Mites were collected primarily from leaf litter samples in the Ozark Mountains of Arkansas (U.S.A.), specifically Buffalo National River and Devil’s Den State Park, and extracted using Berlese-Tullgren funnels. Approximately half of the specimens were slide-mounted with Hoyer’s medium (see Krantz and Walter 2009 for preparation), and half are stored in 95% ethanol at -80°C. Slide-mounted specimens were dissected along the frontal plane. Five paratypes are deposited in the Ohio State University Acarology Collection, Columbus, Ohio. Four paratypes each are deposited in the Field Museum of Natural History, Chicago, Ill., and the National Mite Collection, National Museum of Natural History, Smithsonian Institution, located in Beltsville, Md. All other type specimens are deposited in the Acari Collection of the University of Arkansas, Fayetteville, Ark.

An effort is made to implement terminology that is broadly applicable and well accepted across acariforms despite conventions used among bdelloid authors. Thus, two terms have been renamed herein. First, “hypostome” is used by many (Atyeo 1960; van der Schyff et al. 2004; Hernandes and Feres 2006; Krantz and Walter 2009) to refer to the entire subcapitulum, though it more accurately refers only to the portion anterior to the oral opening (Evans 1992; Krantz and Walter 2009). Thus, in addition to the subcapitulum itself, its setae have been renamed herein to ventral subcapitular setae (vs) and dorsal subcapitular setae (ds).

Second, the major idiosomal divisions of bdelloids are regularly referred to as the “propodosoma” and “hysterosoma” (Bdellidae: Atyeo 1960, Mejia-Recamier and Palacios-Vargas 1999, Hernandes et al. 2007; Cunaxidae: Meyer and Ryke 1959, Swift 1996, Den Heyer and Castro 2009). However, acariform segmentation is a debated topic with recent views favoring a radically reduced podosoma leaving the major idiosomal divisions (when viewed dorsally) the opisthosoma and aspidosoma (Grandjean 1969; Coineau 1974; Evans 1992; Krantz and Walter 2009). With regard to the posterior portion, hysterosoma is already widely used and accurately refers to the idiosoma posterior to the sejugal furrow (metapodosoma and opisthosoma). Moreover, it is independent of segmentation hypotheses. Therefore, we retain the use of hysterosoma. However, propodosoma is hypothesis dependent. The Grandjean (1969) hypothesis of body organization suggests the propodosoma is highly reduced dorsally, rendering propodosoma inaccurate when referring to the anteriodorsal portion of the idiosoma. Instead, aspidosoma accurately describes this region. Therefore, we abandon the use of propodosoma when referring to the dorsal morphology of acariforms. However, until more evidence exists for segmentation homologies, we also avoid the use of aspidosoma for Bdelloidea. Prodorsum is widely used by acarologists, but is usually poorly defined and represents the opposite extreme from aspidosoma--straying too far from morphological hypotheses. An exception to this is in Oribatida (e.g., Sanders and Norton 2004), where it is often used as synonymous with the aspidosomal shield (=aspis) and is therefore hypothesis dependent. Thus we recommend restricting prodorsum to casual use. Proterosoma refers to the body anterior to the sejugal furrow (propodosoma, aspidosoma, and gnathosoma of Grandjean [1969]), and is a segmentation independent term that is already widely used in acarology. Therefore, we adopt proterosoma here.

With regard to hysterosomal setal notation, we follow the chaetotaxic system of (Grandjean (1939, 1947) that has been widely adopted (e.g., van der Hammen 1970; Lindquist 1976, 1977; Kethley 1990). Proterosomal setal notation in this system is currently problematic. Generally, we do not recommend the use of mixed approaches in terminology, especially given our attempts to utilize broadly applicable notations. However, recent suggestions in the proterosomal setal notation of bdelloids have rendered this system unreliable. In the Grandjean system, proterosomal setae are termed internal/external verticals (vi and ve) and internal/external scapulars (sci and sce). In Bdelloidea, sci are always external to sce, leaving the internal/external designations unintuitive. Den Heyer and Castro (2008a) noted this and proposed simply switching the terms when referring to bdelloids so that sce and sci are descriptive, which they have published since (Den Heyer and Castro 2008b, c; Den Heyer 2011). As a result, the literature now has sce and sci referring to both inner and outer scapulars. The homology of these setae with respect to other mites is not known. Therefore, until more evidence is provided for homology, we resort to a modified version of Atyeo (1960) when referring to proterosomal setae, which unambiguously relies on position: anterior and posterior trichobothria (at and pt), and lateral and median proterosomal setae (lps and mps).

