Imparipes burgeri Ebermann & Jagersbacher-Baumann, 2013 (Ebermann et al. 2013) and Scutacarus subquadratus Khaustov & Chydyrov, 2004). Wasps of these families build their nests in sand, are kleptoparasites of other wasps or parasitoids on halictid bees (Witt 1998). Another scutacarid, I. haeseleri, has been reported from a member of Vespidae Hall 2003, Hall andEbermann 2005). The respective host species, Symmorphus bifasciatus, builds its nests in stalks, reed or dead wood (Witt 1998) and animal meat, microorganisms, nest debris and dead larvae are available as food in its nests (Okabe 2013). Finally, Vitzthum (1927) reported random findings of S. acarorum on wasps, not without hypothesizing that these associations may be the rare results of occasional contact between wasps and bumble bees on flowers.

Hymenoptera: ants
The majority of scutacarid mites living in associations with other animals can be found with another family of Vespoidea, the Formicidae, or ants (214 spp.). Most associated scutacarids belong to the genera Scutacarus and Imparipes (Figures 11, 12; Annex V-VII). A great variety of different mite taxa can be found as guests of ants (e.g. Vitzthum 1919, Campbell et al. 2013, and it is also common to encounter more than one scutacarid species within one single ant nest (e.g. Friedl 2000). Scutacarids are either phoretic on the ants or they live in ant nests, or both. Inside the ants' nests, nest debris and (in army ants) temporary provisions are available (Okabe 2013), both of which can become moldy and then serve as food sources for scutacarids. On the ants, scutacarids can be found on the thorax, between the coxae, but also on rather exposed parts like the ant's legs or on the head (e.g. , Rettenmeyer 1961a, Ebermann 1982).
From not further determined "ants", 51 scutacarid species have been reported: Archidispus haarloevi , 13 Imparipes species, Pygmodispus calcaratus and 36 Scutacarus species (e.g. Karafiat 1959; for other references see Annex V). For all other reports of associations between ants and Scutacaridae, at the least the subfamily of the host was given. Most scutacarid species occur together with Formicinae and Myrmicinae (Figure 12; Annex VI, VII).
In the subfamily Formicinae, Archidispus intermissus , 28 Imparipes species, 2 Lophodispus species, 58 Scutacarus species and Thaumatopelvis reticulatus are present as associates (e.g.  for other references see Annex VI). The scutacarids could be detected in 5 ant genera, and the majority of all mite species (52 species) was present in the genus Lasius. Ants of this genus can occur in high population densities (e.g. L. alienus or L. niger), they frequently are social parasites of other ants and often feed through trophobiosis in symbiosis with aphids (Seifert 1996). Scutacarids can also often be found in associations with zoophagous Formica ants, further with the genera Camponotus and Paratrechina. There are even species  associated with desert ants of the genus Cataglyphus, which are highly thermophilic scavengers feeding mostly on dead arthropods (Lenoir et al. 2009).
Thirty-four Imparipes species, Lophodispus irregularis and 38 Scutacarus species have been reported from the subfamily Myrmicinae (e.g. Ebermann 1979, 1980a, b, Khaustov 2008 for other references see Annex VII). The mites have been found in association with ants belonging to 10 genera, and the largest number of mites (25 species) was present in the genus Myrmica. The genera accepted as hosts are characterized by different life styles, ranging from harvester ants feeding on seeds (e.g. Messor) to zoophagous (e.g. Myrmica) and omnivorous ants (e.g. Solenopsis). The colony sizes of the hosts also vary, ranging from 40-120 workers in Stenamma to some thousand workers in Aphenogaster (Seifert 1996).
Scutacarids can also be associated with army ants of the subfamily Dorylinae (following the taxonomic classifaction by Brady et al. 2014). These ants are carnivorous, they raid for food in large groups, build extremely large colonies and the whole colonies emigrate periodically (Rettenmeyer et al. 2011, Okabe 2013. Colonies of Dorylinae are home to several animals belonging to a variety of taxa, but their most abundant guests are mites (Gotwald 1996, Rettenmeyer et al. 2011. Accordingly, Dorylinae also serve as hosts for scutacarids: 39 scutacarid species belonging to seven genera have been reported from five army ant genera (Annex V). The respective mites are 4 Archidispus species, 24 Imparipes species, Pygmodispus dorylini , Rettenmeyerella petropolitana solenifera Mahunka, 1977 Scutacaropsis species, 5 Scutacarus species and 2 Thaumatopelvis species (Rettenmeyer 1961a,b, Ebermann 1980b, Berghoff and Franks 2007, Berghoff et al. 2009, Rettenmeyer et al. 2011. Within ants, Dorylinae display the highest diversity of scutacarids on genus level. Six scutacarid species (3 Imparipes, Lophodispus tapinoma Sobhi & Hajiqanbar, 2017 and 2 Scutacarus) were found in association with ants of the subfamily Dolichoderinae, either with members of the genus Tapinoma or with members of Liometopum , 1982; Annex V). Tapinoma ants are omnivorous and not sedentary, instead they often change the location of their nests, while Liometopum ants are sedentary, associated with trees and shrubs and they can also be minor pests in housing areas (Hoey-Chamberlain et al. 2013).
Only one scutacarid, Imparipes malus , has been reported from Ponera coarctata of the subfamily Ponerinae ; Annex V). The concealed life style of this thermophile ant species makes it difficult to sample (Seifert 1996, Wagner 2014, so intensified collections may reveal a higher number of associated scutacarids.

