Morphological ontogeny of Melanozetes avachai n. sp., a unique member of Melanozetes (Acari: Oribatida: Ceratozetidae)

The morphological ontogeny of Melanozetes avachai n. sp. from Kamchatka Peninsula (Russia) is described and illustrated. The juveniles of this species have a humeral organ and a humeral macrosclerite, which occur in most Sphaerozetinae (Ceratozetidae), but in the larva seta c1 inserts on microsclerite, rather than on the humeral macrosclerite, which is unique in Melanozetes. The larva lacks the gastronotal shield and the gastronotal setae are inserted on microsclerites, which is unique in Melanozetes; other species have these setae inserted on the gastronotal shield. The juveniles of M. avachai also differ from congeners by the length and shape of some gastronotal setae. All instars of M. avachai have femora I and II uniquely oval in cross section; in all other species of Melanozetes at least femur II is flattened, with a ventral carina. The adult of M. avachai differs from congeners by its larger body size and longer prodorsal seta le. The translamella is variable in M. avachai; usually it is absent, but may be incomplete, or present. The diagnosis of Melanozetes is modified and enlarged with the morphological characters of juveniles.


INTRODUCTION
is a well defined genus, if we limit the number of notogastral setae of the adults to 14 pairs, including c 2 and c 3 (Seniczak et al. 2015). This genus is not rich in species. Subías (2015) listed 21 species and two subspecies in this genus, but according to Seniczak et al. (2015) this number is much lower mainly because of the nine species that are unrecognizable as nymphs. Melanozetes species differ from one another mainly by their body size, the length of the lamella and lamellar cusp, the shape of the sensillus, the length and shape of notogastral setae and the size of porose area Aa (Seniczak et al. 2015).
The juveniles of Melanozetes species are less wellknown. Based on the catalogue of Norton and Ermilov (2014) and the publication of Seniczak et al. (2015), the full ontogeny of only four species of this genus is known: (1) Melanozetes azoricus: Seniczak et al. (2015) described and illustrated the morphological ontogeny, including leg setae and solenidia.
The aim of this paper is to propose M. avachai n. sp. based on a collection from Kamchatka, and to describe and illustrate its morphological ontogeny, and then compare it and morphology of the adult with that of other species of Melanozetes.

Leg
Trochanter Note: structures are indicated where they are first added and are present through the rest of ontogeny; pairs of setae in parentheses, dash indicates no additions.
Description of juveniles -Larva oval in dorsal view (Figure 7), main body unpigmented, except light brown sclerites and legs. Prodorsum triangu-lar, with setae ro and le of medium size, in long (Table 1) and ex short; all barbed. Mutual distance of setae ro> mutual distance le > greater than mutual distance in; pair le inserted approximately in midway between pairs ro and in. Opening of bothridium rounded, sensillus clavate with barbed head. Lateral depression present above leg I. Prodorsum porose, porosity denser posteriorly.
Gastronotum of larva (Figures 7, 8A) unsclerotized, except for three pairs of glabrous, aligned macrosclerites (well-formed humeral macrosclerite and two elongated lateral macrosclerites), and 12 pairs of setae, including h 3 inserted lateral to middle part of anal valves. Gastronotal setae (12 pairs) short and barbed, except smooth h 2 and minute h 3 , inserted lateral to middle of anal valves; all on microsclerites, except h 3 ; other microsclerites present in central part of gastronotum. Cupule ia posteroventral to seta c 3 , cupule im posterior to seta lm, 472 cupule ip lateral to seta h 2 , cupule ih lateral to anterior end of anal opening; opisthonotal gland opening gla on microsclerite, lateral to ih and humeral organ (oh) located anterolateral to seta c 3 . Paraproctal valves (segment PS) glabrous.
