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5 October 2020 Morphological ontogeny of Amerus polonicus (Acari: Oribatida: Ameridae), and comments on Amerus Berlese
Stanisław Seniczak, Jarosław Kowalski, Sławomir Kaczmarek, Anna Seniczak
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Abstract

The morphological ontogeny of Amerus polonicusKulczynski, 1902 is described and illustrated, based on the individuals from laboratory culture. The juveniles of this species are unpigmented, with punctated integument. The larva has 11 pairs of gastronotal setae, including h2, the nymphs have 12 pairs. The nymphs are quadrideficient and eupheredermous, i.e. they carry the exuvial scalps of previous instars on the gastronotum, using mainly the inward curving setae la and lm to protect the exuvial scalps against loss. In all juveniles, seta d is present on all femora, genua and tibiae, and setae d on femur I and ft" on tarsus I are very long. All tarsi of juveniles are beaded, with long, thin distal parts. In the adult, seta d is lost on genua and tibiae I and II, but is retained on genua and tibiae III and IV.

Introduction

The morphology of Amerus Berlese, 1896 is relatively well known due to the paper of Avanzati et al. (2003), who redescribed the adult of A. troisi (Berlese 1883), the type species of this genus, and described a new species, A. cuspidatus Avanzati et al., 2003. According to Subías (2020), A. polonicus Kulczynski, 1902 is the third species of Amerus sensu stricto, whereas Amerus lundbladi Willmann, 1939 is included in subgenus Neamerus Willmann, 1939. Amerus belongs to Ameridae, which is rather poor in species. This family includes also six other genera (Subías 2004), of which Hymenobelba Balogh, 1962 has six nominal species, Caenosamerus Higgins & Woolley, 1969 has three species, and Andesamerus Hammer, 1962, Haplamerus J. & P. Balogh, 1992, Ctenamerus J. & P. Balogh, 1992 and Pteramerus Balogh, 1961 have two species each (Subías 2020).

The systematic status of A. polonicus is debated in literature. Some authors considered it as a synonym of A. troisi (Sellnick 1960; Bulanova-Zachvatkina 1975), but in figure of A. troisi by both authors seta lp, h2 and h3 are shorter and more similar to A. polonicus than to A. troisi adding to the confusion. Other authors treated A. polonicus as a separate species (Pérez-Íñigo 1976; Olszanowski et al. 1996; Mahunka & Mahunka-Papp 2000; Siepel et al. 2009; Miko 2016; Murvanidze & Mumladze 2016). Therefore, in this paper we aim to provide better diagnostic characters of A. polonicus to clarify its systematic status.

According to the catalogue of juvenile oribatid mites by Norton and Ermilov (2014) and further literature, the morphological ontogeny of A. polonicus or any species of Amerus is unknown. Grandjean (1965) provided data on hypertrichy of aggenital setae in the nymphs and adult of this genus, but it was not attributed to a specific species.

The aim of this paper is to describe and illustrate the morphological ontogeny of A. polonicus, and compare the morphology of the adult with congeners. As the description of the adult by Kulczyński (1902) is old and general, we also redescribe it.

Material and methods

The adults and juveniles of A. polonicus used in this study were collected on 15 November 2017 by J. Kowalski under oak (Quercus robur L.), at the edge of an old oak forest (Figure 1) near Sierpc (Mazovia Province, Poland, 52°48′32″N, 19°39′35″E). This forest was dominated by oak trees, 85–105 years old, with addition of hornbeam (Carpinus betulus L.), black alder (Alnus glutinosa Gaertn.), lime (Tilia cordata Mill.) birch (Betula pendula Roth), bird cherry (Padus avium Mill.) and Scots pine (Pinus sylvestris L.). Ten samples of volume 500 cm3 each were taken from the upper soil horizon (5 cm deep), including litter and were extracted in modified Tullgren funnels during 10 days into small chambers with water; 12 adults and two deutonymphs of A. polonicus were extracted. Five adults were then cultured successively in a small plastic box (7 cm2 × 5 cm high), with the bottom filled with plaster of Paris and charcoal mixture and covered by a perforated lid. These mites were kept in controlled climate conditions (temperature 25°C, 90% air humidity), and fed with oak litter and fresh champignon mushrooms. To avoid intensive overgrowing of food by mycelium, adults of A. polonicus were cultured with Hafenrefferia gilvipes (C.L. Koch 1839), which feed on mycelium (the juveniles of this species showed extraordinary high feeding activity). Moreover, since the nymphs of H. gilvipes also bear exuvial scalps of previous instars on the gastronotum with many long setae (Seniczak et al. 2018), they easily rip up the mycelium, further aiding with mycelium growth control. While A. polonicus was cultured, we selected five larvae, five protonymphs, and four deutonymphs for the morphological study, and five tritonymphs left, from which one transformed in the adult. Four tritonymphs, six adults from culture, and two deutonymphs and seven adults from the extracted samples were also used in this morphological study. The time of development of A. polonicus was established on the base of the successive first stages that appeared in the culture, but fertility of the adults was difficult to determine because it concerned three females, cultured together with two males (determined in lactic acid after this experiment).