Other insect taxa
Different other insect taxa beside Coleoptera and Aculeata also serve as hosts for a variety of Scutacaridae (Annex I, Figures 6, 7, 13). There are unique reports of scutacarid species found on earworms (Dermaptera) and on bugs (Heteroptera), respectively (Trägårdh 1905. The mite species reported from earworms is Imparipes histricinus, which is a pronounced generalist and which has been reported from ants and beetles, from mammals' nests and from soil ( Table 2). As will be discussed below, I. histricinus might in fact be a cryptic species complex. On the other hand, the scutacarid associated with the heteropteran family Reduviidae, Imparipes nikitensis Khaustov, 2005, has not been reported from any other host .
Reports of Scutacaridae associated with Orthoptera are quite rare (Annex I). Two Heterodispus species have been found on crickets (Mahunka 1964a, and Imparipes rectangulatus  has been reported from grasshoppers . The latter species is another generalist, occurring also together with South American army ants (e.g. Berghoff et al. 2009).

Birds, Aves
Some Scutacaridae have been found in fecal pellets of eagles (Accipitridae) and owls (Strigiformes), in the debris of mutton bird (Procellariidae) nests, on the feathers of "Antarctic birds" and in the nests of other not identified birds (Annex VIII). Strictly speaking, the occurrence of scutacarid mites in birds' fecal pellets does not indicate any further association between the two animals; the respective mites have still been included in the present review because reports of scutacarids in connection with birds are scarce. The mites reported in association with birds (Figure 14) comprise 7 Scutacarus species, Heterodispus longisetosus (Womersley, 1955), Imparipes obsoletus Rack, 1966and Pygmodispus abestus Mahunka & Philips, 1978and unidentified Scutacaridae (Womersley 1955, Philips et al. 1988, Krivolutsky et al. 2004). Curiously, one of the Scutacarus species, S. meansi , has not only been found in bird's nests' debris, but also in dog food . Another species, S. imitans , has been reported from bird's nests, but also from nests of shrew and from the coat of grey squirrels , suggesting that this mite is a pronounced generalist. Apart from the mites from Antarctic birds, all scutacarids associated with birds have been reported from North America.

