Phthiracarus Perty, 1979

Parry, B. W., 1979, A revision of the British species of the genus Phthiracarus Perty 1841 (Cryptostigmata: Euptyctima), Bull. British Mus. nat. Hist., Zool. ser. 35, pp. 323-363 : 324-334

[Genus Phthiracarus Perty ]

External morphology

The following account of the external morphology of Phthiracarus refers to the adult only; full descriptions of the immature stages will form the basis of another paper. The setal nomenclature used in the later works of Grandjean has been followed throughout the account.

Idiosoma

Aspis; Fig. 1B-D; Pl. 1a, e): A pair of oval weakly-sclerotized areas anterodorsally marks the positions of the retracted chelicerae. The ventral margin of the aspis is reflexed to form the aspal rim (a.r.) and there is a distinct lateral ridge (l.r.). The bothridium (b.) has an inner multi- chambered wall and a smooth outer one from which three finger-like chitinous tracheoles arise and are directed mid-dorsally. The margin of the bothridial aperture is thickened (as shown by the arrow in Pl. le) and fianked posteriorly by a pronounced scale. There are three pairs of procum- bent dorsal setae, the rostrals (ro), lamellars (la) and interlamellars (il), and two pairs of setae laterally, the exobothridials (ex) and the sensilli. In most of the species examined, setae il and la are located at the level of the bothridia, la being somewhat shorter than il. The sensilli are variable in form and so provide a useful taxonomic feature. In some species they are short, ovate or lanceolate while in others they are long, narrow and tapering. The sensillar margin may be serrated (Pl. 1a) and in P. serrulatus sp. nov. it bears a number of straight-edged teeth sub- terminally. The sensillus is most easily observed in scanning electron micrographs as in flattened slide preparations its appearance can depend very much on orientation.

Notogaster (Fig. 1A, E; Pl. 1e): The anterior margin of the notogaster is well sclerotized and, following Jacot (1930), can be subdivided into three regions: the thickened collar (C), the pseudo-stigmatic [sensillar] notch (N) (Pl. 1e) and the lappet (L) which projects somewhat anteriorly. Of the 15 pairs of setae, 14 are regarded as being homologues of c1-3 and cp, d1-2, e1-2, h1-3 and ps1-3 of the holotrich nomenclature, and the additional seta as ps4. The distributional pattern of notogastral setae is essentially similar in all the British species but the relative lengths and attitudes of the setae vary considerably from one species to another. The vestiges of setae (f1) and (f2) are thought to be represented by two pairs of subcuticular structures located posterolaterally (Grandjean, 1950). Vestigial f1 normally lies between setae h1 and ps1 and f2 between setae h1 and h2 but in certain small species (for example, P. serrulatus ) f1 is closely associated with the seta h1. There are four pairs of prominent subcuticular fissures: the anteriors (ia) and medians (im) are situated just posterior to seta cp while the posterior pleurals (ip) and infrapleurals (ips) (when present) are situated on either side between setae h2 and h3 and between setae ps3 and ps4, respectively.

Ano-genital region (Figs 2E; 3B; Pl. 1b): On each anal plate there are five setae. Two anal setae an1-2 are located on the paraxial margin and three adanals ad1-3 submarginally; setae ad1-2 are often vestigial. Each anal plate has a prominent hood-like lobe located ventro-anteriorly on its paraxial margin and in the so-called 'left fitting' arrangement (van der Hammen, 1963) the lobe on the right-hand plate overlaps that on the left-hand plate while in the 'right fitting' arrange- ment (as shown by the arrow in Fig. 2E) the reverse is true. Van der Hammen has suggested that the arrangement of these interlocking lobes and the condition of setae ad1-2 (present or vestigial) could be useful taxonomic features. This view is not, however, supported by the present study since these two features have been found to exhibit considerable intraspecific Variation.

On each genital plate there are two well-developed anterior ridges separated by a median furrow. The furrow bears a single aggenital seta ag1 antiaxially (Pl. 1b). There are nine genital setae arranged in two rows. The anterior five setae g1-5 are minute and located on the paraxial border while the posterior four setae g6-9 are moderately short and submarginal. There are three pairs of genital papillae (g.p), the anterior pair being rather small. Elongate oval structures have been observed inside the genital papillae and these may prove to be spermatophores. The first two pairs of genital papillae border the ovipositor, which, when fully extended, can be seen to be a rather short tube divided into a distal and a proximal portion by a weak circular constriction.

