Eulohmannia ribagai Berlese, 1910

Norton K, Roy A. & ErmilovK, Sergey G., 2022, Paedomorphosis and sexuality in Eulohmanniidae (Acari, Oribatida): surprising diversity in a relictual family of oribatid mites, Acarologia 62 (4), pp. 989-1069 : 993-1016

Ontogeny of Eulohmannia ribagai Berlese, 1910

( Figs 1–15 ; all based on New York specimens unless noted)

Background

The most extensive written description of E. ribagai remains that of Trägårdh (1910 ; as

Arthronothrus biunguiculatus ) but it deals only with the adult. Other treatments are brief, vary in characters used, and also vary in whether particular characters are applied to the family,

genus or species diagnoses, since the higher taxa have been long considered monobasic. Adults identified as Eulohmannia ribagai have been illustrated more than a dozen times, based on specimens from around the Holarctic. The figures (and associated text if present) suggest significant variation, especially in body size and the number and size of various body setae

(see R1, R5, R12), which might have led Balogh and Mahunka (1983) to suggest the existence of Palaearctic subspecies. As explained below, some purported variation may not be real, but we suspect the name E. ribagai currently represents a species group. Accumulating molecular evidence has exposed cryptic species in other groups of oribatid mites, both sexual and thelytokous ( Heethoff et al. 2007 ; Schäffer et al. 2019 ; Lienhard and Krisper 2021 ; Pfingstl et al. 2021).

Juveniles of E. ribagai were partly described by Grandjean (1939c), based on material from Brittany, France. This remains an essential reference, but it lacks overall figures of the body, gnathosoma and legs, and it reduces leg setation to numerical formulas. Suzuki (1979)

presented a dorsal habitus image of a nymph from Japan, purportedly belonging E. to ribagai ,

but there are reasons to doubt the species identification (see below).

Below, we offer a narrative description of the ontogenetic development from larva to adult.

The prelarva is unknown, as we have not had E. ribagai in culture and we have never found a prelarva within a female (see below).

Material examined

Our data, figures, and discussions are based primarily on samples from a population in central New York State (Cortland and Onondaga counties) that is morphologically consistent with studied Palaearctic specimens and with the carefully prepared figures of Lebrun and Wauthy (1981) and Weigmann (2006). The provenance of supplementary material is detailed below.

All examined adults were female, except in samples from northwestern North America (see below). Unless indicated otherwise, specimens are in the personal collection of the first author, with a selection of each instar from Cortland Co., NY in the TSUMZ.

USA — New York: Cortland Co., Tully, Heiberg Memorial Forest, 42°46.19′N, 76°04.61′W, humus and upper soil in young hardwood forest, R.A. Norton col., 29-v-2017 (~80 Ad, 11 Tn, ~80 La); same, 2-vii-2017 (4 Ad, 2 Tn, 6 Dn, 14 Pn); same, 01-viii-2017 (8 Tn, 12 Dn, 3 Pn);

same, J. Cianciolo col., 17-vi-1999 (3 La); Onondaga Co., Clark Reservation St. Park, Glacier Lake basin, south slope, 42°59.65′N, 76°05.42′W, R.A. Norton col., from deep humus on north-facing talus slope, in Acer , Betula alleghaniensis forest, 26-vi-2002 (11 Ad 1 La); same, 4-x-2002 (4 Ad); same 7-vii-2009 (5 Ad); same, 02-xi-2016 (39 Ad, 4 Tn, 1 Dn); St. Lawrence

Co., Cranberry Lake Biological Station, Barber Island, 26-vii-1983, R.A. Norton col., from dense mat under Lycopodium obscurum , in beech forest (1 Ad). Alaska: Fairbanks, University of Alaska Campus, West Ridge, 14-iv-2021, R. Andrews col., from soil-litter in boreal forest

(8 Ad: 6 females, 2 males, 4 undetermined, UAM).

Canada — Alberta: Kananaskis Country, Fortress Mountain, 12-vii-1983, V. Behan-Pelletier col., from litter under semi-prostrate alpine fir in alpine area (2 Ad, CNC). Newfoundland: Gross Morne National Park, Berry Hill area, near Rocky Harbour, 29-vii-1976, E. Lindquist col., from spruce-fir litter above seashore (1 Ad, CNC); 2 mi. N of Eddieʹs Cove, 11-viii-1976, E. Lindquist col. from crowberry-bilberry mat and litter above seashore (1 Ad); St. Anthony, 12-viii-1976, E. Lindquist col., from moss, grass, herbs and substrate by seepage, lighthouse area (1 Ad, CNC). New Brunswick: Kouchibouguac National Park, 19-vi-1978, R. Cope col., from den in mixed woods (1 La, CNC). Nova Scotia: Cape Breton Highlands National Park, Clyburn Brook, 7-ix-1983, V. Behan-Pelletier col., from red oak litter at base of very old tree on rocky slope (1 Ad, CNC); same, but Pleasant Bay, 7-ix-1983, from thick Fagus litter (1 Pn, 1 Tn); same but Mica Mountain, Barren, 12-ix-1983, from Arctostaphylos , Ledum , Vaccinium and Alnus litter (1 Ad). Northwest Territories: Reindeer Station, edge of Caribou Hills, 68°42′N, 134°07′W, 8-vii-1987, R.A. Norton col., from litter, roots under Spiraea and birch ( Betula papyrifera ) (~ 7 cm to permafrost) (10 females, 8 males ; 2 Tn). Ontario: Lanark Co., Tennyson, 14-vi-1970, B. Stewart col., from moss on rock in deciduous bush (2 Ad, CNC).

Europe — Austria: Vorarlberg, Vandans, Lüner See, southern slope of Seekopf, 2030 m a.s.l., 47°03′12.2″N, 9°44′37.3″E, 30-vi-2010, I. Schatz col., from sieving alpine grassland and cushion plants (1 Tn, 1 Pn); Vorarlberg, Rheintal valley, Batschuns near Rankweil, 590 m a.s.l., 47°16′45.3″N, 9°39′51.1″E, H. Schatz col., from soil in pasture grass with roots (2 Tn); Salzburg, Badgastein, Stubner Alm, on base of Stubnerkogel, ~ 1815 m a.s.l., 47°07′N, 18°07E, vii-1977, H. Schatz col., from soil in cultivated pasture near timberline (1 Tn). Germany: Saxony, District of Görlitz, Ostritz, Neissetal, 1967, H.D. Engelmann col., from deciduous forest (11 Ad; see Engelmann 1972); Bremen, Teufelsmoor, 53°16ʹN, 8°54ʹS, 3-xii-1987, H.