Mammals, Mammalia
Scutacarids have been reported from several "small mammals" (52 spp.), and in all cases where host species have been nearer identified in literature, they turned out to be soil dwelling and to belong either to the group of Eulipotyphla or Rodentia. Apparently, the nests of these mammals offer a favourable environment for mites (Ebermann 2004). Like in birds, most associated scutacarids belong to the genus Scutacarus (Figure 15). All in all, the following scutacarids have been reported: Diversipes exhamulatus (Michael, 1886), 5 Heterodispus species, 15 Imparipes species, Lophodispus irregularis, Pygmodispus zicsii , 28 Scutacarus species and Reductacarus singularis Mahunka, 1963(e.g. Mahunka 1967a, 1975b, 1977d for more references see Annex VIII). Most species occur in the nests of rodents, only 12 Scutacarus species and Reductacarus singularis were found in the nests of Eulipotyphla. While most scutacarids live inside their mammal hosts' nests, five species, namely H. foveatus, I. cupes, I. spickai , S. geomyi Mahunka, 1977, S. imitans and S. missourensis Mahunka, 1977 were found phoretic on their hosts (Drummond 1957, Basolo and Funk 1974, Mahunka 1977d, Spicka 1981, Estébanes-González and Cervantes 2005, Jagersbacher-Baumann and Ebermann 2012a. Amongst these, only I. cupes has not been reported from the coat of its host, the pocket mouse Chaetodipus spinatus lambi, but from its dorsal skin, where the mite apparently lives in small cavities that appear as scabs (Estébanes-González and Cervantes 2005). This report is extraordinary as it is the only one describing the occurrence of scutacarid mites "inside" its host and not attached to external structures like setae, hairs or feathers.
All of the scutacarids associated with mammals except the African H. foveatus and the Mexican I. cupes have been reported from the Holarctic. Unidentified scutacarids have also been reported from mammal carrion (Early and Goff 1986) and scutacarid mites can frequently be found in mammal dung (e.g. Mahunka 1964b, Truck 2012. It would have exceeded the scope of the present review to include all of these dung inhabiting species as, in contrast to Figure 15 Number of scutacarid species belonging to different genera associated with mammals (Mammalia). "Other genera" combine Diversipes, Lamnacarus, Lophodispus, Nasutiscutacarus, Parascutacarus, Reductacarus, Rettenmeyerella, Scutacaropsis, Symbolacrasis and Thaumatopelvis. scutacarids in bird's faeces, there is a high number of them and as they cannot be regarded as real associates of mammals.
Last but not least, there are even studies in which Scutacaridae have been found in connection with humans. Of course these encounters are no examples of associations and should only be considered as random or accidental, but they still are worth mentioning in this paper: Paik et al. (1992) found not identified Scutacaridae in low numbers in house dust in Seoul, Korea, and Modak (1991) found members of the genera Imparipes and Scutacarus in house dust in West Bengal, India. Modak even described a new species "Imparipes tropicus" from house dust, but as the description is only available in a PhD thesis and has not been published properly, it has to be considered a nomen nudum.

Genus specific trends
Some scutacarid genera show clear preferences towards special host taxa, while other genera are evenly distributed amongst a variety of hosts (Figure 16). Strong preferences can be detected in the genera Archidispus, Imparipes and Scutacarus. The vast majority of all known Archidispus species (85%) occurs phoretic on beetles, and within this taxon predominately on ground beetles (Carabidae). Less than 10% of the known Archidispus species have been reported in association with other hosts-ants, "insects", bees and termites. The term "insects" has been adopted from the original literature and indicates that the hosts had not been identified in detail. "Insects" might therefore also include beetles, meanining that the number of Archidispus associated with Coleoptera may even be higher than given here. There are no reports of Archidispus in association with birds or mammals.
Both Imparipes and Scutacarus clearly prefer ants as hosts: 63% of all known Imparipes and 58% of all known Scutacarus species have been found on ants or in ant nests. Both genera have also been reported from associations with beetles, bees, termites, other insects, small mammals and birds. Within these other host taxa, most species of both genera can be found on mammals and bees (or in their nests), with Imparipes showing a trend towards preferring bees and Scutacarus preferring mammals as host. Scutacarus is, moreover, the only scutacarid genus occurring in associations with other acari.
Pygmodispus can be found on ants, beetles, other insects, small mammals and birds, and there is a tendency towards "insects" as hosts since most species (63%) were found phoretic on this taxon.

