Redescription of Chribellphoruraallanae (Christiansen & Bellinger, 1980) (Collembola, Onychiuridae), with comments on the systematic position of the genus.

Chribellphoruraallanae (Christiansen & Bellinger, 1980), a poorly known Nearctic springtail, is redescribed and important morphological characters are illustrated (Figs 1-7). The genus is characterized by the following characters: postantennal organ with one vesicle divided into five lobes, antennal segment IV with apical vesicle closely flanked by two papillae, sense organ of the third antennal segment with four papilla, four guard setae, two straight sensory clubs and two sensory rods, very similar in shape and length, labial palp of 0-type, abdominal sternum IV divided ventrally into two subsegments, furcal remnant as a finely granulated area with three rows of setae posteriorly, tibiotarsi with clavate setae in distal whorl and anal spines present. The taxonomic status of this Chribellphorura is also discussed.


Introduction
The monotypic genus Chribellphorura was established by Weiner (1996) to accommodate Onychiurus (Archaphorura) allanae described by Christiansen and Bellinger (1980) from localities in Oregon, USA. Weiner erected the new genus in the tribe Oligaphorurini and emphased as its distinguishing characters the presence of an apical vesicle on the antennal segment IV and clavate dorso-distal setae on the tibiotarsi.
In connection with a phylogenetic analysis of the Oligaphorurini (Paśnik and Weiner 2017) and in order to clarify the status of Chribellphorura, its type species has been reexamined. As the original description of C. allanae is incomplete and we have found differences between the original description and the specimen examined, the current paper presents a redescription of the C. allanae, illustrations of its most important taxonomic features, and notes on its taxonomic position.

Abbreviations
Antennae thin, shorter than head, not club-like, their base not marked. Ant. I with 10 setae, Ant. II with 16 setae. AIIIO consisting of four papillae, four guard setae, two smooth, straight sensory clubs and two smooth sensory rods in the middle, subequal in length to sensory clubs (Fig. 4), ventro-lateral s-microsetae (ms) present (Fig. 2). Ant. IV with small subapical organite and apical vesicle with two closely flanked papillae (giving the impression of a trilobed vesicle) (Fig. 3) and s-microsetae (ms) located in the middle of the segment (Fig. 2).
Abd. sternum IV elongated, divided ventrally into two subsegments, below this division with four setae in the middle of segment (Fig. 6). Furcal vestige reduced to field of fine granulation. Chaetotaxy of manubrial field with four setae in ma-row, four setae in mm-row and five setae in mp-row (Fig. 6). Anal valves with numerous acuminate setae, each lateral valve with a0 and 2a1; upper valves with setae a0, 2b1, 2b2, c0, 2c1, 2c2.

Discussion
Christiansen and Bellinger's description of the Chribellphorura allanae is very abbreviated and does not contain features of taxonomic value introduced for this group in later years. In addition, several of the characters in the original description do not match those of the specimen we examined. These differences are shown in Table 1.
Recent phylogenetic studies of Oligaphorurini (Paśnik and Weiner 2017) recover Chribellphorura as a monophyletic group. The genus was placed in the basal position on the tree as a sister group to the remaining taxa. This specialised genus has several features which are absent or rare in the subfamily Onychiurinae: antennal segment IV with apical vesicle closely flanked by two papillae; sense organ of the third antennal segment with sensory elements consisting of two sensory clubs and two sensory rods, which are straight and smooth, and very similar in shape and length; labial palp of 0-type and tibiotarsi with clavate setae in distal whorl.
The genus Chribellphorura shares only one feature with the other members of Oligaphorurini, namely the shape of the postantennal organ. However, the use of postantennal organ (PAO) as the main feature to divide Onychiurinae into tribes has already been criticized as a feature that often divides related genera into separate evolutionary lines (e.g. Pomorski 1998). In addition, the definition of tribes based on a single character is insufficient.
Traditionally, the tribes of Onychiurinae have previously been defined by character combinations, which mainly include: the shape of the postantennal organ, the build of the sense organ of the third antennal segment, the presence/absence of pseudocelli and anal spines, the reduction of furca, the chaetotaxy of tibiotarsi, and the distribution of sensory setae on the body. Unfortunately, most of these characters are variable, scattered among the genera, and no longer distinguish the different tribes even when used in combination with other features.
At present, the systematic position of Chribellphorura within Oligaphorurini is uncertain and should be studied in greater detail. Table 1. Differences between the original description of Chribellphorura allanae and the studied paratype.

Character
Original description Christiansen and Bellinger (1980) Paratype studied