Main body of protonymph and legs light brown. Compared to larva, prodorsum relatively shorter, setae slightly longer (Table 1), and sensillus relatively slimmer. Gastronotum with 15 pairs of setae due to appearance of p-series ( Figure 9A), which are retained by subsequent nymphs ( Figures  10A, B); all setae smooth, except barbed c 1 and c 2 . Seta c 1 inserted on humeral macrosclerite, c 2 and c 3 on microsclerites. Gastronotal macrosclerite uniform, reticulate, with 10 pairs of setae (d-, l-, h-series and p 1 ); setae p 2 and p 3 on microsclerites. In protonymph one pair of genital setae on genital valves, and two pairs are added in both deutonymph and tritonymph. Opisthonotal gland opening gla lateral to seta lp, with dark, porose sclerite, humeral organ oh located anterolateral to seta c 3 . Deutonymph with one pair of aggenital setae and three pairs of adanal setae ( Figure  10A), remaining in subsequent instars; all short and smooth. Tritonymph with two pairs anal setae on anal valves ( Figure 10B). Prodorsum of tritonymph relatively shorter ( Figure 10; Table 1) than in other nymphs, but setae similar. Lateral integument with five macrosclerites, four elongated and one smaller ( Figure 8B). Cupules ia and im placed as in larva, cupule ip between setae p 1 and p 2 , cupule iad lateral to anterior part of anal valves, cupules ips and ih displaced posterolateral to iad. Setae of p-series (three pairs) longer than ad-series (three pairs) and an-series (two pairs); all smooth, p 2 and p 3 on microsclerites ( Figure 10B). Genital setae (five pairs) and aggenital setae (one pair) short and smooth. Femora I and II oval in cross section, without ventral carina. Location and shape of solenidia on tarsus I of tritonymph ( Figure 11) similar as in adult, but some setae on leg segments (d on femora I and II and genu IV, l" on genua I and II and tibia II, l' on genu and tibia III and tibia IV, v' on tibia III and IV) thicker.
Summary of ontogenetic transformations -Setae ro and le are of medium size, in is long and ex is short in all juveniles, whereas in the adult ro and le are long, but ex remains short. The sensillus is relatively thicker in the larva than in the nymphs and adult. The larva has 12 pairs of gastronotal setae, including h 3 , the nymphs have 15 pairs. A humeral macrosclerite is present in all juvenile instars , but in the larva it is glabrous and in the nymphs it bears seta c 1 . Gastronotal macrosclerite is absent in the larva and is well-formed in the nymphs. The notogaster of adult loses seta c 1 . The number of leg setae and solenidia increases during the ontogeny ( Table  2), but some setae of tritonymph are thicker than in the adult.
Ecology and biology -We found M. avachai n. sp. in one sample (1 dm 3 ) in dense moss in the floor of deciduous forest, with alder dominating and some birch. Density of oribatid mites was 345 individuals per 500 cm 3 , with M. avachai the dominant oribatid mite (59.8 %). In M. avachai, the adults dominated (67 % of all individuals), and the stage structure of this species was the following: 22 larvae, 38 protonymphs, 3 deutonymphs, 6 tritonymphs and 137 adults. The mean sex ratio (females to males) was 1:1.5. Most females (87 %) were gravid and carried 4-5 large eggs.
Type deposition -The holotype and 6 paratypes (in 70 % ethanol) are deposited in the University Museum of Bergen (Norway) and 6 paratypes are deposited in the Department of Evolutionary Biology, Kazimierz Wielki University, Bydgoszcz, Poland.
Etymology -The species epithet avachai refers to the Avacha volcano (Kamchatka, Russia), the type locality.
It is evident that the juveniles of M. avachai have several morphological characters that are typical of Melanozetes (Table 3), but the larval lack a gastronotal shield, and seta c 1 located on a microslerite, outside the humeral macrosclerite, are unique character states in Melanozetes; in other species the gastronotal shield is present and c 1 is inserted on the humeral macrosclerite. The tritonymphs of M. sellnicki and M. crossleyi have a clavate sensillus and seta c 1 inserted on the humeral macrosclerite, but in the former species other setae of the c-series are placed on microsclerites, as in M. avachai, whereas in the latter species they are on unsclerotized integument, as in M. azoricus.
The juveniles of M. avachai have femora I and II oval in cross section and clearly slimmer than those of M. azoricus, and without ventral carina ( Figure  12), whereas in the latter species these femora are wide, flattened, with a large, ventral carina. The tritonymph of M. sellnicki and M. crossleyi also have femora I and II flattened, with ventral carina. The nymphs of M. avachai have solenidion ω 2 on tarsus I longer than ω 1 and located anterolateral to solenidion ω 1 , as in those of M. azoricus.
The adult of M. avachai differs distinctly from M. azoricus, M. meridianus and M. mollicomus by larger body size (Table 3), and from M. interruptus it differs mainly by shorter lamella and lamellar cusp, larger porose area Aa and a clavate sensillus; in M. interruptus the sensillus is fusiform.
Considering the shape of lamellar cusp, M. avachai is most similar to M. tanana, but the latter species is distinctly smaller and more stocky (females -mean length 553, mean width 353; malesmean length 545, mean width 351) than M. avachai. Additionally, in M. tanana the lamellar cusps are thicker and closer together (Behan-Pelletier 1986) than in M. avachai, porose area Aa is smaller and the clavate sensillus has longer barbs than M. avachai.
