Morphology of adult and juvenile instars of Galumna obvia (Acari, Oribatida, Galumnidae), with discussion of its taxonomic status

Abstract The adult instar of the oribatid mite, Galumna obvia (Berlese, 1914), is redescribed in detail, on the basis of specimens from Finland. The morphology of juvenile instars of G. obvia is described and illustrated for the first time, and compared to that of other species of the family Galumnidae. The position of the insertion of the lamellar seta in adults proved variable in studied European populations, being either on or medial to the lamellar line. Since the genera Galumna and Pergalumna are currently distinguished only by the relative positions of the seta and line, specimens of G. obvia in some populations show an intermediate situation between other studied Galumna species – with lamellar seta on or lateral of lamellar line – and Pergalumna with lamellar seta at a distinct distance medially of lamellar line. A detailed reevaluation of the two genera is needed.


Introduction
The oribatid mite Galumna obvia (Oribatida, Galumnidae) was described by Berlese (1914) as Oribata obvius. This species has a semicosmopolitan distribution, being known from the Palearctic and Neotropical regions, i.e. from the USA, South Africa, Santa Elena Islands, Vietnam, and Hawaii (references were summarized by Subías 2004Subías , updated 2013. The original description and several redescriptions of adult G. obvia (see below, Taxonomic history) were incomplete, lacking measurements of morphological structures and information about leg setation and solenidia, and morphology of the gnathosoma. Further, lateral and ventral views of the idiosoma, which have important traits in this family, were insufficiently studied and illustrated. The present paper provides a detailed description and illustrations of G. obvia on the basis of 10 specimens collected in Finland as a reference population. Our data and a literature review show variation in the insertion of lamellar setae, relative to the lamellar line; since this insertion is considered important in distinguishing Galumna from Pergalumna, we discuss the generic position of G. obvia.
Comparative material for the taxonomic discussion originates from one Portuguese and some German locations: - Specimens were mounted in lactic acid on temporary cavity slides for measurement and illustration. All body measurements are presented in micrometers. Body length was measured in lateral view, from the tip of the rostrum to the posterior edge of the ventral plate. Notogastral width refers to the maximum width in dorsal aspect. Lengths of body setae were measured in lateral aspect. Formulae for leg setation are given in parentheses according to the sequence trochanter-femur-genu-tibia-tarsus (famulus included). Formulae for leg solenidia are given in square brackets according to the sequence genu-tibia-tarsus. General terminology used in this paper mostly follows that summarized by Grandjean (see Travé and Vachon for references), Weigmann (2006), and Norton and Behan-Pelletier (2009).
There is some confusion regarding the validity of Galumna obvia, which was declared as junior synonym of G. elimata (described as Oribates elimatus Koch, 1841 in CMA 31.5) firstly by Jacot (1929: 4) in the context of the discussion on the type species of Zetes Koch, 1835, a question which is not relevant for the synonymy of the species. The original figure of Koch's O. elimatus shows clearly long interlamellar setae, but the original description of G. obvia by Berlese (1914, tav. 10: 1) shows clearly very short interlamellar setae, confirmed by the type study of Mahunka (1992) who refered to the redescription of Shaldybina (1975: 353, fig. 887) being in accordance to Berlese's species. Berlese himself (Berlese 1914: 122, Tav. 10: 7) published his interpretation of G. elimata (Koch) with the remark that it is different from G. obvia. Mahunka (1992: 242, fig. 57) figured G. elimata Koch (sensu Berlese) after a slide in the Berlese collection and declared the specimen in Berlese's slide 153/29 as lectotype. We follow the interpretations of Mahunka (1992) as did Weigmann (2006) that G. elimata Koch and G. obvia Berlese are distinct species. Consequently, the synonymization of G. obvia with G. elimata as senior synonym by Jacot (1929) must be rejected. The interpretations of van der Hammen (1952) and others (see above) under the name "G. elimata" are based on Jacot (1929) and refer to G. obvia.
Integument. Body color brown to brownish-black. Body surface microfoveolate (well visible under high magnification); foveolae rounded (diameter up to 1) or represented by short lines (on prodorsum and pteromorphs). Posterior part of ventral plate with long, light furrow (f), located posterior and lateral to anal plates. Genital plates with one strong longitudinal fold located medial to genital setae; additional, short, weakly visible folds present in several specimens.