John col., from litter under ferns and scattered birch in coal mining area (1 Tn). Sweden: Torne Lappmark, Abisko, 3-vii-1975, A. Edler col., from ‘reindeer moss’ ( Cladonia lichen) on stone and clay (1 Ad); Skåne, Traneröds Bog, 30-viii-1974, A. Edler, col., substrate unknown (7 Ad).

Asia — Russia: Far East, Khabarovsk Territory, Bikin District, 9 km SSE Boitsovo (46°49′N, 134°23′E), 4-ix-1991, R.A. Norton col., from fern litter, rhizomes in moist ravine under Picea litter (1 Ad); same, Khabarovsk District ; Bolshekhekhtsirsky State Nature Reserve, mixed forest, 48.275656 135.047959, 17-ix-1988, V. Behan-Pelletier col., from Sorbus and Pinus koraiensis litter (8 Ad, CNC); same, from litter under dead fallen tree (1 Ad, 1Tn); same, edible mushrooms under Betula (1 Ad). China: Beijing Administrative Zone, Men Tou Gou District, Donling Mountain, ca 1200 m a.s.l., 39°58′N, 115°26′E, 6-x-1997, R.A. Norton col. from moist litter at edge of small stream in Populus , Juglans forest, with much herbaceous litter (1 Ad).

Description of ontogeny

Dimensions — In the New York population, the various instars (n = 10 of each) had a range of total length × maximum width as follows: La 340–369 × 116–132; Pn 427–448 × 126–149;

Dn 494–543 × 145–182; Tn 553–660 × 165–204; Ad (all females) 679–752 × 209–223. Most examined adults and juveniles from other studied populations were within these ranges (R1).

Facies and proportions — The elongated, almost cylindrical form of the body develops gradually ( Figs 1-4 ), by changing proportions (proterosoma more than 3/4 length of hysterosoma in La but less than 2/ 3 in Ad), especially by relative elongation of the hysterosoma (slightly more than 1.5 times its maximum width in La but about twice its width in Ad). In all instars the basal region of the proterosoma comprises a ring-like cervical collar col () that inserts into the hysterosoma when the mite is fully contracted, but the collar gradually becomes more waist-like during ontogeny. Also in all instars, the paraprocts are oriented almost vertically,

such that defecation occurs posteriorly, rather than ventrally. This seems advantageous for a species inhabiting narrow pore spaces in soil.

Integument ( Figs 1 , 2 ) — Juvenile instars are colorless ( Fig. 1A, B ); mature adults are light orangish-yellow when living ( Fig. 1C ), with teneral and long-preserved specimens being paler, straw-colored ( Fig. 1D ). Except for articulations and appendages, the epicuticle

(epiostracum of Grandjean 1956b) is reticulated in all instars by tessellating, 5- or 6-sided

(rarely 4) polygons that are almost flat but circumscribed by sharp, depressed lines. Mostly the polygons are rather regular in form and size (honey-combed, with width usually 10-13 um in Ad) but locally they can be smaller or more elongated ( Figs 1H , 2 , 7 , 8B ). In juveniles, but not adults, the epicuticle commonly detaches with lactic acid treatment ( Fig. 1E, F ), leaving the underlying procuticle without surface pattern. With light microscopy several types of inner cuticle can be distinguished. The general body cuticle of juveniles seems somewhat leathery and elastic, but to characterize it as hardened in some manner—as did Grandjean

(1969; ‘chitinized’) and Woas (2002 ; ‘sclerotized’)—seems inappropriate. In polarized light or strong DIC illumination there is distinct layering and glowing in the procuticle ( Fig. 1F ; see

R8), which is unaffected by clearing (lactic acid). In the adult, differentiated body sclerites have a relatively thin, dense layer under the epicuticle that we assume is an exocuticle ( Fig.

1G), which causes adult cuticle to be more rigid than that of juveniles, though still relatively elastic, deformable. The melanization typical of adult oribatid mite exocuticle is not apparent.

Beneath the adult exocuticle only several, relatively thick underlying endocuticular layers are distinctly visible in light microscopy. Electron micrographs show the thinner external layers in adult cuticle ( Alberti et al. 1981, their Fig. 7A ; 1997, their Fig. 1A ). Pore canals are unusually dense and branched in sclerites of the adult, but ultrastructure of juvenile cuticle has not been studied. Segments of appendages ( Figs 7D , 11F ) have thick, dense exocuticle in all instars and no endocuticle layers noticeable in light microscopy. Most articulations, such as those of appendages and the sejugal articulation, which allows telescoping of protero- and hysterosoma,

lack both the epicuticular reticulation and the distinct procuticular layering ( Fig. 5 E-G); by contrast, the main articulations between hysterosomal plates of the adult, which seem less supple, show the layering ( Fig. 9E ).

Prodorsum ( Figs 2-4 ) — In all instars, the outline of the prodorsum is roughly ovate in dorsal view. Its outline broadens at mid-length, more noticeably in the adult than in juveniles,

such that the prodorsum is widest just posterior to seta in. A distinct post-bothridial groove pbg (;

Fig. 2E ) dorsally delimits the cervical collar col (), which is overlapped by the hysterosoma when the mite is fully contracted. Overall, the collar integument has the general form of reticulation ( Figs 2E , 5H ) but on the ventral surface in the adult (not juveniles) the anterior part usually has elongated parallel ridges ( Fig. 8B ). The rostral tectum is well-developed in all instars, with a distinct rostrophragma rph (; Fig. 5D ), but it is relatively short and effaces laterally, leaving the gnathosoma partly exposed. The rostral margin is not extended anteriorly by a distinct limb. A medial mucro mu () is present in all instars, which can appear like a rounded lobe when seen obliquely. Seen flat, the projection forms a small equilateral triangle in the larva and a more elongated mucro—set off by a pair of shallow notches—in nymphs and adult ( Fig. 2D ). Otherwise, the rostral margin is nearly smooth or weakly scalloped. In the adult, a narrow, solid marginal band lacks the reticulate pattern ( Fig. 2D , black arrow); on each side it meets a raised carina pdc (), which runs posteriorly, effacing below seta exp ( Figs 2B ,

5H ).