We measured the total length of mites (tip of rostrum to posterior edge of notogaster) in lateral aspect and width (widest part of notogaster) of mites in dorsal aspect, and length of anal and genital openings and setae perpendicularly to their size in µm. The illustrations of A. polonicus are limited to the body regions that show substantial differences between instars, including the dorsal and lateral aspect and some leg segments of the larva, tritonymph and adult, ventral regions of all instars, and the palp and chelicera of the adult. Illustrations were prepared from individuals mounted temporarily on slides in lactic acid, using the open-mount technique (Grandjean 1949). In the text and figures, we used the following abbreviations: rostral (ro), lamellar (le), interlamellar (in) and exobothridial (ex) setae, bothridium (bo), bothridial seta (bs), notogastral or gastronotal setae (c-, d-, l-, h-, pseries), cupules or lyrifissures (ia, im, ip, ih, ips, iad), humeral apophysis (ha), scalps of larva (L), protonymph (Pn) and deutonymph (Dn), subcapitular setae (a, m, h), cheliceral setae (cha, chb), palp setae (sup, inf, l, d, vt, cm, acm, lt, ul, su) and solenidion ω, pedotecta (Pd), discidium (Dis), epimeral setae (1ac, 2a, 3ac, 4ac), adanal and anal setae (ad-, an-series), aggenital seta (ag), leg solenidia (σ, φ, ω), famulus (ε) and setae (bv, ev, d, l, ft, tc, it, p, u, a, s, pv, pl, v). Terminology used follows that of Grandjean (1949, 1953, 1965) and Norton and Behan-Pelletier (2009). The species nomenclature follows Subías (2004, updated 2020).

FIGURE 1.

General view of the edge of old oak forest, in which Amerus polonicus was found (enlarged view of litter in bottom-right corner).

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For scanning electron microscopy (SEM), the mites were air-dried and coated with Au/Pd in a Polaron SC502, sputter coater and placed on Al-stubs with double-sticky carbontape. Observations and micrographs were made with a ZEISS Supra 55VP scanning electron microscope. The photos of the adult of A. polonicus were prepared using microscope Leica DM3000 and camera Leica DFC420.

Amerus polonicus Kulczynski, 1902
Amerus troisi (Berlese 1883) sensu: Sellnick 1960; Bulanova-Zachvatkina 1975.
Amerus polonicus: Olszanowski et al. 1996; Mahunka and Mahunka-Papp 2000; Siepel et al. 2009, Miko 2016; Murvanidze and Mumladze 2016.

  • Diagnosis

  • Adults large (923–1021), oval, dark-brown, integument smooth, but some parts of body covered with thicker granular cerotegument. Rostrum with two deep incisions. Bothridial seta setiform, dorsosejugal furrow absent. Ten pairs of notogastral setae present, lm longest, p3 shortest, all curved; setae lp, h2 and h3 clearly shorter than other dorsal setae. Epimeral seta 1b longer than other ventral setae, genital setae (6 pairs), aggenital setae (12–13 pairs), adanal setae (3 pairs) and anal setae (2–3 pairs) short. Seta l′ from palpal tibia absent, formula of palp setae (and solenidion): 0-2-1-2-9(1). Femora I and II with 5–6 setae. Seta d from all genua and tibiae I and II absent, but on genua and tibiae III and IV present.