Legs. Ontogeny of leg setae and solenidia given in Table 3.

(a), s
See Table 1 for explanations. Setae are listed only for the instar in which they first appear.
-From Pilogalumna (P. crassiclava, P. ornatula, P. tenuiclava) by: the length of prodorsal setae (ro longest in G. obvia versus in longest in Pilogalumna species); length of gastronotic setae of c-series (c 3 of medium size, c 1 and c 2 short, c 3 > c 1 ≈ c 2 in G. obvia versus c 3 and c 2 of medium size, c 1 short, c 3 > c 2 > c 1 in Pilogalumna species); and number of gastronotic setae and length of setae h 1 in larval instar (11 pairs -h 3 absent, h 1 short in G. obvia versus 12 pairs -h 3 present, h 1 of medium size in Pilogalumna species).
-From Acrogalumna (A. longipluma) by: the length of prodorsal setae (ro longest in G. obvia versus le longest in A. longipluma); the length of gastronotic setae of c-serie (c 3 of medium size, c 1 and c 2 short, c 3 > c 1 ≈ c 2 in G. obvia versus c 3 and c 2 of medium size, c 1 short, c 3 > c 2 > c 1 in A. longipluma).
-From Allogalumna (A. alamellae) by: the length of prodorsal setae (ro > le > in in G. obvia versus in > ro ≈(>) le in A. alamellae); the length of gastronotic setae of c-serie (c 3 of medium size, c 1 and c 2 short, c 3 > c 1 ≈ c 2 in G. obvia versus c 3 and c 2 of medium size, c 1 short, c 3 ≈ c 2 > c 1 in A. alamellae).
-From Galumna species: from G. alata by the length of prodorsal setae (ro > le > in in G. obvia versus in > ro > le in larva, in ≈(>) le > ro in nymphal instars in G. alata), the length of gastronotic setae of c-series (c 3 of medium size, c 1 and c 2 short, c 3 > c 1 ≈ c 2 in G. obvia versus c 3 and c 2 of medium size, c 1 short, c 2 > c 3 > c 1 in G. alata); from G. zachvatkini by the length of gastronotic setae (c 3 of medium size, c 1 , c 2 and other dorsal setae short, c 3 > c 1 ≈ c 2 in G. obvia versus c 1 , c 2 , c 3 and other dorsal setae well developed, of medium size, c 1 ≈ c 2 ≈ c 3 in G. zachvatkini), and number of gastronotic setae in larval instar (11 pairs -h 3 absent in G. obvia versus 12 pairs -h 3 present in G. zachvatkini).
-From Pergalumna (P. nervosa) by: the length of prodorsal setae (ro > le > in in G. obvia versus le ≈(>) ro > in P. nervosa); the length of gastronotic setae of c-serie and number of gastronotic setae and length of setae h 1 in larval instar (c 3 of medium size, c 1 and c 2 short, c 3 > c 1 ≈ c 2 , 11 pairs setae present -h 3 absent in G. obvia versus c 3 and c 2 of medium size, c 1 short, c 3 ≈ c 2 > c 1 , 12 pairs setae present -h 3 present in P. nervosa).
Thus, the diagnostic morphological characters of Galumnidae juvenile instars are not numerous and can be summarized as: the length of rostral, lamellar and interlamellar setae; the number of gastronotic setae in larval instar; the length of gastronotic setae of c-series, dp, h 1 ; the presence or absence of a transverse furrow on gastronotic shield and genital and adanal macrosclerites on the ventral side in nymphal instars); and body size.