Prodorsal setae are similar in all instars ( Figs 3A , 4C ). Except for the bothridial seta they are simple, smooth (or nearly so) and attenuate. Setae exp and le are smallest, in is longest, and exa and ro are intermediate in length ( Table 1). Pair ro are nearly adjacent, separated by

2-3 alveolar diameters, and insert immediately behind the medial mucro ( Fig. 2D ). Pair le are distinctly shorter than their mutual distance in all our material. Seta exa has a consistent, unusual position anterior to and well removed from the bothridium.

In all instars the bothridial seta bs () arches dorsolaterally, then curves ventrad; it is thin, distally attenuate, and conspicuously pectinate on the outer curvature, with 11 to 13 long dorsal tines (alternate tines slightly divergent), and often with minute barbs in a separate row closer to the inner curvature; the basal quarter to third may appear glabrous but often has similar minute, inconspicuous barbs. The bothridium opens on a low elevation and has a slightly raised rim ( Fig. 2E ); there are two internalized chambers in the larva and three in nymphs and adult ( Fig. 5A, C ), with at least the most internal chamber having several distinct raised rings. As described in detail by Grandjean (1939b) there are two porose saccules that invaginate from the bothridial wall in the narrow, curved region between the inner chamber and the setal insertion (R2). In the larva and nymphs, the two saccules are similar ( Fig. 5B ), but in the adult one saccule is elongated and flattened ( Fig. 5C ).

Digestive system ( Fig. 6 ) — The structure of the gastrointestinal tracts was studied only superficially, in glycerine preparations of uncleared specimens, but its general structure does not appear to vary during ontogeny. The esophagus leads to a barrel-like, thick-walled ventriculus (ven). From the posterolateral region of the ventriculus arise a pair of conspicuous, egg- to sausage-shaped midgut caeca cae () that occupy much of the lateral region of the hysterosoma posterior to the ventriculus; like the hysterosoma in general, their proportions change during ontogeny, from about twice as long as wide in the larva to about three times in the adult. The ventriculus opens posteriorly to a tubular colon (cln) that is separated by a constriction from a similarly shaped postcolon (pco).

Hysterosomal dorsum ( Figs 2-4 , 7 , 9 ) — In juveniles, the gastronotum is not clearly circumscribed from the ventral regions; the reticulated cuticular pattern continues around the circumference of the mite between the level of the legs and the paraproctal region, broken in nymphs only by the genital valves. Depending on treatment, the gastronotum of some juveniles may show a transverse crease at mid-length, but this is an artifact ( Fig. 4E ; R3).

A notogaster becomes defined in the adult with the appearance of two paired articulations. Using Grandjean’s (1956b) terminology and notations, one is band na, a substantial, weakly curved plicature band of flexible cuticle separating the notogaster NG () from the aggenital region (AG) of the uniform hysterosomal venter ( Fig. 7H ); it runs posteriorly from near the insertion of leg IV to merge with the similarly curved ventral plicature band bpv () articulating the notogaster and adanal plates ( Fig. 9A ). The effect is to create an obtuse angle at the meeting of these curves such that the notogaster projects slightly into the space between genital and adanal plates from each side. This unusual form has been the source of incorrect observations and confusing terminology (R4).

The second, anterior articulation ( Fig. 7C ; ncx) delimits the notogaster from the coxisternum; it is a very narrow, inconspicuous lateral scissure that probably allows only slight flexing. Scissure ncx begins at the sejugal articulation in the humeral region, where it marks a change in the nature of that articulation: dorsal to ncx the notogastral margin bears a narrow but distinct tectum in adults (ngt ; Fig. 5E, F )—only weakly defined on the juvenile gastronotum—that overhangs the cervical collar when the mite is fully contracted, while ventral to ncx no tectum exists ( Fig. 5G ). The scissure runs posteriorly, passing over the insertion of leg III where it bends ventrad, as if to pass behind leg IV; but it effaces in the vicinity of gland opening g4 ( Fig. 7C ), such that ncx and na do not meet. In some instances, epicuticular reticulations may align so as to give a false impression of a longer scissure ( Fig. 7D ), but the notogaster is consistently fused to the coxisternum in the short intervening distance. Based on all adults we examined, regardless of provenance, the statement by Grandjean (1956b) and implication by Hammen

(1959), that band na continues anteriorly to the sejugal articulation, is incorrect.

There is no evidence of an opisthonotal gland or its opening gla () in any specimen we studied. The ontogenetic appearance of lyrifissures follows the usual pattern, with ia, im, ip,

ips and ih occupying the adult notogaster. Lyrifissures are slit-like in all instars ( Fig. 1H ); only for ih of the larva and ips of the protonymph—i.e., when these lyrifissures first appear—do they have a narrowly elliptical cupular form ( Fig. 11B ). Lyrifissure ia has a slightly more lateral position in the larva than in later instars (cf. Figs 3A , 4C ); ip also shifts position, being posteroventral to seta f 2 in the larva but dorsal to f 2 in nymphs and adult. Both ih and ips make the usual small migrations seen in acariform mites with the addition of paraproctal segments

(in Pn and Dn, respectively). In the adult, lyrifissure ips comes to lie close to and parallel to the notogastral margin, a short distance anterior to the angular projection, but the position ih of is more variable. Grandjean (1956b, his Fig. 1A ) and Lebrun and Wauthy (1981, their Fig. 2 )

showed ih with an orientation similar to that of ips but slightly more anterior on the notogastral margin. In our material ih is sometimes more removed from the margin, and oriented more vertically.

In the larva gastronotic setae are minutely, asymmetrically barbed, but in later instars they are smooth, or nearly so ( Fig. 7A, B ); while they may become proportionally thinner during ontogeny their size relative to the body decreases somewhat, concomitant with body elongation

(cf. Figs 3A , 4C ). Absolute sizes are given in Table 1. The chaetome of the adult notogaster has been reported differently in the literature (R5), but in all our material, regardless of provenance,

there are 15 pairs, with no indication of setal vestiges to indicate which seta is absent from a holotrichous (16 pairs) chaetome. The gastronotic chaetome of juveniles is consistent with that of the adult, i.e., no setae are added or deleted from the gastronotum other than the usual anamorphic addition of segment PS and its setae in the protonymph: so, there are 12-15-15

pairs in the larva, nymphs and adult. The larval count assumes an absence of inguinal seta hi and the notations applied to the dorsal setation assume the missing seta f is 1 (R6).