  • Juveniles unpigmented, integument punctated, some parts of body covered with thicker granular cerotegument. In all juveniles, prodorsal setae ro and le of medium size, and ex short, whereas seta in long in larva and short in nymphs. In all juveniles, bothridium rounded, protruding above surface, and bothridial seta setiform. Most gastronotal setae long and curved, with thin apical ends. Larva with 11 pairs of gastronotal setae, including h2, nymphs with 12 pairs. Nymphs quadrideficient and eupheredermous and carry scalps of previous instars, using mainly curved inwards setae la and lm that protect exuvial scalps against loss. In all juveniles, seta d on all femora genua, tibiae present, and seta d on femur I and ft″ on tarsus I very long. In juveniles, all tarsi beaded, with long, thin distal parts.

  • Redescription of morphology of adult

  • Measurements. Mean length (and range) of females – 966.7±2.16 (923–1021, N= 6) and males – 967.3±1.38 (range 934–1004, N= 7); mean width of females – 555.8±7.06 (520–637) and males – 539.5±2.31 (527–566).

  • Integument. Dark-brown, main body smooth, some parts of body covered with thick granular cerotegument, especially in dorso-sejugal depression, and on basal parts of legs.

  • Prodorsum. Subtriangular, rostrum with two deep depressions (Figs. 2a, 2b). Seta ro slightly shorter (140–144) than le (147–150), and inserted laterally (Figs. 2a, 3a, 4a), pair le inserted dorsally between pair ro. Seta in short (33), seta ex longer (105–107), all prodorsal setae smooth and slightly curved. Bothridium rounded, protruding above surface, bothridial seta long (193–197), setiform, barbed, distally thin and curved, in some individuals two bothridial setae present in one bothridium. Dorsal sejugal suture absent.

  • Notogaster. Anterior part of notogaster with deep depression, pair of humeral apophysis (ha) and 10 pairs of setae (including c2) of different length (Figs. 2a, 4a, 5a, 5c, 5d, 6a, 6b, Table 1); setae lp, h2 and h3 clearly shorter than other dorsal setae. From dorsal setae, lm longest (192–195) and lp shortest (54–57), setae of p-series short (33–41). Lyrifissures ia, im, ip, ih, ips and opisthonotal gland opening in normal positions.

  • Gnathosoma. Subcapitular seta h slightly longer (59) than m (56) and a (46), all smooth (Figs. 3a, 3b). Chelicera relatively thin (205 × 66) chelate, seta cha longer (25) than chb (19), both smooth (Fig. 4b). Palp relatively short (154–157), palpal setae on femur, genu and tibia relatively long and barbed, l′ on tibia absent, palpal seta cm relatively long, other setae shorter, all smooth (Fig. 4c). Solenidion ω and eupathidia relatively long, seta acm short, separated from solenidion. Formula of palp setae [trochanter to tarsus (+ solenidion ω)]: 0-2-1-2-9(1). Axillary saccule of subcapitulum short (9).

  • Ventral and lateral regions. Epimeral seta 1b long (127–130), 1c, 3c and 4c shorter (72–105), other setae short (35–50, Fig. 3a). Six pairs of genital setae, g1–2 in anterior position; all short (28–50) and smooth. Aggenital setae (12–13 pairs), adanal setae (three pairs) and anal setae (2–3 pairs) short (32–37) and smooth. Among 13 adults investigated, two individuals with three pairs of anal setae, one individual with two pairs and one unpaired seta, and other individuals with two pairs. Lyrifissure iad short, located lateral to anterior part of anal plates. Seta ad3 inserted laterally, relatively distant from iad, seta ad1 inserted posterior to anal plate, and seta ad2 between setae ad1 and ad3. Pedotectum I large, oval (122 × 99), pedotectum II smaller (56 × 77), discidium well-formed, distally rounded (Figs 3a, 4a).