taxonomic discussion: the position of seta le in species of Galumna and Pergalumna
In the Finnish population of G. obvia, the lamellar seta (le) inserts medial to the lamellar line, at a distance of about 5 μm; no distinct variability is observed. The conventional definition of the genus Galumna includes the differential character "lamellar seta on (at) the lamellar line" in contrast to the definition of the genus Pergalumna Grandjean, 1936, originally as subgenus with the differential character "lamellar seta in some distance medially to the lamellar line" (Grandjean 1936;cf. keys of Sellnick 1960, Pérez-Iñigo 1993, Weigmann 2006. Following a strict interpretation of the le position, the Finnish population of G. obvia could be regarded as a Pergalumna species. To resolve this will require a detailed reevaluation of Galumna and a comparison with Pergalumna. We compared the characters of the Finnish population of G. obvia with those in some other European populations, especially from northwest to northeast of Germany; and we found no convincing character combinations to exclude the Finnish population from Galumna obvia, regarding body size (indicated for German populations with 705-845 μm total body length: Weigmann 2006), setation, shapes and positions of porose areas on notogaster, sensillus shape and other characters which are used in literature to define G. obvia. Concerning the position of setae le relative to the lamellar line, we found remarkable variability. In some populations or specimens, the seta le has some distance, continuously up to 6 μm, in median direction from the lamellar line; in other populations or single specimen le inserts strait at the lamellar line on the median side (cf. Figs 37 and 38): In populations "Oder valley" (n=2) -0-1 μm, "Berlin 1" (n=3) -5-6 μm, "Berlin 2" (n=6) -1-5 μm, "Berlin 3" (n=3) -0-1 μm, "Oldesloe" (n=6) -0-4 μm. In all populations we observed a small variability, partly with different ranges. Unfortunately, we have no information about Berlese's typical population in Italy.
These slight differences of the position of seta le raise a similar question with regard to other Galumna species. Since species descriptions, redescriptions and illustrations are often not sufficiently precise, we give only selective examples. In the type species of Galumna, G. alata (Hermann, 1804), Grandjean (1936: 97) figured in his very accurate redescription the setae le on the lamellar line. In material of one of the coauthors (G.W.; location "Berlin 3") of G. alata, the insertion of le is a short distance laterally from the lamellar line. The latter position of le is described in several other Galumna species, e.g. G. asiatica Grishina, 1981 (Bayartogtokh andWeigmann 2005); G. dimorpha Krivolutsky, 1952 (Bayartogtokh and Weigmann  2005); G. gibbula Grandjean, 1956(Grandjean 1956b, 145 as G. tarsipennata gibbula); G. lanceata Oudemans, 1900 (from three urban sites "Berlin 4", actually studied within this project); G. paragibbula Weigmann 2011(cf . Fig. 39); G.
Comparing the position of seta le in strict lateral aspect in Pergalumna nervosa (Fig. 40) and in Galumna obvia (Figs 6,37), there seems to be less difference: in both species the seta seems to be inserted a short distance medially from the lamellar line. Yet in dorso-frontal aspect without parallactic error, the distance between le and the lamellar line is about 27 μm in P. nervosa (Fig. 41), in G. obvia at most 6 μm.
As a preliminary conclusion, most studied Galumna species have the seta le inserted a short distance lateral to the lamellar line; in G. alata the seta is positioned on the line or lateral to it. Galumna obvia is the only species observed with a le insertion medial to the lamellar line or in some specimens on it. The latter two species both show some variability of the le insertion.
The differentiation of the genera Galumna and Pergalumna, defined by Grandjean (1936) by means of the insertion of seta le, is called into question by the variable character state in Galumna obvia. This single differentiation character is of questionable value to discriminate genera as monophyletic entities, and the character is a simple one with a tendency to variability. Nevertheless, it is convenient to split the Galumna-Pergalumna complex in two parts for determination in keys: in Galumna "le is inserted on the lamellar line", in Pergalumna "medially at an obvious distance". We propose to maintain both genera provisionally until a desirable multifactorial phylogenetic analysis is performed.
An analogous case in the family Malaconothridae relates to the single argument to differentiate Malaconothrus Berlese, 1904from Trimalaconothrus Berlese, 1916, by the typological characters "monodactylous or tridactylous legs". This character state is easy to distinguish but obviously without phylogenetical value. Colloff and Cameron (2013) provided a multifactorial analysis for several species of both genera. They found no reasonable pattern to confirm either genera as monophyletic: as a result Trimalaconothrus was considered a junior synonym; Malaconothrus and Tyrphonothrus Knülle, 1957, could be established as valid taxa without the number of claws being a key character.