The six coronal (k) setae which Grandjean (1956) found on the constriction in Heminothrus targionii (Berlese) and in the 'higher' oribatid mite Eremaeus hepaticus C. L. Koch are apparently absent in all the British Phthiracarus species. The surfaces of both portions of the ovipositor are strongly pleated. Distally, three eugenital lobes Surround the opening of the ovipositor: an unpaired ventral lobe and a pair of laterodorsal lobes (as shown by the arrows in Fig. 3B). The ventral lobe is triangular in anterior view and bears two pairs of setae distally (psi1-2), the posterior pair (psi2) being the shorter. The two laterodorsal lobes are larger, compressed laterally, and each bears seven setae (tau1-7) antiaxially.

Feider & Suciu (1957) figured two pairs of setae, presumably (psi1) and (psi2), on the ovipositor of P. lentulus (C. L. Koch) , and in P. parabotrichus Feider & Suciu , a dorsal group of seven setae and a smaller ventral group of three setae were shown, possibly (tau) and (psi) respectively. Harding (1976), in his description of P. murphyi , identified 16 setae on the ovipositor: three setae on each of the laterodorsal lobes, two setae on the ventral lobe and six setae which he considered as the coronals.

Gnathosoma

Infracapitulum (Fig. 2D; Pl. 1c): The lateral Ups (L) bear three pairs of adoral setae (or1-3), the anterior pair (or1) being brush-like (Pl. 1c) and the two posterior pairs weakly serrated. The infracapitulum is 'sternarthrous' (Grandjean, 1957) and the rutella (RU) are without atelobasic expansions. There are three pairs of infracapitular setae: an anterior (a) and a median pair (m) of long smooth setae located on the genae (G) and a rather short posterior pair (h) located on the hysterostoma (H). Laterally there is a single pair of barbed supracoxal setae (e).

Pedipalps (Fig. 2A; Pl. 1d): The pedipalps are only three-segmented. The basal segment, formed from the fused trochanter, femur and genu, bears two setae, the tibia two setae and the tarsus seven setae and a solenidion. The three most distal of the tarsal setae are eupathidial: the anteroculminal acm, anterior ultimal ul' and posterior ultimal ul". The subultimal seta sul is a minute spine-like process at the base of seta ul' (as shown by the arrow in Pl. 1d) and also appears to be eupathidial.

Chelicerae (Fig. 2B, C): Both the fixed and the movable digits are dentate. The movable digit has four teeth and the fixed digit carries five. The latter are arranged in two rows, an outer one of two and an inner one of three teeth. The large principal segment which terminates in the fixed digit, bears a number of short conical spines on the antiaxial surface and a larger number of sharply pointed spines paraxially; the spines are distributed extensively on the paraxial surface but are restricted to a more compact zone antiaxially. There are two cheliceral setae, an anterior seta chb inserted on the antiaxial surface and a posterior seta cha located dorsally. Both setae are serrated, cha being somewhat longer than chb.

Legs

Legs 11 to IV are approximately equal in length while leg I is longer and more robust. The indivi- dual epimera are separate and, except for epimera II, each bears a single seta, 1a, 3a and 4a respectively (Fig. 3A). All the legs have five segments: the trochanter, femur, genu, tibia and tarsus, and terminate in a single claw bearing two ventral teeth and an antero- and posterolateral row of serrations (Pl. 2c).

Solenidia (Pl. 2a, b, e): The solenidiotaxy (12-1-3; II 1-1-2; III 1-1-0 and IV 0-1-0) is constant in the 13 species examined and typical of that found in other Phthiracaridae . All the tarsal solenidia are transversely striated as described by Grandjean (1935) for Oribotritia berlesei (Michael) . On tarsus I the solenidion omega1 is closely associated with the famulus e (Pl. 2a) which is short and rugose. Solenidion omega2 is the longest of the three tarsal solenidia and has a small distal coupling seta (Pl. 2b). Harding (1976) described such a setal/solenidial association in P. murphyi and in P. nitens but noted that solenidion omega2 was apparently free in P. anonymum . In the present study the scanning electron microscope has revealed the presence of a distal coupling seta in all the British species. Although usually short and simple, in P. rectisetosus sp. nov. this seta is long, prominent and apparently divided into two parts by a longitudinal constriction, the distal part being produced into a scabre-shaped process reminiscent of that found in species of the genus Steganacarus (see Parry, 1978). On all legs the tibial solenidion phi is coupled with a reduced dorsal seta (PI. 2e) while on genu I solenidion sigma2 is coupled with a-reduced lateral seta.