  • Legs. Trochanters III and IV and all femora oval in cross-section, dorsal parts with porose areas. Apical ends of trochanters III and IV with clear collars. All leg setae relatively long and finely barbed (Figs. 4a, 5a–c, 5e, 6a, 6b, 7), except for smooth ventral setae on all femora and setal pairs p on all tarsi. All tarsi relatively thin and long, especially tarsus IV. Seta d on genua and tibiae I and II absent, but on genua and tibiae III and VI present; on latter segments, d inserted at some distance from proper solenidia (Fig. 7, Table 2). Formulae of leg setae (and solenidia, from trochanter to tarsus): I—1-(5-6)-3(1)-4(2)-20(2), II—1-(5-6)-3(1)-4(1)-15(2), III—2-3-3(1)-4(1)-15, IV—1-2-3-4(1)-12. Legs monodactylous.

  • Description of juvenile stages

  • Larva oval in dorsal aspect (Fig. 8a) and unpigmented, some parts of body covered with granular cerotegument. Prodorsum subtriangular, rostrum rounded, cerotegument irregularly punctated. Prodorsal seta in long, ro and le of medium size (Table 1), seta ex short; most with short barbs, except for smooth ex. Mutual distance between setal pair in almost as long as that between pair ro, and between setal pair le nearly half of that between pair ro. Seta le inserted closer to ro than to in. Bothridium large, rounded, bothridial seta long, setiform and barbed.

  • Gastronotum of larva with 11 pairs of setae, including h2 inserted laterally to posterior part of anal valves (Figs. 9a, 10a). Most gastronotal setae long and with short barbs, except for short and smooth c2; all curved, distally thin and inserted on apophyses, apophyse at seta dm larger than at other setae. Anal valves (segment P) glabrous (Fig. 9a). Cupule ih lateral to anterior part of anal valves, other cupules and gland opening not observed in punctated cerotegument. Seta d on femur I and ft″ on tarsus I very long, seta d on tibia I, l′ on genu III and most setae on tarsi long, other setae of medium size or short; most barbed, except for smooth d on tibia I and p on tarsi (Figs. 8a, 10a 11). Basal part of all tarsi short, beaded, with long and thin distal parts, especially of tarsus I, bearing setae p and u, other setae inserted on basal parts of tarsi. Solenidion φ1 on tibia I long, adjoined to coupled seta d, other solenidia and famulus ε of medium size or short.

  • Nymphs oval in dorsal aspect, shape of prodorsum, prodorsal setae, bothridium and bothridial seta as in larva, but seta in short and smooth. Gastronotum of protonymph with 12 pairs of setae because setae h3 and setae of p-series appear (Fig. 9b), remaining in deutonymph and tritonymph (Figs. 12a, 12b, 13), setae of d-series lost and remaining absent in all nymphs. In protonymph, seta p1 long, other setae of p-series of medium size; all barbed (Fig. 9b), in deutonymph and tritonymph, all these setae long (Figs. 12a, 12b). In all nymphs, gastronotal setae on small apophyses. All nymphs carry exuvial scalps of previous instars, using mainly the inwards curving seta la and lm to protect the exuvial scalps against loss (Figs. 6c, 6d, 10b, 13, 14a, 14c, 15a). Prodorsum of tritonymph, exuvial scalps and legs with granular cerotegument. In protonymph, one pair of setae appears on genital valves, and three pairs in deutonymph and one pair in tritonymph added (Figs. 9b, 12a, 12b); all short and smooth. In deutonymph, two pairs of aggenital setae and three pairs of adanal setae appear, and in tritonymph five pairs of aggenital setae added; all short and smooth. Anal valves of protonymph (segment AD) and deutonymph (segment AN) glabrous, in tritonymph 2–3 pairs of anal setae present (Fig. 12a, 12b), all short and smooth. Among four tritonymphs investigated, one individual with three pairs of anal setae, on individual with two pairs and one unpaired seta, and two individuals with two pairs. In protonymph, cupule ips located lateral to anterior part of anal valves, and cupule ih displaced posterolateral to ips, in deutonymph and tritonymph cupule iad located posterolateral to anterior part of anal valves, and cupule ips displaced posterolateral and cupule ih anterolateral to iad. Other cupules and gla opening not observed in punctated cerotegument. In tritonymph, seta d on femur I and ft″ on tarsus I very long (Figs. 6c, 6d, 10b, 13, 14a, 14b, 15c, 15d, 16), seta d on femora II–IV and genu and tibia IV, l′ on genu III and IV, and most setae on tarsi long, other setae of medium size or short; most barbed, except for smooth d on tibia I and p on tarsi (Fig. 16). Basal part of all tarsi short, beaded, with long and thin distal parts, especially of tarsus I, bearing setae p and u, other setae inserted on basal parts of tarsi. Solenidion φ1 on tibia I long, adjoined to coupled seta d, other solenidia and famulus ε of medium size or short.