Leg chaetotaxy follows Grandjean’s system as reviewed by Norton (1977). However, leg chaetotaxy is poorly studied among Eupodina, and only distal tarsal setae are denoted presently, which has been adopted by other eupodine authors (e.g., Jesionowska 2010). Nevertheless, we believe Grandjean’s system can be employed with other leg setae, and will readdress this in a more detailed forthcoming study.

Most species descriptions include only a few image types; line drawings are most common in acarology (e.g., Mejia-Recamier and Palacios-Vargas 1999; Den Heyer and Castro 2008a, b, c; Łochyńska 2008). We believe image diversity increases accuracy, accessibility, and attractiveness of taxonomic products, and have therefore included many images and a range of imaging types. Other acarologists have also begun including image diversity in taxonomic works (e.g. Mąkol 2010; Wohltmann 2010; Pešić et al. 2011).

Line drawings were created digitally with Adobe Illustrator CS5 and a Wacom Cintiq 21UX tablet using procedures outlined in Fisher and Dowling (2010). Compound light micrographs were taken with a Leica DFC300 FX camera and a Leica DM2500 DIC light microscope. Stereomicrographs were taken with a Leica MZ 16 stereoscope and a Leica DFC 290 camera. Photographs were enhanced using Adobe® Photoshop CS4.

Low-temperature scanning electron micrographs (LT-SEM) were made using an S-4700 field emission scanning electron microscope (Hitachi High Technologies America, Inc., Pleasanton, Calif.) equipped with a Quorum CryoPrep PP2000 (Energy Bean Sciences, East Grandby, Conn.) cryotransfer system. To prepare specimens, mites were placed on 12 mm diameter ultra smooth carbon double sided adhesive tabs (Electron Microscopy Sciences, Hatfield, PA) which were adhered to flat specimen holders consisting of 16x30mm copper plates that were tacked on the edges to the tabs with a small dot of Tissue Tek (OCT Compound, Ted Pella, Inc., Redding, Calif.), which acted as the cyro-adhesive upon freezing. The samples were frozen conductively, in a Styrofoam box, by placing the plates on the surface of a pre-cooled (-96°C) brass bar whose lower half was submerged in liquid nitrogen (LN2). After 20–30s, the holders containing the frozen samples were transferred to a LN2 Dewar for future use or cryotransferred under vacuum to the cold stage in the pre-chamber of the cryotransfer system. Removal of any surface contamination (condensed water vapor) took place in the cryotransfer system by etching the frozen specimens for 10–15 min by raising the temperature of the stage to -90°C. Following etching, the temperature was lowered below -130°C, and a magnetron sputter head equipped with a platinum target, was used to coat the specimens with a very fine layer of platinum. The specimens were transferred to a pre-cooled (-130°C) cryostage in the SEM for observation. An accelerating voltage of 5kV was used to view the specimens. Images were captured using a 4pi Analysis system (Durham, N.C.). Images were sized and placed together into figures using Adobe® Photoshop 7.0 and CS4.

Trachymolgus recki was described from two specimens from Georgia (Gomelauri 1961). Unlike other Trachymolgus, the integument of Trachymolgus recki was described as unarmored (despite having foveolate indentations and proterosomal crown) and yellowish. There was one tooth on the fixed digit, one pair of eyes, and the pedipalps were inconclusively described as “nearly fused” though they were completely fused in the illustration (Gomelauri 1961).There were 6–7 genital setae, and coxal field setae 3-4-4-5.

We have found that purple Trachymolgus purpureus immatures lose color more readily than adults when slide-mounted, and some immatures are yellowish-white in life (Fig. 12b). Also, though deuto- and tritonymphal shields are foveolate, they are not as heavily sclerotized as in adults, which could give the appearance of being un-armored. Adults of Trachymolgus purpureus and Trachymolgus jesusi have higher setal counts with >20 and 10 genital setae, respectively, and coxal field setae 7-7-9-8 (female Trachymolgus purpureus), 6-6-6-10 (male Trachymolgus purpureus), and 8-5-11-10 (Trachymolgus jesusi). Larvae, proto- and deutonymphs of Trachymolgus jesusi were described as having highly reduced eyes. Gomelauri observed only one pair eyes in Trachymolgus recki. This offers significant evidence to suggest the specimens used to describe Trachymolgus recki were immature. Since these specimens were said to have three pairs of genital papillae, we suggest the description of Trachymolgus recki was based on tritonymphs. Therefore, Trachymolgus recki is excluded from the key below.

Aspects of the morphology and development described for Trachymolgus jesusi (Mejia-Recamier & Palacios-Vargas 1999), suggest major deviations from what is known from other Bdellidae. Unfortunately, we were unable to obtain type specimens of this species.