Leg setae (Figs 4; 5; 14; Pl. 2d): In all the larger species examined the formulae for the leg setae are: I (1-4-2-5-16-1); II (1-3-2-3-12-1); III (2-2-1-2-10-1) and IV (2-1-1-2-10-1). This complement will be referred to as the 'complete chaetotaxy type' (Figs 4; 5). On tarsus I only 16 of the 20 setae regarded by Grandjean (1940) as being characteristic of 'higher' Oribatei are present, the primilaterah and postlarvals always being absent. Four setae Surround the base of the claw, namely, a dorsal pair of prorals (p) and a ventral pair of unguinials (u). Three pairs of setae are located posterodorsally to the prorals: the iterals (it), tectals (tc) and fastigials (ft). Ventrally, behind the unguinials, there is an unpaired subunguinial seta s and a pair of primiventral setae (pv). A single pair of anterolateral setae (a) is located laterally behind the prorals and the unguinials.

On tarsi II to IV there is a reduction in the number of setae to 12, 10 and 10 respectively: setae (it), a' and e are absent on tarsi II to IV, seta pv" on tarsus III, seta a" on tarsi III and IV and seta ft" on tarsus IV. Furthermore, the tarsal setae exhibit considerable variety in form. On tarsus I six of the setae (s, (it), (p) and a') are hollow eupathidia. On all four tarsi setae (ft) and (pv), together with a" on tarsi I and II, are generally more or less straight, circular in section and bear two or three rows of lateral serrations. In certain species (for example, P. globus sp. nov.) seta ft" on tarsus II is hooked distally. The other tarsal setae, (tc) and (u) on tarsus I and (tc), (u), (p) and s on tarsi II to IV, are ribbon-like, hooked distally and covered with whorls of spicules in the middle third. Such setal ornamentation is, however, only discernible in the larger species of the genus.

The setation of the four proximal leg segments is shown in Table 1. Apart from tibia I which bears a whorl of live setae (d, l', l", v' and v"), each of the other segments bears an incomplete whorl of one to four setae. Seta d on femur I is somewhat thickened, serrated and curved distally (Pl. 2d) in all the species examined except P. clavatus sp. nov. and P. globus sp. nov. where it is rather long, straight and only weakly serrated. On all segments setae (l) and (v) carry two or three rows of serrations.

In all the smaller species of the genus (for example, P. tardus Forsslund ) there are fewer setae on legs I, II and IV (Fig. 14): tarsus I bears 15 setae (a' absent), tarsi II and IV usually bear 11 and 9 setae respectively (s absent), femur I bears 3 setae (v' absent) and genu IV is without any setae (l' absent). Thus the setal formulae (referred to here as the 'reduced chaetotaxy type') are: I (1-3-2-5-15-1); II (1-3-2-3-11-1); III (2-2-1-2-10-1) and IV (2-1-0-2-9-1). In P. anonymum (a variant of the 'reduced chaetotaxy type') the number of setae on tarsus IV is further reduced by the absence of seta pv'. Although the total number of setae on leg III is constant throughout the genus, in species of the 'complete chaetotaxy group' seta pv" is absent and seta s present while the reverse is true in the 'reduced' group.

Review

Perty proposed the genus Phthiracarus in 1839 and two years later created the 'family' Phthiracarea (now Phthiracaridae ) for the Single species P. contractilis . Perty's original specimens are presumed to be lost and his figures (subsequently published by Claparede, 1868) and description are such as to make the specific identity of contractilis impossible to determine.