  • Summary of ontogenetic transformations

  • In all instars of A. polonicus, the prodorsal setae ro and le are of medium size. In all juveniles, seta ex is short, and seta in is long in the larva and is short in the nymphs, whereas in the adult seta in is short and seta ex is of medium size. In all instars, the opening of bothridium is rounded and the bothridial seta is setiform. The larva has 11 pairs of gastronotal setae, including h2, and the nymphs have 12 pairs (h3 and p-series appear, d-series is lost in protonymph). The notogaster of adult loses setae c1 and c3, so that 10 pairs remain. The formula of gastronotal setae of A. polonicus is 11-12-12-10 (from larva to adult). Formulae of epimeral setae is 3-1-2 (larva, including scaliform 1c), 3-1-3-1 (protonymph), 3-1-3-2 (deutonymph) and 3-1-3-3 (tritonymph and adult). Formula of genital setae is 1-4-5-6 (protonymph to adult), aggenital setae is 2-7-(12–13) (deutonymph to adult), and segments PS–AN is 03333-0333-0(2-3)(2-3). Ontogeny of leg setae and solenidia is given in Table 2.

  • Distribution, ecology and biology

  • According to Subías (2004, 2020), A. polonicus has a Mediterranean distribution, but on the base of hitherto obtained data (Pérez-Íñigo 1976; Olszanowski et al. 1996; Mahunka & Mahunka-Papp 2000; Siepel et al. 2009; Miko 2016; Murvanidze & Mumladze 2016) and the type locality of Kulczynski (1902), this species occurs in Poland, Hungary, the Netherlands, Czech Republic and Georgia. Amerus polonicus inhabits Alpine meadows, forests and urban soils (Murvanidze & Mumladze 2016). In forests, this species prefers beech (Fagus sp.) litter (Beck & Woas 1991; Beck et al. 2007), and fermentation layer (Wunderle 1992). Probably therefore, in old beech forests this species was clearly more abundant in moder type of humus (plant association Luzulo-Fagetum) than in mull humus (plant association Galio-Fagetum, Beck et al. 2007).

  • In our investigation, A. polonicus was found in an old oak forest near Sierpc (Mazovia Province, Poland), but was not abundant. From 10 large samples (500 cm3 each), we extracted 12 adults and two deutonymphs in total. During this extraction, the mites left the samples relatively quickly (in 1–3 days), and were present only in three of the total of 10 samples, so constancy of this species in these samples was 30% and mean density was 1.4 individuals per 500 cm3.

  • In laboratory culture (temperature 25°C, 90% air humidity), the development of A. polonicus lasted 68 days, including the following stages: 13 days as egg, eight days as larva (+ one day as immovable pre-moulting form), seven days as protonymph (+ two days as pre-moulting form), 10 days as deutonymph (+ four days as pre-moulting form), 14 days as tritonymph (+ nine days as pre-moulting form), from which the adult appeared. While moulting, the tritonymphal hysterosoma broke around the opisthonotum posteriorly, and the adult slowly left the tritonymphal moult in the same direction (Fig. 5f). This action from breaking tritonymphal hysterosoma lasted about nine hours, and a new adult was light brown and moved slowly. From this culture, we obtained five larvae, five protonymphs, four deutonymphs, four tritonymphs and one adult in total. Among the adults obtained from field samples and culture, males were more abundant than females (sex ratio 1:1.2), and 50% of females were gravid. The gravid females usually carried three large eggs, each 337 × 182, which constituted about 35% of the total body length of females.

  • FIGURE. 2–3.

    Amerus polonicus, male, legs partially drawn, scale bars 100 µm. 2. (a) Dorsal aspect, (b) rostrum (enlarged). 3. (a) Ventral aspect, (b) part of hypostome (enlarged).

    img-z4-7_1840.jpg

    TABLE 1.