Generalists and specialists
In most of cases, the host range of single scutacarid species apparently is rather narrow. One species usually uses hosts belonging to one genus or family, and several species have been reported from only one host species (Annex). A similar trend can also be recognized in other phoretic mite taxa. For example, when analyzing mites associated with bees, Eickwort (1994) reported that few mite species can be found on only one host species, but that there also few occurring on hosts belonging to more than one subfamily; the majority of mite species are in association with many congeneric host species which are available in the range of the mite.
Examples for scutacarids that have been reported only from one host species are Imparipes adleri  found on the termite Reticulitermes virginicus , Pygmodispus zicsii from the rodent Cricetulus migratorius  or Scutacarus sabinaesimilis Khaustov & Chydyrov, 2004 from the ant Tapinoma simrothy . However, Scutacaridae in general are weakly studied and several reports of phoretic scutacarids are random findings. Comprehensive or more focused studies on the mites' hosts like the one by Ebermann et al. (2016) on I. dispar might thus reveal more host species.
Some scutacarid species, however, already seem to be pronounced generalists and can be found on a variety of hosts belonging to very different taxa ( Table 2). For most of these host taxa, one important ecological similarity can be encountered: a lifestyle strongly connected to the substrate "soil". Because of this shared habitat, it probably is rather easy for mites with a generalist life style to switch between different hosts. For other host taxa, like flies and ants, it is more difficult to find clear points of contact between them and thus to explain the occurrence of a mite species on both groups.
While some of the presumed generalists have been investigated thoroughly and thus their species status is quite certain (e.g. Jagersbacher-Baumann and Ebermann 2012a, Ebermann et al. 2016), the status of other generalists is questionable. As will be discussed below, the respective species could have been misidentified or might be complexes of cryptic species.

Discussion and future perspectives
The present work shows that most animal taxa that are accepted as host by scutacarid mites have a strong connection to the habitat "soil": either they are highly mobile, epigaeic species, or they are endogaeic species building nests in the ground. As Scutacaridae are also soil inhabiting mites, it seems probable that associations between them and their host evolved through the shared habitat. The genus specific trends towards different taxa also indicate that the associations between scutacarids and their hosts evolved independently several times within the family, and most probably also within the genera.
Exceptions from typical soil living hosts are bark beetles and some Apoidea and Vespoidea that thrive in dead wood, reeds and stalk; moreover not ground dwelling birds which can harbour scutacarids in their nests. These hosts, however, still offer favourable conditions in form of mouldy debris in their dwellings which contains the mites' nutritional fungi.
Based on the compiled information on scutacarids and their hosts given in the present review, their associations can be grouped into three categories with typical ecological characteristics: 1. Phoresy only Host within this group have high dispersal abilities and are only used for phoresy. They can transport the scutacarids to new, suitable habitats, but they themselves do not provide theses respective habitats (that is, they do not build nests that offer favourable conditions for the mites). Typical for this group are epigaeic, fast running Carabidae and several other beetles like Staphylinidae or Hydrophilidae, moreover flying insects like Orthoptera and Diptera.
2. Shared habitat only This group includes soil-dwelling hosts whose nests have been invaded randomly by the scutacarid mites through the surrounding soil (as described by Okabe 2013). The mites live in the dwellings of their hosts as facultative commensals, or, if the association is already more evolved, as obligate inquilines. In this category, hosts are not used for dispersal. This type of association is not very common; typical hosts are mammals and ant species with large and temporarily stable nests, like some members of the genus Crematogaster.
3. Shared habitat and phoresy In this category, the hosts build nests which are more or less temporary. The scutacarids live in the hosts' nest as obligatory commensals and when the time is right they use their hosts for phoresy in order to reach new habitats, that is, new nests. Examples for hosts in this group are ant species with fissioning colonies, bumble bees with annual colonies or wild bees which build single brood cells. In fact, the majority of associations between scutacarid mites and their hosts belong to this category.

Future perspectives Behavioural aspects
Although many associations between various species of Scutacaridae and other animals have been reported, most times also including information about the type of association (inquilinism, phoresy, or both), few details are known about the behaviour of the mites and the interactions with their hosts. At has been mentioned in the introduction, the mechanisms of host finding, host identification, host switch and identification of suitable habitats are only speculative today and should be subject of future studies. Behavioural experiments which could shed a light on this issue can, however, be rather complicated because of the mites' extremely small size which makes them more difficult to handle.
The cohabitation of mites and their hosts also raises questions: What fungi exactly do the mites feed on in the shared habitat, are they food specialists or generalists? The nutritional fungal species have only been identified for very few scutacarid species (Jagersbacher-Baumann and Ebermann 2013). Have the scutacarids any influence, positive or negative, on their hosts? Most likely, the tiny mites do not bother or harm their hosts in any way. A strong support for this assumption is the fact that hosts usually do not remove their mites (Eickwort 1994). It is possible that Scutacaridae even have a positive effect on their hosts as they could play a sanitary role by feeding on potentially harmful fungi. Such sanitary roles have been hypothesized for other mite taxa before (Okabe 2013), but so far only one study provides statistically supported evidence for it: Biani et al. (2009) observed significant correlations between the presence of Laelaspoides mites (Mesostigmata) in a bee nest, the absence of fungi in brood cells and a decrease in bee mortality. Studies on scutacarids about this topic are still lacking to date. Preliminary observations revealed that scutacarid mites can consume considerable amounts of fungal hyphe in laboratory cultures (Baumann unpublished), so a sanitary effect indeed seems probable.