Acarus piger Scopoli, 1763, the oldest species currently classified in Phthiracarus , was originally assigned to the genus by Oudemans (1915). The mite described by Scopoli, for which there is no type material available, is undoubtedly ptychoid but there is no evidence to suggest that it is a species of Phthiracarus - it is probably a member of the Euphthiracaroidea (see Jacot, 1930).

The systematic position of Oribates dasypus Duges , 1834 is also somewhat uncertain although the species is evidently ptychoid. Michael (1888) recorded dasypus in the British Isles. The specimen labelled Hoplophora* dasypus from Theydon Bois in the Michael Collection (deposited in the BMNH) is P. clavatus .

In 1841 ten species of Hoplophora , nine† of which are currently classified in Phthiracarus , were described by Koch from woodland habitats near Regensburg. While it seems probable that these mites have been correctly assigned to Phthiracarus (with the ecxeption of H. testudinea which is possibly a member of the Euphthiracaroidea ), only two of Koch's descriptions refer to 'key characters' which might permit certain reidentification. In comparison with the other Regensburg species, H. globosa is very 'globular' while in H. laevigata the notogaster is 'angled' at the level of seta c1. The remaining six species can only be divided into two groups on the basis of their notogastral setae; crinita , ferruginea and longula are each characterized by 'long setae' while the other three species ( lentula , lucida and straminea ) are all described as being 'sparsely setose'. Despite the inadequacy of Koch's descriptions and the apparent absence of any type material, various interpretations of his species have been published, amongst others by Jacot and van der Hammen. In Les Phthiracaridae de Karl Ludwig Koch, Jacot (1936) redescribed six species from topotypic material but neglected details of the leg and notogastral chaetotaxy, now regarded as being essential for the Separation of Phthiracarus species. More recently, van der Hammen (1963), in one of his series of papers on the Phthiracaridae , has published a detailed description of P. laevigatus (from material collected at Regensburg) and has designated a neotype. Van der Hammen (personal communication) also believes that he has topotypic material of Koch's seven other species but until these specimens have been examined it seems advisable to postpone any decision concerning their taxonomic Status.

Hoplophora nitens Nicolet (1855), recorded as common in the woods around Paris, is a Phthiracarus species. The true identity of nitens appears to be doubtful, although van der Hammen's redescription (see Hammen, 1964) from topotypic material is generally accepted.

Hoploderma italicum Oudemans (1907), recorded from Tiarno, Italy, is not based on a type specimen but on Berlese's description of H. dasypus which Oudemans regarded as being distinct from O. dasypus Duges . Van der Hammen (1952) considers the specimens of both Berlese and Oudemans as being useless for reidentification purposes.

Hoploderma boreale Traegardh (1910) and Hoploderma affine Hull (1914) are both Phthiracarus species. This is evident from the figures of these mites and has been confirmed by examination of 'cotype' and syntype material respectively. The characteristic features of these two species are now certain.

Oudemans (1915) proposed the name Phthiracarus undatus for the 'larve' of Hoplophora stricula (sensu Nicolet, 1855). While Nicolet's figures and description appear to refer to an adult oribatid mite, there is no evidence to suggest a Phthiracarus species.

Berlese (1920 & 1923) described six species of Phthiracarus : rotundus , roubali and subglobosus are European while P. nigerrimus was collected from Argentina, P. curtulus from the United States and P. pudicus from South Africa. The type of each of these species has been examined by Dr J. G. Sheals (BMNH) who has found that their condition is such as to make any chaetotactic characters impossible to discern. In 1959 van der Hammen reviewed all Berlese's species of primitive oribatid mites deposited in the 'Stazione di Entomologia Agraria', Florence, but could not confirm the identities of any of the Phthiracarus species.

Jacot (1928-1939) described 13 species from North America and a single species (P. insular is) from the Marquesas Islands. Of these, type material is available for eight species (see Appendix 1), seven of which are represented by a number of 'cotypes' while only P. brevisetae is based on a holotype. All Jacot's Phthiracarus specimens are mounted in Canada Balsam, the majority being entire and uncleared. The shape of the sensillus and the relative lengths and attitudes of the dorsal notogastral setae can usually be seen in such preparations but the leg chaetotaxy is extremely difficult to study. In view of the condition of Jacot's material, it seems advisable to postpone any decision regarding the identities of his Phthiracarus species until all the type specimens have been dismounted and cleared (an exercise which will be complicated by the presence on each 'cotype' slide of additional species of this and other genera). It can, however, be noted that P. brevisetae appears to be close to if not conspecific with P. laevigatus , while the differences between anonymus amicus and Grandjean's anonymum are evidently not sufficient to Warrant subspecific ranking. P. insularis and P. setosellum bryobium possibly have affinities with two British species, P. murphyi and P. clavatus respectively.