    Measurements of some morphological characters of juvenile stages of Amerus polonicus (mean measurements of 4–6 juveniles and 10 adults in µm); Nd—not developed.

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    TABLE 2.

    Ontogeny of leg setae (Roman letters), solenidia and famulus (Greek letters) of Amerus polonicus.

    img-z6-3_1840.gif

    FIGURE 4.

    Amerus polonicus. (a) Adult, lateral aspect, legs partially drawn, scale bar 100 µm; mouthparts, right side, antiaxial view, scale bars 20 µm; (b) chelicera (Trägårdh organ indicated in ‘transparent’ area); (c) palp.

    img-z7-2_1840.jpg

    FIGURE 5.

    Amerus polonicus, male, photos, dorsal view. (a) Whole body, (b) anterior part of body, (c) notogaster, (d) central part of notogaster, (e) part of legs I and II, (f) male while and after moulting; scale bars: (a) 200 µm, (b–e) 100 µm, (f) 400 µm.

    img-z8-2_1840.jpg

    FIGURE 6.

    Amerus polonicus, adult, SEM micrographs. Adult, (a) dorsal view, (b) lateral view; tritonymph, (c) dorsal view, (d) lateral view.

    img-z9-1_1840.jpg

    FIGURE 7.

    Amerus polonicus, adult, part of femur to tarsus, lateral aspect, seta on the opposite side not illustrated, but indicated in the legend, scale bars 50 µm. (a) Leg I, tarsus (pl′); (b) leg II; (c) leg III; (d) leg IV.

    img-z10-1_1840.jpg

    FIGURE. 8–9.

    Amerus polonicus, legs partially drawn, scale bars 50 µm. 9. (a) Larva, dorsal aspect, (b) shape of seta da (enlarged). 10. Ventral side of hysterosoma, (a) larva, (b) protonymph.

    img-z11-1_1840.jpg

    FIGURE 10.

    Amerus polonicus, lateral aspect, legs partially drawn, scale bars 50 µm. (a) Larva, (b) tritonymph.

    img-z11-3_1840.jpg

    FIGURE 11.

    Amerus polonicus, leg segments of larva (part of genu to tarsus), right side, seta on the opposite side are not illustrated, but indicated in the legend, scale bar 20 µm. (a) leg I, tarsus (pl′), (b) leg II, (c) leg III.

    img-z12-1_1840.jpg

    FIGURE. 12–13.

    Amerus polonicus, legs partially drawn, scale bars 50 µm. 12. Ventral side of hysterosoma, (a) deutonymph, (b) tritonymph. 13. Tritonymph, dorsal aspect.

    img-z12-3_1840.jpg

    Comparison of morphology of Amerus polonicus with congeners and remarks

    Avanzati et al. (2003) redescribed the adult of A. troisi and described A. cuspidatus Avanzati et al., 2003, which differs from the former species mainly by the shape of rostrum, wider separated interlamellar seta in the latter species and on molecular level. These two species differ clearly from A. polonicus studied herein mainly by longer setae lp, h2 and h3 on the notogaster; in A. polonicus, these setae are clearly shorter than seta h1, whereas in other species these setae are of similar length. The palp of A. polonicus has more setae on tarsus (formula 0-2-1-2-9) than those of other species (formula 0-2-1-2-7, Avanzati et al. 2003).

    FIGURE 14.

    Amerus polonicus, tritonymph, SEM micrographs. (a) Anterior part of body, dorsal view, (b) anterior part of body, dorsolateral view, (c), exuviae of previous instars on the gastronotum, lateral view (d) bothridium and bothridial seta, dorsolateral view.

    img-z13-4_1840.jpg

    FIGURE 15.