Cryptic species complexes?
In scutacarid species with a broad host spectrum and/or a wide geographical distribution, doubts about the status of the respective species should arise for two reasons, as has already been discussed in Ebermann et al. (2016).
(1) The frequent occurrence of a species on certain hosts tempts to neglect a proper identification. For example, S. acarorum is widely distributed and is also the most common scutacarid associated with bumble bees. Comparisons between presumed S. acarorum individuals from spatially distinct populations (European localities and New York) confirmed their conspecificity even after thorough morphometric analyses (Jagersbacher-Baumann 2015). Because of its abundance, apparently all scutacarids encountered on bumble bees often are being identified as S. acarorum without any closer inspection. However, there are at least three other morphologically similar scutacarid species associated with bumble bees: S. deserticolus, S. mendax and S. occultatus . They all belong to the acarorum speciescomplex as they share a very similar phenotype and can also be encountered syntopically in the nests of large bumble bee species (Jagersbacher-Baumann 2014). Re-inspections of presumed S. acarorum individuals thus already revealed incorrect classifications: for example, specimens of "S. acarorum" from Hamburg, Germany, were identified as S. deserticolus (Ebermann 1991a), and "S. acarorum" reported from bumble bees in Argentina turned out to be S. mendax or a new species close to S. mendax (Revainera et al. 2014 and pers. comm.). A not nearer identified scutacarid from Brazilian bumble bees ) also turned out to be a variation of S. mendax (Baumann unpublished).
(2) Thorough morphological and molecular genetic analyses of scutacarids will most likely reveal the existence of several "cryptic" species. The term cryptic species describes species which are impossible or extremely difficult to distinguish by traditional (morphological) means, and due to advanced techniques, many of them have been described in the recent years in mites (Knee et al. 2012, Skoracka et al. 2015. Although it had been stated before that host-race formation of mites might be a main driving force for cryptic speciation, this hypothesis could not be supported by Skoracka et al. (2015). They showed that not only strong host relationships can induce speciation, but also abiotic or other host-independent environmental factors can do so. An example for a cryptic species complex could be present in the scutacarid I. histricinus, which apparently is one of the most frequent guests of ants (Annex V-VII; , Okabe 2013. Closer inspections of the mites identified as I. histricinus might reveal the existence of several new species with a similar phenotype. In fact, there are already several reports of scutacarids only being similar to I. histricinus: for example,  reported I. cf. histricinus from ants of the genus Formica. The respective specimens have not yet been described as separate species. Additional, all other scutacarid species with a broad host spectrum ( Table 2) or species with a wide geographic distribution also could in reality be complexes of cryptic species.

Phoretomorphism
The factors determining the morphs in dimorph/phoretomorph scutacarid genera like Archidispus or Scutacarus are another point which needs further clarification. Ebermann (1991a) already showed that morph determination might happen in the larval stage and might be induced by the quality and/or quantity of available food, but detailed and statistically supported studies are still pending.

Scutacarid fauna
Last but not least, the knowledge of the scutacarid fauna in different countries is very fragmentary, depending on the individual interests of different researchers and made difficult by the generally low abundance and small size of the scutacarids. While some countries like Austria, Hungary, Iran or Russia have been and/or are still investigated intensely by researchers like Ebermann, Hajiqanbar, Khaustov, Loghmani and Mahunka, others, like Spain or Portugal (Ferragut 2015) are practically "terra incognita" when it comes to Scutacaridae. From other countries, soil living scutacarids have been reported, but few or no associations with other animals are known yet. For example, although several scutacarid mites are known from Australia (e.g. Mahunka 1967c, 1974d, only three species have been reported from associations with other animals (Womersley 1955, Mahunka 1975c, Seastedt et al. 1986). Investigating Scutacaridae from new geographical areas as well as closer looks on other possible host taxa will most likely bring more associated/phoretic scutacarid species to light, several of which might be new to science.