Lateral podosoma and coxisternum ( Figs 2-4 , 7 , 8 ) — In all instars, the prodorsum and the epimeral region of the proterosoma merge without clear separation. This is easily seen in the uninterrupted cuticle of the cylindrical cervical collar. Anterior to the collar two linear structures appear to intervene between the prodorsum and coxisternal epimeres, but neither is an articulation. The more dorsal is carina pdc, noted above ( Fig. 2B, D ). The other is the podocephalic canal (cpc ; Figs 2B , 3C ), which runs from the gnathosomal articulation posteriorly, just above trochanter II, to the groove pbg () delimiting the cervical collar. Development of the podocephalic canal and its associated glands was illustrated and described in detail E. for ribagai by Grandjean (1939b, 1939c, 1968, 1971 ; see also Hammen 1982 ; Alberti and Coons

1999). Our observations are consistent with his, including the presence of a small separate gland opening (g4) just above the insertion of leg IV in nymphs and adult ( Fig. 7C, D ). Juveniles have no thickened, rib-like lateral ‘nervure’ such as that found in some other mixonomatans ( Grandjean 1966, Norton and Sidorchuk 2014). Claparède’s organ of the larva ( Figs 3C, D ,

8A ) was described in detail by Grandjean (1939c, his Fig. A): it is elongated and clavate, with a piriform head and a stalk having three or four distal annuli that seem to allow bending.

The propodosomal part of the coxisternum is a single unit, lacking distinct articulation between or within epimeres I and II. While there are no borders per se, the epimeres appear delineated more or less into four regions in transmitted light by the complement of coxisternal apodemes, which is similar in all instars. On the propodosoma, apodemes 1 ap (. 1) and 2

(ap.2) are large, thin vertical lamina ( Figs 3C , 4A, D ; illustrated only in cross section). Pair ap.1 are strongly cupped posterolaterally; they are well separated medially in the larva, but their curved medial portions become noticeably closer in nymphs and adult. Pair ap.2 meet medially but are there deflected straight posteriad to form what can be considered a subunit: the bilayered sternal apodeme ap (. st), which ends at the strong vertical contour vc () marking the edge of the constricted cervical collar ( Fig. 8B, C ). Extrinsic leg musculature that attaches to these apodemes (partly seen in Fig. 8C ) does not change noticeably during ontogeny. The articulations of trochanters I, III and IV with the body are fully exposed, as in most macropyline taxa, but trochanter II appears partially recessed, protected anteriorly by a cuticular fold. This fold is indistinct in juveniles but in the adult it seems to form a rudimentary acetabulum, with the cotyloid wall (co) formed by a strong declivity posterior to ridge r2 ( Fig. 8C, D ; cf. Grandjean 1952b, his Fig. 1C ).

Epimeres of the metapodosoma likewise have no distinct borders in any instar, and epimere IV merges seamlessly with the hypertrophied aggenital region behind it. Two pairs of apodemes exist in all instars, associated with epimere III (no apodeme is associated with epimere IV in any instar); these are relatively small, extending only slightly into the hysterosoma and therefore are inconspicuous, especially in juveniles. One, which we consider the sejugal apodeme ap (. sj)

as it invaginates from the presumed anterior margin of epimere III, is posteroventrally cupped.

It is most easily distinguished in the adult, at the front of the hysterosoma just below scissure ncx ( Fig. 7E, F ) and in dorsoventral view it tapers posteriorly to efface at the level of seta 3d

( Fig. 7G ). Part of the extrinsic musculature of leg III ( Fig. 8C ) inserts on its posteroventral face. In lateral view, fascicles of the anterior dorsoventral muscle (m.adv) may seem directed toward ap.sj ( Fig. 6D ), but almost certainly they insert on the centrally located endosternite (not illustrated; see Akimov and Yastrebstov 1991, their Fig. 3 ) from above. The second is a small apodeme 3 (ap.3) projecting internally from the body wall just anterior to the insertion of trochanter III; it is a simple vertical lamina associated with leg musculature and in all instars it is visible by transparency in dorsoventral view ( Figs 3C , 7G , 8C ).

In nymphs a pair of strong, conspicuous dorsoventral muscles m (. mdv) originate medial to gastronotic seta d 2 ( Fig. 6A ) and each inserts via tendon just medial to the leg IV insertion (t.mdv ; Figs 4A, B, D ). The larva, with its proportionally shorter hysterosoma, lacks m.mdv

but the posterior dorsoventral muscles (see below) have an analogous middle position.(cfFig.

3C; t.pdv). The adult also lacks m.mdv ( Fig. 6C ), perhaps due to different force requirements associated with the presence and organization of sclerites.

Setation of the propodosomal venter is normal for oribatid mites, with most setae being smooth or weakly barbed (lengths given in Table 1); in all instars epimeres I and II have three and one pairs, respectively. Seta 1c has the usual scale-like form in the larva ( Fig. 8A ), covering the tip of Claparède’s organ when the latter is retracted, and is a simple seta in later instars. In all instars, supracoxal seta eI is inserted posterodorsal to leg I; it is small, with an ;

isodiametric stem and distal bifurcation of two acuminate tines. The symmetry of the tines and relative proportion of tines to stem varies within populations, but the most equal tine: stem ratios were seen in some NY specimens ( Fig. 8G , bottom), while proportionally short tines were more often seen in European specimens (e.g., Fig. 8J , top; see Fujikawa 2014), and in northern Canada ( Fig. 8I , bottom). Tines sometimes were broken ( Fig. 8H , bottom) and in one case a tine was abnormally short and bulbous.