    Amerus polonicus, tritonymph, SEM micrographs. (a), (b) Granular cerotegument on exuviae, (c) legs I and II, dorsal view, (d) fragment of leg I, dorsal view.

    img-z14-1_1840.jpg

    Willmann (1939) included Amerus lundbladi in Neamerus mainly based on some skeletal characters of one individual. However, the figures of this individual indicate that the body shape of A. lundbladi differs slightly from the other species, but the shape of rostrum and shape and distribution of prodorsal setae are similar in all species of Amerus. The notogastral seta c2 is missing in the figure by Willmann (1939), but setae la and lm are long, as in other species of Amerus, and setae lp, h2 and h3 are slightly longer than in A. polonicus. In A. lundbladi, most ventral setae are missing, but the epimeral seta 1a is long and 1c slightly shorter, as in A. polonicus. In all species, the bothridial seta is setiform, but in A. lundbladi it is barbed, whereas in other species is smooth. Willmann (1939) drew leg I of A. lundbladi, which has three relatively thick ventral setae on femur, as in A. polonicus. Therefore, more investigations are needed on A. lundbladi, including the juvenile stages, to contradict or support its membership in Neamerus.

    The morphology of the adult A. polonicus varies in different geographical regions. For example, the individual from Iran (Akrami et al. 2008) has setae lp, h2 and h3 relatively longer and thicker than A. polonicus from Poland studied herein, especially seta lp. Also individuals from Turkey (Baran & Kilic 2013) and Hungary (Mahunka 1996) have these setae slightly longer than A. polonicus from Poland. In A. polonicus by Kunst (1971), seta lp is lacking, seta h3 is longer, but seta h2 is as short as in A. polonicus studied here. The individuals from Hungary have 13 pairs of aggenital setae (Mahunka 1996), whereas those from Poland have 12–13 pairs of these setae.

    The shape of leg segments and setae of adult of A. polonicus is similar to that studied by Mahunka (1996). The leg setae are relatively strong, three smooth setae are present on the ventral part of femur I, and seta d is present on genu and tibia IV. The juveniles of A. polonicus have beaded tarsi with long and thin distal parts, similar tarsi occur in species of Damaeidae (Norton 1978, 1980; Seniczak & Seniczak 2011, 2013; Seniczak et al. 2013, 2016; Miko 2015). However, the amerid and damaeid species represent systematically distant groups, which suggest that the shape of leg segments is convergent.

    FIGURE 16.

    Amerus polonicus, leg segments of tritonymph (part of femur to tarsus), right side, setae on the opposite side are not illustrated, but indicated in the legend, scale bar 50 µm. (a) Leg I, tarsus (pl′); (b) leg II, femur (l′); (c) leg III; (d) leg IV.

    img-z15-1_1840.jpg

    The nymphs of A. polonicus carry the exuvial scalps of previous instars, using mainly setae la and lm that are curved inwards and protect the exuvial scalps against loss. The nymphs of other species of Ameroidea sensu (Norton & Ermilov 2014) are highly differentiated and use different manners of carrying the exuvial scalps. For example, Mongaillardia granjeani Călugăr & Vasiliu, 1984 (Amerobelbidae) has rather short gastronotal setae that do not cooperate with the scalps (Călugăr & Vasiliu 1984), Damaeolus asperatum (Berlese 1904) from Damaeolidae use relatively strong marginal setae on the gastronotum (Seniczak et al. 2020), whereas Basilobelba parmata Okayama, 1980 (Basilobelbidae) and Caleremaeus monilipes (Michael 1882) from Caleremaeidae use a cornicle (Okayama 1980; Seniczak & Seniczak 2019). By contrast, the nymphs of Gymnodampia setata (Berlese 1916) from Ameridae are apheredermous, i.e. they retain setae of dseries and do not bear the exuvial scalps (Chen et al. 2004). These results show a great differentiation of morphology of Ameroidea and encourage for more studies on this superfamily, including the juvenile stages.

    Acknowledgements

    We thank two anonymous reviewers for helpful suggestions that improved the scientific value of this paper. This study was done under the program of the Polish Minister of Science and Higher Education “Regional Initiative of Excellence” in 2019–2022 (Grant No. 008/RID/2018/19).

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    © Systematic & Applied Acarology Society
    Stanisław Seniczak, Jarosław Kowalski, Sławomir Kaczmarek, and Anna Seniczak "Morphological ontogeny of Amerus polonicus (Acari: Oribatida: Ameridae), and comments on Amerus Berlese," Systematic and Applied Acarology 25(10), 1840-1856, (5 October 2020). https://doi.org/10.11158/saa.25.10.8
    Received: 16 June 2020; Accepted: 29 July 2020; Published: 5 October 2020
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