In 1933 Grandjean published the first of a series of detailed works on the external morphology of P. anonymum , a species he recorded from rotting wood in his cellar at Perigueux, Dordogne, France. By Clearing whole and dissected specimens (Grandjean, 1949) he was able to observe the patterns of setae on the body shields (Grandjean, 1933, 1934 & 1950), which provided new criteria for the identification of species of this genus. Hitherto, species differentation had been based almost entirely on body shape and colour, two characters now known to be uniform in many Phthiracarus species. Grandjean (1935, 1940 & 1946) also undertook the first detailed studies of the leg chaetotaxy of oribatid mites, introducing the System of nomenclature now in general use. However, in recent years the majority of Phthiracarus species have been defined solely in terms of characters visible in undissected material. Leg chaetotactic characters have only rarely been studied (van der Hammen, 1963; Sheals, 1965; Ramsay, 1966; Harding, 1976) probably due to difficulties in Interpretation.

Further Phthiracarus species have been described by Willmann (1932, 1939 & 1951), Woolley (1954) and Forsslund (1956) (see under Descriptions of species). However, P. peristomaticus , recorded by Willmann (1951), from a number of habitats including leaves, turf and subsoil under buckthorn, guelder-rose and alder, nördlich Moosmühle, near Vienna, Austria, cannot be identified either from Willmann's figure or from his description. Moreover, there are no specimens of P. peristomaticus in Willmann's Collection (Dr W. Hirschmann, personal communication). Feider et al. (1957, 1958 & 1968) recorded eight species from Rumania, none of which were described in sufficient detail to permit certain reidentification, although P. baloghi , collected from oak leaves in Iasi, appears to be unique among described species of the genus in having notogastral setae of two markedly different lengths. Unfortunately, it has not been possible to borrow the types of any of the Rumanian species for study. This was also the case with species described by Balogh (1958, 1962, 1963 & 1977) and Krivolutsky (1966 & 1975). Other Phthiracarus species have been described from Japan (Aoki, 1958 & 1963), Chile (Hammer, 1962), Nepal (Sheals, 1965), New Zealand (Ramsay, 1966), Spain (Perez-lnigo, 1969), Tahiti (Hammer, 1972), the Tonga Islands (Hammer, 1973) and the British Isles (Harding, 1976) (see Appendix 1). Of these, P. robertsi , recorded by Sheals from rhododendron litter in Nepal, is of particular interest for, although having certain affinities with Phthiracarus , it shows a general similarity to Steganacarus . The arrangement of setae on the genital and anal plates, while reminiscent of Phthiracarus , is nevertheless 'unusual' for setae ad1-3 are almost marginal. The only diagnostic feature whicli P. robertsi appears to share with all species of the genus is the presence of a coupled solenidion on tibia IV. The general shape of the aspis and the form of the integumental ornamentation are characteristic of Steganacarus species and it is questionable whether P. robertsi should have been classified in Phthiracarus (see Sheals, 1969).

There have been relatively few reviews of the British species. In his Synonymic catalogue of British Acari, Turk (1953) listed only five species of Phthiracarus : piger (Scopoli) (= Hoplophora dasypus Duges sensu Michael), affine (Hull) , anonymum Grandjean , ligneus Willmann and spinosum (Sellnick) (now classified in Steganacarus ). Turk does not give sources for individual records but the above are presumably based on the following published records: Michael, 1888, Haibert, 1915 and Hull, 1916 ( dasypus ); Hull, 1914 ( affine ); Murphy, 1954 ( anonymum ); Macfadyen, 1952 ( ligneus and spinosum ). Of these, only P. affinis and anonymum are recognized in the present revision since the taxonomic Status of the other two Phthiracarus species is doubtful.