On the metapodosoma, the setation of epimere III also appears to have a development normal for oribatid mites: in all studied specimens, there are two pairs in the larva, three in the protonymph and four in subsequent instars (R11). The setation of epimere IV is complicated by a progressive neotrichy that appears to begin with the first formation of the epimere in the protonymph, where there are three pairs in the vicinity of legs IV ( Fig. 4A , all marked ‘4’); this contrasts with the usual complement in oribatid mites, where the protonymph has a single pair of epimere IV setae ( Grandjean 1934c). The two pairs near the genital aperture (x, y) probably do not belong to epimere IV (R11, R13). In subsequent instars, setae are added to the region of epimere IV but the absence of epimeral borders makes the number equivocal ( Fig. 4B, D ).

Genital-aggenital region ( Figs 3 , 4 , 9 ) — Beginning with its appearance in the protonymph, the small genital aperture is positioned unusually far posteriorly, at about two-thirds the length of the hysterosoma and removed by its length, or less, from the anal aperture. Collectively, the valves have an oval or slightly obovate outline, without marginal tecta. The aperture length increases during ontogeny from less than a third that of the anal aperture (as seen in ventral view, so slightly foreshortened) to about half the anal aperture length in tritonymph and adult (cf. Fig. 4A, D ). In the adult, a pair of parenthetic arms of the sclerotized aggenital region partially envelop the genital aperture but do not close behind it; the end of an arm often is partially or fully separated (usually asymmetrically) as a small island-like sclerite ( Fig. 9A ; Grandjean 1956b, his Fig. 1A ). Behind the genital plates is a transverse band of unsclerotized cuticle that merges laterally with articulations bpv and na, and like them it shows layered procuticle in polarized light ( Fig. 9A, E ). Lateral to the genital valves (and in a similar position in the larva) tendons of the paired posterior dorsoventral muscles t.pdv () insert via minute hardened projections (one per muscle fiber) at a location halfway between the midline and the lateral contour ( Figs 3A , 4A, B, D ). The muscles (m.pdv ; Figs 6B, C , 9C ) originate in the region ventral to notogastral setae f 2 and h 3, so probably function in controlling hemocoel pressure. There is a single pair of tendons in the larva, proto- and usually the deutonymph, but two adjacent tendons and insertion points in the tritonymph and adult. When two are present, their insertions may be separate or connected ( Fig. 9B ). In the adult, these lie in tandem along the margin of the aggenital plate ( Fig. 9A ).

Setae in this region are relatively small ( Table 1), attenuate, smooth or weakly barbed. Those of the genital valves follow an ontogenetic formula (Pn to Ad) of 1-4-7-9. We encountered no variation in this development in juveniles, or in females from Nearctic or European populations, but literature reports of the adult genital setation vary significantly (R12), and we have noted variation (7-9 pairs) in studied males (see below). Setation in the aggenital region develops as part of the increasing ventral plate neotrichy, and no aggenital setae can be unequivocally identified. Allowing for the posterior displacement of the genital aperture, setae x and y of the larva and protonymph ( Figs 3C , 4A ) might be precocious aggenital setae (R13), but subsequently they become lost in the increasing overall neotrichy of the ventral plate ( Fig. 4B, D ). Collectively for epimere IV and the aggenital region, there are about 9-10 setae on each side in the deutonymph, 14-15 in the tritonymph and 16-21 in the adult. While this neotrichy has been called a ‘plethotrichy’ ( Grandjean 1956b), the setae seem mostly ordered in nearly symmetrical pairs, rather than being randomly placed or ‘chaotic’ (see Hammen 1980).

The female ovipositor ( Fig. 9E, F ) is short and lacks coronal k () setae, but the three distal lobes have a normal setation of six pairs ( Grandjean 1956a ; Ermilov 2011), all of which appear to be eupathidial. The unpaired ventral (posterior) lobe bears the long seta ψ 1 (28-32) and the shorter ψ 2 (12-15); the paired dorsal (anterior) lobes have four short pairs, τ 1 (12-13) and the slightly shorter (10-11) τ 2, τ 3 and τ 4. Based on males from apparently sexual populations in northwestern North America (Reindeer Station and Fairbanks; see below), the small spermatopositor ( Fig. 9D ) occupies less of the genital vestibule than does the ovipositor,

and seen ventrally it is elliptical, ~ 20-25 long (relative to 65-70 for genital plates). The soft cuticle at its base is not plicate but otherwise the structure is similar in form to that of

Perlohmannia ( Grandjean 1958a, his Fig. 3D, E ). The spermatopositor is difficult to study due to small size and obscuring internal structures, but two examples were relatively clear. There are seven pairs of setae, all small (6-8) and apparently eupathidial. A conspicuous feature is that the two pairs of short ψ setae are closely adjacent in a curving transverse row, most easily located by their alveoli ( Fig. 9D ). The five other pairs are longitudinally arranged, but the posterior pair often are difficult to find and seem to be at a slightly more proximal level on the structure; the latter may be a single pair of remaining k setae (absent from the ovipositor), with the other four representing τ 1 - τ 4. As in Perlohmannia , there is a weakly defined sclerotized support near the central midline.

Paraproctal region ( Figs 3 , 4. 11 ) — Since the anal aperture is nearly terminal throughout ontogeny, the paraprocts—segments PS, AD, AN respectively in La, Pn, Dn—develop in a rather idealized form, as a series of parenthetic valves that are not distorted by the usual strong

‘caudal bend’ of acariform mites. In each instar, there is a small, vertical preanal apodeme

(ap.pa) extending from the anterior end of the paraproctal valves ( Fig. 9A, E ) to which muscles from the genital valves insert; these are the ‘anoprogenital muscles’ of Grandjean (1971) or

‘constrictors of the genital valve’ of Akimov and Yastrebstov (1991). Segment PS has four pairs of setae in the larva, but when it becomes part of the gastronotum in the protonymph there are only three pairs. We follow Grandjean’s (1949b) interpretation, that the most anterior ps seta is inguinal (psi = ps 4), and is lost in the protonymph. Paraproctal setae are attenuate and smooth or weakly barbed, with length in a particular row increasing posteriorly ( Table 1). There are two exceptions. Seta ps 1 is conspicuously thickened and barbed in the larva ( Figs 3C , 11B, C ),

but is thin, attenuate in later instars. The setation of segment AD develops similarly— ad 1 is thickened and acute in the protonymph ( Figs 4A , 11C ), but normal in later instars—except that it bears four pairs of setae in nymphs and adult (R14). Segment AN also has four pairs of setae from the time it forms. Lyrifissure ips is absent when the segment is paraproctal but appears in the protonymph, when the segment is incorporated into the gastronotum. Lyrifissures iad and ian also are delayed one instar, appearing in the deuto- and tritonymph, respectively. In the adult, iad and ian are inconspicuous, lying on the lateral margin of the plate close to the anterior seta ( Fig. 11A ); for ian often the canal is easier to see than the small slit. Grandjean (1956b ;

his Fig. 1A ) did not illustrate ian, but it is present in all Palaearctic adults and tritonymphs that we examined.

