Two new Oribatid mites from Costa Rica, Mixacarus turialbaiensis sp. n. and Paulianacarus costaricensis sp. n. (Acari, Oribatida, Lohmanniidae)

Abstract In this paper we describe two new species belonging to the family Lohmanniidae: Mixacarus turialbaiensis sp. n. and Paulianacarus costaricensis sp. n. from Costa Rica.


Introduction
Approximately three years ago, the authors commenced the study of materials housed at the Museum d'Histoire Naturelles de Genève (MHNG), which was collected from the Turrialba forest in Costa Rica. In this initial paper, we describe two new species belonging to genera Mixacarus and Paulianacarus of the family Lohmanniidae. The taxonomy of the first species, Mixacarus turialbaiensis sp. n., was problematic as taxonomically important characters of related species were not adequately described in most prior studies. For the second species, Paulianacarus costaricensis sp. n., the situation is similar, but with seemingly misinterpreted original descriptions an aggravating factor.

Material and methods
Specimens studied by means of light microscopy were macerated in lactic acid, and observed in the same medium using the open-mount technique (cavity slide and cover slip) as described by Grandjean (1949), Krantz and Walter (2009). Drawings were made using a Zeizz GFL (Germany) compound microscope equipped with a drawing tube. Specimens preserved in ethanol, studied under Scanning Electron Microscope (SEM), were carefully rinsed by sucking them several times into a Pasteur pipette, after which they were transferred to buffered glutaraldehyde (2,5%) in Sörensen phosphate buffer: pH 7,4; 0,1 m for two hours. After postfixation for 2hr. in buffered 2% OsO4 solution and being rinsed in buffer solution; all specimens were dehydrated in a series of graded ethanol and dried in a critical point apparatus. After mounting on Al-stubs with double sided sticky tape, specimens were gold coated in a sputter apparatus (Alberti and Fernandez 1988, 1990a, 1990bAlberti et al. 1991Alberti et al. , 1997Alberti et al. , 2007Fernandez et al. 1991). For SEM observations, a FEI-Quanta Feg 250 Scanning Electron Microscope; with 10 Kv and working distant (WD) variable was used.
Measurements taken: total length (tip of rostrum to posterior edge of notogaster); width (widest part of notogaster) in micrometers (μm). Setal formulae of the legs include the number of solenidia (in parentheses); tarsal setal formulae include the famulus (ε).
Lateral region. Prodorsal margin present on either side of cavities housing legs I-IV when retracted. Anterior notogastral zone presenting conspicuous tectum and clearly defined unsclerotized lateral longitudinal line, terminating almost posterior to level of ip lyrifissure and delimiting unpaired dorsal notaspis and pleuraspis (paired narrow lateral zones) ( Figure 11). In posterior notogastral zone, when unsclerotized line does not exist, notaspis and pleuraspis not delimited ( Figure 11). Each pleuraspis presenting an anterior rounded lobe between legs II and III, where lyrifissure ia is observed. Posteriorly, at level of d 3 and e 2 setae, well delimited edges form canopies over cavities in which legs III and IV are housed when retracted, with a protruding angle between them.
Anal and adanal plates with four pairs of adanal and two pairs anal setae ( Figures  16, 18). Band BPAD clearly visible in specimens immersed in lactic acid for lengthy period; lyrifissure ips present near margin of this band ( Figure 10).
Legs. Two types of femora can be distinguished. Femora of legs I and II displaying large ventral blade (Figures 24, 25), femora of legs III and IV lacking ventral blade (Figures 26, 27).

Discussion
The genus Mixacarus was proposed by Balogh (1958); but later Balogh and Balogh (1987) proposed another new genus, Phyllolohmannia. Currently Mixacarus is divided into two sub-genera, Mixacarus and Phyllolohmannia, and includes 22 species (Subias 2017). A comparison between Mixacarus turialbaiensis sp. n. and Mixacarus exilis Aoki 1970 is quite complex, as some aspects of the initial description is detailed and others are deficient, for example: lateral observations are ignored and for the ventral region, drawings are referred to, but in a preceding paper by Wallwork 1962. Other problematic aspects include the absence of any reference to the porose areas of M. exilis. Making use of different study methods and technology, the authors were able to observe structures evidently not previously observed, such as the particular microsculpture in depressed areas (Figure 7), and transversal bands on notogaster (Figures 2,5,6,13,14,16,17,19,21,23). Porose areas ( Figure 8) were discernible on transversal notogastral band in the zone of this microsculpture.
Mixacarus turialbaiensis sp. n. is close to Mixacarus exilis Aoki 1970, but is differentiated by the depressed areas with particular microsculpture; all prodorsal setae have similar characteristics and length; ribbon-like bands on prodorsum distributed very differently; notogastral setae slightly barbate; nine transversal bands; epimeres with large number of depressed areas; variable chaetotaxy in genital and epimeral zone.
Paulianacarus was proposed by Balogh (1960) from Madagascar. Subias (2004) considered Millotacarus to be a subgenus of Paulianacarus. At present there are 15 species allocated to Paulianacarus and Millotacarus. The taxonomy of these genera are complex, and considering one a subgenus of the other is complicated by the lack of a detailed comparative study of type materials. Several authors have expressed their opinions (Mahunka 1985;Coetzee 2001;Chen et al. 2012, Fernandez et al. 2014, and these considerations highlight the incongruences in the descriptions, indicating that some do not consider that these are different genera, or do not consider one to be a subgenus of the other, while other researchers accept both subgenera. An analysis of these opinions is not repeated here in order to avoid redundancy. The only way to solve the problem is the study and comparison of type material, which was not possible in this instance.
The new species Paulianacarus costaricensis sp. n. was described using optical and SEM microscopy. These techniques allowed us to understand some of the complex structures also observed in Paulianacarus rugosus Balogh, 1961, a species close to the newly described species.
P. costaricensis displays the following characters: elevated transversal thickening (tr.e.t) with transverse bands: 1) some cross the transverse medial notogastral plane, others do not; 2) some are rectilinear, others oblique; 3) some present superficial rounded protuberances, others are smooth; 4) smooth thickenings either with complete furrow running the entire length, or partial furrow; 5) tr.e.t are associated with transversal furrows (S); 6) transversal furrows are related to one or both sides of the elevated transversal thickenings. Variable number and disposition of genital and epimeral setae, difficulty in observing lyrifissures. These are only some of the characteristics of this species, but they emphasize the need for detailed studies.