Gerdia n. gen. from eastern North America, with redescription of the type species, Porobelba parki Jacot, 1937 (Acari, Oribatida, Damaeidae)

A new mite genus, Weigmannia (Oribatida: Damaeidae), is proposed for the North-American species Porobelba parki described by Jacot (1937). It is distinguished from Porobelba Grandjean, 1936 based on differences in prodorsum morphology, the absence of an unpaired porose area on the notogaster and the development of notogastral setae. Porobelba and Weigmannia may be sister genera in a group that also includes Dameobelba Sellnick, 1929. Weigmannia parki n. comb. is redescribed in detail based on numerous adults and its ontogeny is described for the first time. Eleven states (USA) and provinces (Canada) are added to its known distribution, which now encompasses a region from Manitoba to New Brunswick in the north and Missouri to North Carolina in the south.


INTRODUCTION
The inclusion of the North-American species Porobelba parki Jacot, 1937 within the oribatid mite genus Porobelba Grandjean, 1936 was recently rejected (Miko, 2008), based on the absence of several diagnostic characters of this otherwise European genus. The most apparent is the absence of an unpaired cuticular porose area that occurs posteriorly on the adult notogaster, between insertions of setae ps 1 and h 1 . This porose area represents a secretory organ (Alberti et al., 1997) that is unique within Damaeidae; while variable in size and shape it is always present in Porobelba (Grandjean, 1954). Other characters of the prodorsum and notogaster, considered commonly as genus-level diagnostic traits within Damaeidae, support the proposal of a new genus for the North-American species.
Our aim is to propose such a new genus, to provide a modern redescription of adults of Porobelba parki and describe its ontogeny for the first time. We also include information about its known distribution and the relationship of the new genus to Porobelba.
We dedicate the new genus to our friend and good colleague, oribatologist Gerd Weigmann from Berlin, Germany.
Ventral characters (Fig. 2B). Venter covered by layer of cerotegument. Discidium (dis) short, tubercular, but distinct, directed slightly forwards or perpendicular to body. Epimeral setation 3-1-3-4; setae relatively short, setiform, thin, lateral ones longer than others, seta 1c longest. Epimeral setae insertions of first two epimeres on more or less distinct papillar protuberances, setae 4c and 4d inserted close together. With six genital, one aggenital, two anal and three adanal pairs of setae present,  as usual in Damaeidae; all similar in shape and size. Genital aperture well separated and quite distant from anal opening; longer and broader than anal.
Gnathosoma (Characters were studied only on non-dissected individuals, and therefore are not described in full detail). Type and form of subcapitulum typical of Damaeidae. Palps relatively long, with elongated tarsus; setation (tarsal solenidion not included) 0-2-1-3-8, three distal setae on tarsus eupathidial (Fig. 3C). Setae of rutella and mentum similar in shape to other ventral setae. Chelicerae without unusual characters.
Legs (Figs. 7, 8 K-L). Overall form typical of family. Ontogeny of setae and solenidia given in Table 1; no variation observed. Most setae with several small barbs. Seta d of all tibiae and all genua (except IV which lacks solenidion) closely coupled with respective thin, isodiametric solenidion. In most cases, d 1.5 to 2 times longer than respective solenidion, with following exceptions. Seta d on larval genua regressed, much shorter (≈2, 4, 5 µm on I-III, respectively) than solenidion; latter curves ventrodistad around posterior face of seg-ment (both becoming normal in nymphs). Also, tibia I seta d in all immature instars about half length of flagellate (tactile) φ 1 (but, seta d absent on both tibia I of observed tritonymph, Fig. 7 E,F); φ 2 similar to other tibial solenidia but slightly thinner. Tarsal solenidion ω 1 of leg I ceratiform (Fig. 8K), 2-3 times as thick and half-again as long as ω 2 ; both solenidia of tarsus II thin, similar to ω 2 of tarsus I. Famulus minute, sunk in deep sclerotized cup in all immature instars, not noticeably projecting beyond edge; with straight or slightly curved root-like internal extension from cup, about equal to cup in length (Figs. 8L; 7D). Proral setae (p) of tarsus I appear eupathidial in all instars; subunginal seta of tarsus I normal and proximal to antelateral setae (a) in immatures, becoming eupathidial and distal to pair (a) only in adult; no other setae eupathidial.

Material examined
From the records indicated below, the known distribution of Weigmannia parki encompasses a region from Manitoba to New Brunswick in the north and Missouri to North Carolina in the south. Most collections are from forests, but these vary widely in composition, from rather pure coniferous or deciduous woodlands to mixed forests. The species seems to have an affinity for decaying woody substrates, including stumps, logs and rot-holes in standing trees, since the larger samples of adults and the few immature specimens were collected from such microhabitats. Specimens obtained from leaf litter in low numbers might have been dispersing adults.
Other studied material from the USA. The following records relate to specimens in the collection of R.A.N. or in the Field Museum; they are adults unless noted otherwise. Data are presented in their  (Jacot). Structures are indicated where they are first added and are assumed present through the rest of ontogeny, unless noted otherwise; setae in parentheses represent pairs; dash indicates no additions.

Larva
Protonymph Protonymph original form. Individuals used for drawings in this redescription were from Indiana and New York, in the following overview they are in bold. Illinois. Cook Co., Carle Woods, (unknown col.),18-IV-1936, cornicle is shorter and much less curved in all immature instars of W. parki. Also notable is the form and length of interlamellar seta in deuto-and tritonymphs of W. parki; they are short, only slightly tapered, with a terminal brush of small spines, compared to their normal, setiform shape in P. spinosa.
The hypertrophied dorsal setae of leg femora is apparent even in nymphs of W. parki. The ontogeny of leg setation is almost identical, with differences only in tibial dorsal setae. In W. parki, setae d of tibia II and III are always retained in adults. In P. spinosa, these setae are normally lost in the adult (Ermilov and Łochyńska, 2009). Nevertheless, some races of P. spinosa from southwest and west Europe may lose setae d of tibia II and III as in W. parki (Grandjean, 1954). In observed tritonymphs of W. parki, setae d of tibia I were missing from both legs. Perhaps they were broken, but this also may represent variability of development of tibial d setae, in which case it would further indicate independence of tibia I and tibia II-III development. This is supported also by an observation of Grandjean (1954), who reported a rare presence of seta d on tibia I even in adults of P. spinosa.
In our view, these differences are sufficient to recognize Weigmannia as a separate genus. In particular, the unusual pattern of notogastral setae and hypertrophied femoral setae d are considered apomorphies of Weigmannia, while the unique porose area is an apomorphy of Porobelba. These genera seem close enough to be sister-taxa, or at least part of a well defined group together with Dameobelba minutissima (Sellnick, 1920), as suggested by Miko (2006). A formal cladistic analysis remains to be done.

ADDENDUM
The original version of this paper proposed the genus name Gerdia for Porobelba parki Jacot, 1937 and this name appeared in the pre-publication online version. However, Dr. Pavel Klimov kindly informed us that it would be a junior homonym of Gerdia Menge, 1869, a fossil spider genus in the family Hersiliidae, so Gerdia was replaced with Weigmannia.