Gnathosoma ( Figs 10 , 11 ; Alberti and Coons 1999, their Fig. 92B) — Other than absolute size (see Table 2) there are few changes to the gnathosoma during development. In all instars the subcapitulum is noticeably longer (~1.3×) than wide (slightly flattened in figures) and distinctly stenarthric. The mentum H (), with transversely elongated reticulation, forms a relatively small, equilateral triangle that occupies little more than a third the subcapitular length.

Each gena (G) is smooth, elongated and gradually tapering distally. The rutellum RU) (—nearly vertical in its natural position—is about half the length of the gena, from which it is clearly demarcated dorsally and laterally (manubrial line αf). The rutellum is about as long as wide and is atelobasic, leaving the adoral lips exposed in ventral view. The distal margin includes a strong, thumb-like lateral process ending in two unequal cusps, a short but strong, pigmented tooth close to its base, and a flat cutting edge occupying the medial half, with a small point at the medial end ( Fig. 10A, C, D ). The dorsal face of the rutellum has several fine carinae running proximally from the distal margin, and in all instars there are two oblique rutellar brushes (ciliary combs; br): a short distal brush with long cilia near the medial side, and a second at mid-length that is parallel with the first but longer and with smaller cilia. The labrum

(LS) has a typical form, narrowly triangular but distally rounded in all instars, supported by a conspicuous pair of narrow, strut-like embedded sclerites ( Fig. 11D ; scl). The distal quarter has a row of narrow, contiguous denticles around its margin, with a second row dorsally near the tip; four inconspicuous transverse rows of narrow denticles are distributed along its dorsal face ( Fig. 11E ), and the ventral face has several fine transverse grooves. The lateral lips lack dorsal cilia or denticles and we observed no distinct ventral sclerite. The larva has two pairs of adoral setae: or 1 usually tapers only distally and appears thickened by coarse barbs, or 2 is acicular to acuminate and nearly smooth. In the protonymph or 3 is added, similar to or 2 but shorter, and both setae are acuminate to attenuate, with inconspicuous barbs. Setae of the hypostomal mentum (h) and gena a (, m) all are similar: thin, attenuate, smooth or with few minute barbs.

The postpalpal seta (ep) is isodiametric and distally forked ( Fig. 15C ), generally similar to supracoxal seta eI, but depending on viewing angle the branches can be superimposed ( Fig. 11F ); rarely one branch is minute or even absent.

The palp has four segments in all instars: the femur and (glabrous) genu are fully fused, lacking articulating cuticle, vestigial suture, or even change of thickness ( Fig. 11F ) to mark their juncture. Femoral seta inf forms in the protonymph; otherwise, the setal complement is unchanging, with nymphs and the adult having the formula 0–[2+0]–2–7(+ ω). Setal forms are shown on Figs 10 (C, E, F) and 11 (F, G). On the tarsus, pair ul () are eupathidial in all instars, terminal and distally directed, inserted almost in tandem vertically. The unpaired acm is semi-erect, a normal, acicular seta in the larva ( Fig. 10F ), but in nymphs and adult it is eupathidial. There is no evidence of seta su (= sul) or its vestige in any instar. Tarsal solenidion

ω is relatively thin, long, ceratiform, nearly straight and directed distally, closely parallel to seta ulʺ ( Fig. 11G ).

In all instars the chelicera has a typical, chelate-dentate form ( Fig. 10G, H ). The adaxial face of the cheliceral body is emarginated in its proximal half and there is a distinct paraxial oncophysis (opʹ) and lamellated organ l. (or). The edge of l.or can extend distally, producing a line that can be misinterpreted as belonging to Trägårdh’s organ, which is absent. Slightly above the center of the adaxial face there usually is a single small spine with minute teeth on its sides or distally, and above that are 3-4 vertical rows (differing in length) of small, uniform denticles; the movable digit has an oblique row of uniform, minute denticles at about mid-length on the abaxial face. The cheliceral frame attachment (line en) is somewhat distal, such that about a quarter of the chelicera is inserted through the body wall. Cheliceral seta chb inserts dorsolaterally in the distal third of the fixed digit—relatively short, acicular and nearly smooth; cha is absent in all instars.

Legs ( Figs 12-15 ) — Legs are relatively short, with leg I (the longest) averaging 0.38-, 0.41-, and 0.44-times body length for larva, nymphs and adult, respectively. Proportional leg length changes slightly during development. Legs I-III average 1: 0.89: 0.98 in the larva; in nymphs, leg IV starts relatively small, but becomes proportionally larger, equaling leg I in the adult (legs I-IV average 1: 0.80: 0.85: 0.75 in Pn, 1: 0.80: 0.85: 0: 85 in Dn, 1: 0.80: 0.85:

0: 98 in Tn, 1: 0.85: 0.83: 1 in Ad). Leg form changes little during ontogeny: most notable are (a) a slight elongation of femur I, (b) development of a more pronounced proximal stalk on tarsus and tibia I, and (c) relative elongation of femur IV, which is about equal in length to the trochanter when formed in the protonymph, but about 1.3-1.4 longer in the adult. Throughout development tarsus I is swollen, compared to other tarsi. In all instars segments may have slight cuticular undulations, particularly ventrally, as well as a microsculpture of short, dense vermiform striae ( Fig. 15M, N ). There are no porose areas, and the so-called ‘genual pore’ ( Grandjean 1940) is absent in all instars.