Other species of Melanozetes are also smaller than M. avachai. The body length of females and males of M. sellnicki is 421 -473 and 421 -447, respectively, that of M. crossleyi is 600 -624 and 557 -596, respectively), and both species have distinctly longer lamella and lamellar cusp than M. avachai. The length of M. altaicus is 537 -559 and M. orientalis is 516 -602, and the former species has smaller area porose Aa and distinctly longer lamella and lamellar cusp than M. avachai, whereas the latter species has distinctly shorter notogastral setae than M. avachai. Melanozetes exobothridialis is smaller (length 616 -688) than M. avachai, and has longer lamella and lamellar cusp and seta ex than M. avachai, but the shape of translamella also varies in it (can be absent or incomplete), as in M. avachai.
The leg femora I and II of adult of M. avachai are slimmer than in M. azoricus, and have no ventral carina, whereas the latter species has these femora flattened, and with ventral carina. Flattened femora I and II are also observed in M. sellnicki, M. tanana and M. crossleyi, whereas in Melanozetes meridianus Sellnick, 1928 sensu Behan-Pelletier (1986) only femur II is flattened, with ventral carina. The adult of M. avachai has the solenidion ω 2 on tarsus I longer than ω 1 , and located anterolateral to solenidion ω 1 , as in M. azoricus. Similar shape and location of these solenidia was observed in other species of Melanozetes studied by Behan-Pelletier (1985, 1986, 2000.

DISCUSSION
Melanozetes avachai n. sp. is an interesting member of Melanozetes because the larva lacks the gastronotal shield, which is present in other species of Melanozetes with known ontogeny (Seniczak 1989a;Seniczak 1993d;Seniczak et al. 2015). The larva of M. avachai has seta c 1 inserted on a microsclerite, whereas in other species of Melanozetes it is located on a humeral macrosclerite.
Considering the larva of M. avachai, it seems to be more similar to that of Fuscozetes fuscipes (C. L. Koch, 1841) than to other species of Melanozetes. The larvae of both species have a humeral macrosclerite with no seta c 1 and in both species the gastronotal shield is absent and gastronotal setae are inserted on microsclerites (Seniczak 1989b). In some species of Fuscozetes we observe reduction of gastronotal shield size in the larvae. For example, in F. setosus (C. L. Koch, 1941) this shield is uniform (Seniczak 1989b), in F. tatricus Seniczak, 1993 it is divided in two parts (Seniczak 1993c), whereas in F. fuscipes the gastronotal shield is reduced to three pairs of medium sized macrosclerites (Seniczak 1993d). In the larvae of Sphaerozetes, the gastronotal shield can be present (Seniczak et al. 2016a) or absent (Seniczak et al. 2016b). In Trichoribatinae sensu Shaldybina (1975) the gastronotal shield is usually absent (Behan-Pelletier 1985;Seniczak 1980aSeniczak , d, 1993aSeniczak & Solhøy 1987), but in some species is present (Seniczak 1993d). In Ceratozetinae sensu Shaldybina (1975), the gastronotal shield of juveniles is reduced in size, especially in the larvae, and in some species only microsclerites remain on the larval gastronotum, some with gastronotal setae (Behan-Pelletier 1984). All these observations indicate that the presence or absence of gastronotal shield in the larvae of Sphaerozetinae has specific value.
In all instars of M. avachai femora I and II are slimmer than in those of M. azoricus, especially in the juveniles, and without ventral carina, which is unique in Melanozetes, and more similar to those of Fuscozetes setosus than Melanozetes (Seniczak et al. 2015). The shape of femora I and II of juveniles of other species of Melanozetes is poorly known, but in the adults these femora are wide, flattened, and with ventral carina (Behan-Pelletier 1985, 1986, and flattened femur II with ventral carina was considered typical of Melanozetes (Behan-Pelletier 1985).
Some similarities of the larva of M. avachai to that of F. fuscipes, and similar shape of femora I and II to those of F. setosus may raise a question if M. avachai belongs to Melanozetes. Our answer is positive because this species has three morphological characters of generic value: (1) the adult has 14 pairs of notogastral setae, including c 2 and c 3 , as other species of Melanozetes; (2) the nymphs have notogastral macrosclerite with seta c 1 , as other species of Melanozetes; (3) the nymphs and adult have solenidion ω 2 on tarsus I longer than ω 1 and located anterolateral to solenidion ω 1 , as other species of Melanozetes (Seniczak et al. 2015). Melanozetes is sometimes confused with Fuscozetes, but latter genus has 10-13 pairs of notogastral setae in the adult, including c 2 , and the nymphs and adult have solenidion ω 2 on