Juveniles have a monodactylous pretarsus; the empodium is a strong claw with a pair of rows of minute barbs on its dorsal curvature and a distinct, straight, ventral spine close to its base ( Fig. 15A ). In the adult, the empodium regresses, being represented by two minute, blunt,

,

ventral view (upper right insert = enlarged seta ps 1); C – same, protonymph ( Austria); D – adult, labrum, focused on embedded sclerites; E

– same, slightly oblique, surface of distal two-thirds showing transverse bands of denticles ( Sweden); F – adult, right palp and subcapitulum abaxial view (seta sup out of focus); G – same, tip of palp tarsus, abaxial (upper; seta ltʺ out of focus) and adaxial (lower) surface. Scale bars

,

10 µm (A-C, F, G); 5 µm (D-E).

tandem, basally-fused spines emerging from the basilar piece ( Fig. 15B ; see also Alberti and Coons 1999, their Fig. 112D) that perhaps correspond to the claw and basal spine in juveniles;

the pair of strong lateral claws are equal in size, with weak barbs on the dorsal curvature. There are no condylophores.

In general, normal (non-eupathidial) leg setae are relatively thin, attenuate and with small, inconspicuous barbs; on tarsus I adaxial setae of the c -row, particularly c 1 ʹ, usually are more distinctly and densely barbed ( Fig. 15M ). Numerical formulas for the chaetome of each instar are given in Table 3, and the homologies identified in Table 4. Unusual or otherwise notable features include the following. Femora. On femora I and II, lateral setae exhibit strong vertical displacement (basculation), with lʹ higher and lʺ lower (almost at level of vʺ) than the typical lateral position. Genua. On genu I–III, seta d is represented in the larva by an alveolus with a minute setal vestige (dv ; Figs 12B , 15G ); the seta is fully formed in all subsequent instars (R15). On genu I, seta lʹ is small and coupled with solenidion σʹ in all instars (see below). Tibiae. The verticil on tibia I has five setae— d, (l), (v)—lacking primitive seta c ʺ. Seta vʺ of tibia IV is present in the protonymph (R16). Tarsi. From the larva, primitive seta mʺ is present on tarsus

I, with mʹ forming in the deutonymph (R17). The iteral pair forms on tarsi I–III but develops differently on each leg (R18). From the larva, tarsus I has the usual primilateral pair pl (). On tarsus II, adaxial seta plʹ forms in the larva, but not pl ʺ; however, a lateral seta does form on the posterior face in the tritonymph, and we interpret this as a delayed primilateral seta (R19). Three setae of the primitive c -row form on tarsus I (R20), in successive instars. Discounting the posterior lateral seta of tarsus II, no proximal accessory setae of l - or v -rows form on any tarsus.

On tarsi, disjunctions (offsets) of the pseudosymmetrical pairs of setae are consistent across our material and also through ontogeny, whenever the particular pair is present. For tarsi I-IV, the disjunctions are: (ft) = o-p-p-p; (tc) = po-po-p-p; (it) = ao-po-p-x; (p) = o-o-p-p; (u) = o-o-p-p; (a) = ao-a-p-p; (pl) = a-x-x-x; (pv) = ao-o-p-p; (m) = a-x-x-x; (c 1) = ao-x-x-x. This differs much from the simple pattern common to most Brachypylina and Nothrina, where posterior disjunction is the rule except for the primiventral pair (pv), which have anterior disjunctions ( Grandjean 1958b, 1960 ; Wauthy and Fain 1991). It also differs from the pattern of positional analogy, as seen in the mixonomatan family Collohmanniidae , where except for pair (ft) disjunctions are mostly adaxial ( Norton and Sidorchuk 2014).

Compared to most oribatid mites, tarsus I is moderately rich in eupathidia—probable taste receptors ( Alberti 1998)—with a total of 10 in the adult. Transformation from normal to eupathidial form takes place at different times ( Table 5 ; R21): the proral setae p () are eupathidial in all instars, but most eupathidia transform one instar after they first appear u [) (

and s in Pn, (it) in Dn, mʹ in Tn]. Only the fundamental antelateral pair breaks this pattern, with aʹ transforming in the deutonymph and aʺ in either the tritonymph or adult (R21). The famulus of tarsus I ( Fig. 15 D-F) is similar in all instars: strongly proclinate, bacilliform (~ 10-12 in adult), weakly curved, with a weakly formed conical head; vague annular rugosity is often visible.

The solenidial complements of tarsi and tibiae are typical of early-derivative oribatid mites,

with adult formulas (legs I-IV) of 3-2-0-0 and 1-1-1-1, respectively, while that of the genu,

3-1-1-1, is exceptional (R22). The developmental aspects of these complements, shown in

Table 4, was thoroughly discussed and compared by Grandjean (1964c). The shape of particular solenidia is essentially constant through ontogeny ( Figs 12-14 ): those of genua are attenuate

(piliform) while those of tibiae and tarsi (except piliform ω 3 on tarsus I and φ on tibia IV) taper only slightly, or at least remain noticeably thickened distally (ceratiform); none are flagellate.

There is no coupling of a solenidion with seta d on any genu or tibia: d inserts well proximal to the respective solenidion in all postlarval instars. Beginning in the larva, genu I seta lʹ is imperfectly coupled to σʹ, with adjacent but separate insertions ( Figs 12B , 15G ; R23).

Biological notes

Figures

Figure 1 Eulohmannia ribagai(Berlese): A – larva, lateral view, epi-illumination; B – tritonymph, same; C – living adult, stereomicroscope (arrow to cervical collar); D – gravid female, with single egg (provenance uncertain, photo D.E. Walter); E – tritonymph, lateral view of gastronotum contour, with epicuticle separated by clearing; F – same, closeup optical section in polarized light; G – adult, notogastral cuticle at edge of sagittal section; H – tritonymph, right gastronotic setaec1, c2 and lyrifissureia (lower left insert = im from adult, right =im from deutonymph, showing canal). Scale bars 100 µm (A-D); 20 µm (E); 10 µm (H); 2 µm (F, G).

Figure 2 Eulohmannia ribagai(Berlese), adult, SEM images: A – lateral view; B – closeup of posterior proterosoma; C – dorsal view; D – frontal view (black arrow to narrow solid rostral rim); E – sejugal region, dorsal view. Scale bars 100 µm (A, C); 20 µm (B, D, E).

Figure 3 Eulohmannia ribagai(Berlese), larva (appendages incompletely shown): A – dorsal view B – posterior view of hysterosoma; C – ventral view; D – lateral view. Scale bar 100 µm.

Figure 4 Eulohmannia ribagai(Berlese), nymphs (appendages incompletely shown): A – protonymph, ventral view (prodorsum incomplete); B – deutonymph, ventral view of hysterosoma; C – tritonymph, dorsal view; D – tritonymph, ventral view; E – tritonymph, partial dorsal view of gastronotum showing artifactual transverse crease (see text, R3). Scale bars 100 µm: A, B to same scale, C-E to same scale.

Figure 5 Eulohmannia ribagai(Berlese): A – larval bothridium, dorsomedial view (only one of two saccules visible); B – deutonymph, bothridial saccules; C – adult, bothridium, dorsomedial view; D – adult, rostral tectum, sagittal view; E –sejugal articulation of extended specimen, dorsal midline (sagittal section), anterior to right; F – same, but contracted specimen; G – as in E, but ventral midline; H – bothridial region, dorsolateral view. Scale bars 10 µm (H); 5 µm (A-G).

Figure 6 Eulohmannia ribagai(Berlese), uncleared specimens in glycerine: A – deutonymph, lateral view (musclem.pdv out of focus); B – adult hysterosoma, dorsal view, showing major organs; C – same, lateral view (one fascicle of musclem.pdv out of focus); D – as in C, closeup of humeral region; E – adult, anterior hysterosoma, showing some proterosomal retractor muscles. Scale bars 50 µm (A-C, E); 20 µm (D).

Figure 7 Eulohmannia ribagai(Berlese): A – seta c1 of larva; B – seta c1 of adult; C – adult, metapodosomal region, lateral view of cleared specimen; D – same, closeup of leg IV region; E – same, surface closeup of leg III region; F – same as E, but deeper focus to show apodemes; G – left sejugal apodeme and apodeme III, dorsal view; H – adult, ventral aspect, region near leg IV insertion (specimen from Alberta). Scale bars 20 µm (C, H); 10 µm (A, B, D-G).

Figure 8 Eulohmannia ribagai(Berlese): A – larva, region of right leg II insertion, ventral view, showing Claparède’s organ (insert = closeup of protective scale, seta 1c); B – adult, ventrosejugal region; C – adult, podosomal region, ventral view (uncleared specimen in glycerine * = extrinsic leg III muscle inserting onap.sj); D – insertions of legs I, II, ventrolateral view, showing declivity bordered by ridger2 (* = articulation of trochanter); E – cleared adult, cross section at level of apodeme 1, anterior view; F – same, at level of apodeme 2, posterior view; G – supracoxal seta eI of two different adults from New York; H – same, from eastern Canada (top Nova Scotia, bottom New Brunswick with one broken tine); I – same, from northwestern North America (from top: Alberta, two examples from Northwest Territories, Alaska); J – same, from Palaearctic populations (from top: two examples from Sweden, Germany (tritonymph), Khabarovsk). Scale bars 20 µm (C-F); 10 µm (A, B); 5 µm (G-J, to same scale).

Figure 9 Eulohmannia ribagai(Berlese): A – genital region of female, ventral view (focused inside genital vestibule; white arrow on soft cuticle partially isolating part of aggenital sclerite); B – tritonymph, insertions of tendons from two fascicles of posterior dorsoventral muscle (separate insertions at top, partly attached insertions at bottom); C – posterior half of hysterosoma, ventrolateral view, showing two fascicles of posterior dorsoventral muscles (adult female from China); D – male spermatopositor (specimen from NW Territories, Canada); E – sagittal view of female genital region in polarized light, showing ovipositor (setaeτ3, τ4 out of focus) and layered articulating cuticle between genital and anal plates; F – ovipositor, unpaired ventral lobe seenen face. Scale bars 20 µm (C); 10 µm (A, B); 5 µm (D-F).

Figure 10 Eulohmannia ribagai(Berlese): A – larva, subcapitulum, ventral view, palps omitted; B – protonymph, adoral lip; C – deutonymph, subcapitulum, ventral view, one palp omitted; D – tritonymph, rutellum, ventral view; E – larva, palp, abaxial view; F – same, enlarged tarsus; G – larva, chelicera, abaxial view; H – tritonymph, chelicera, adaxial view. Scale bars 20 µm (A-E, G, H); 5 µm (F).

Figure 11 Eulohmannia ribagai(Berlese): A – adult, left anal and adanal plates, ventral view; B – larva, paraproctal region, slightly flattened

Figure 12 Eulohmannia ribagai(Berlese), larva: A – right leg I, abaxial view; B – left genu I, dorsal view; C – left leg II, adaxial view (slightly rotated dorsad); D – right leg III, adaxial view (slightly rotated dorsad). Scale bar 20 µm.

Figure 13 Eulohmannia ribagai(Berlese): A – protonymph, right leg I, abaxial view; B – protonymph, left leg IV, abaxial view; C – deutonymph, right tarsus I, abaxial view (slightly rotated dorsad); D – deutonymph, left leg IV, abaxial. Scale bars 20 µm (A, B, D to same scale); 10 µm (C).

Figure 14 Eulohmannia ribagai(Berlese), tritonymph: A – right leg I, abaxial view (distal segments slightly ventral), with insert showing position of variable femoral seta vʹ; B – left leg II, ventro-adaxial (slightly twisted, some setae in distorted positions); C – left leg III, abaxial; D – left leg IV, abaxial. Scale bar 20 µm.

Figure 15 Eulohmannia ribagai(Berlese): A – pretarsus of leg IV, deutonymph, lateral view; B – distal part of tarsus IV, adult (insert = enlargement of vestigial empodial claw); C – postpalpal seta of NY larva (top) and adult from Sweden (bottom); D – famulus of tritonymph abaxial view (with insertion of broken setaft”); E – same, adult from Cape Breton; F – same, adult from Sweden; G – larva, partial dorsal view of right genu I; H – larva, left tarsus I, ventral view (setaea” andpl” out of focus); I – same, deutonymph (setaplʹ out of focus); J – same, tritonymph; K – adult from Sweden, left tarsus I, adaxial view; L – same, but right tarsus, abaxial view; M – adult, lower portion of left tarsus I, adaxial view; N – microsculpture of tarsus I. Scale bars 20 µm (K, L); 10 µm (H-J, M); 5 µm (A, B, D-F); 2 µm (C, G, N).