Linyphiidae Blackwall, 1859 is a species-rich family of spiders which has species-specific genitalia but more conservative somatic features in general. The tracheate system in linyphiids consists of two pairs of trachea; “desmitracheate” and “haplotracheate” are the two terms referring to the two main tracheal conditions having the median pair highly branched and unbranched, respectively (Blest 1976, Millidge 1984). The seven subfamilies currently accepted in Linyphiidae are delimited largely based on genital characters, except for Erigoninae Emerton, 1882, which was originally defined by having a simple type male palp (Merrett 1963, Millidge 1977), and then redefined on the basis of its desmitracheate system (Blest 1976, Millidge 1984). Such a classification for “erigonines” has long been puzzled by the observations that some “erigonine” species have genitalia of simple type, but possess haplotracheate systems (Hormiga 2000, Miller and Hormiga 2004), while some “micronetine” species have genitalia of complex type, but possess desmitracheate systems (Millidge 1984, Dupérré 2013, Yan et al. 2015). Results of phylogenetic analyses based on molecular data recovered the monophyly of the Linyphiidae, in which the taxa of “micronetines” and “erigonines” nest (Arnedo et al. 2009, Wang et al. 2015); however, all “micronetine” species with a desmitracheate system form a monophyly (addressed as “desmitracheate micronetines” clade hereafter). Ancestral state reconstructions indicate that both tracheal features and genital characters used in subfamily classification are homoplastic. Furthermore, conversions between haplotracheate and desmitracheate conditions have taken place multiple times, and usually at the generic level. Until now, no evidence indicated that both tracheal conditions co-occur among congeneric species.

Macrargus Dahl, 1886 is a typical “micronetine” genus with a haplotracheate system (Blest 1976). However, Macrargus alpinus Li & Zhu, 1993, occurring in China, was found to have a desmitracheate system, different from its congeners. The “desmitracheate micronetines” clade resulting from phylogenetic analyses is distantly related to Macrargus (Wang et al. 2015). Some putative synapomorphies for the desmitracheate “micronetine” genus Nippononeta Eskov, 1992 and for the clade Nippononeta+Agyneta proposed by Yan et al. (2015) can also be found in M. alpinus, yet none of them is present in other Macrargus species. This implies that the generic placement of M. alpinus is questionable.

To test the phylogenetic placement of Macrargus alpinus and its relationships with other desmitracheate “micronetines”, we added the newly sequenced DNA sequence data of M. alpinus into the dataset of Wang et al. (2015). In the present study, we propose a new generic placement for M. alpinus based on the result of phylogenetic analysis of the new dataset. We present a redescription of M. alpinus and comparisons with closely related groups. Putative synapomorphies for Nippononeta and the desmitracheate “micronetine” groups proposed by Yan et al. (2015) are revised for further studies.

With the data on Macrargus alpinus and an additional Nippononeta species added to their dataset, the Maximum Likelihood analysis recovered the general topology of Wang et al. (2015): the monophyly of Linyphiidae, its sister relationship with Pimoidae, the seven main clades and relationships among them within linyphiids (all with bootstrap > 95%), with variations on the placement of some weakly supported lineages within these clades (Fig. 1). “Micronetines” remain paraphyletic, forming clades A and F, as well as some basal lineages of clade B, nesting with “erigonines”. Three desmitracheate clades are included within clade B. Besides the “distal erigonines” clade (bootstrap = 69%) and one other “desmitracheate erigonines” lineage (bootstrap = 100%), all those “micronetine” species of a desmitracheate system form a well-supported clade, the “desmitracheate micronetines” clade (bootstrap = 100%), sister to the Helophora clade (bootstrap = 91%). Macrargus rufus and Microneta viaria form one of the “haplotracheate micronetines” lineages within clade B (bootstrap = 69%). Meanwhile, Macrargus alpinus falls into the Nippononeta clade (bootstrap = 63%), sister to N. kantonis (bootstrap = 99%), but distantly related to Macrargus rufus. The monophyly of Agyneta and its relationship with the Nippononeta clade remain unresolved.

Our results show that all desmitracheate “micronetines” form a monophyly, and Macrargus alpinus falls into the Nippononeta clade, distantly related to Macrargus rufus (Fig. 1). These suggest a new generic placement for this species, Nippononeta alpina comb. n. The two “micronetine” species, N. alpina and M. rufus grouped into clade B is consistent with the putative synapomorphies based on somatic characters proposed for the seven-clade division (Wang et al. 2015). It is also in accord with the tracheal characters to group N. alpina with other desmitracheate “micronetines”, rather than with the haplotracheate genus Macrargus.

According to Wang et al. (2015) and Yan et al. (2015), some putative synapomorphies based on genital characters support the relationships among the desmitracheate “micronetines” and other linyphiids. The presence of a median plate is a synapomorphy for all linyphiids, but is secondarily lost in Helophora, Nippononeta, Agyneta and Mesasigone, as well as in N. alpina (Fig. 4B). Therefore, the absence of a median plate as a putative synapomorphy supports the sister relationship between the “desmitracheate micronetines” clade and Helophora (Fig. 1). The highly branched median pair of the tracheae, the protruding epigynum modified into a scape and epigynal cavity (Fig. 4A–D), and male palp with complex embolus (Fig. 1E) are putative synapomorphies for the former (Fig. 4G), while the moderately branched (Arnedo et al. 2009), the protruding epigynal plate without forming a scape and epigynal cavity (Tu and Hormiga 2010: fig. 7), and by the flagelliform embolus of the male palp (Chamberlin and Ivie 1933: fig. 85, Tao et al. 1995: fig. 69) are putative synapomorphies for the latter. Furthermore, the monophyly of the clade including Nippononeta and Agyneta (unknown for Mesasigone) is supported by four putative synapomorphies: by the presence of a serrated distal tibial apophysis and a retrolateral tibial process on the male palp (Fig. 3D, see also Yan et al. 2015: fig. 1D); by the dorsally opened epigynal cavity resulting from a secondary loss of the median plate (Fig. 4B, see also Yan et al. 2015: fig. 2B); and by the fertilization tracts changing from groove to duct state (Fig. 4E, see also Tu and Hormiga 2010: fig. 6c, Yan et al. 2015: fig. 5D).

The monophyly of the Nippononeta clade is supported by the following four putative synapomorphies: the pointed apophysis on the paracymbial proximal arm (Fig. 3D, Yan et al. 2015: fig. 1D); the embolus thumb modified into spine-like projections on the male palp (Fig. 3H, Yan et al. 2015: fig. 1G); and the presence of a transverse dorsal fold and finger-like stretcher (Fig. 1K, Yan et al. 2015: fig. 2A). Furthermore, the new placement of Nippononeta alpina implies that several putative synapomorphies previously proposed for Nippononeta by Yan et al. (2015) have to be reviewed: the narrowed epigynal basal part and the expanded lateral epigynal shoulder (Yan et al. 2015: fig. 2A–D) are not present in N. alpina. Meanwhile, N. alpina also has some apomorphies not shared with other Nippononeta species: the two pairs of scape lateral wings and the uplifting stretcher (Fig. 4A–D).

The monophyly of Agyneta is not supported and its relationships with other desmitracheate “micronetines” remain unresolved. Nevertheless, morphological studies show that Agyneta species are easily distinguished by some genital characters: e.g. the presence of a conical cymbial elevation and a sickle-shaped embolus with a large thumb, and the scaped epigynum with a pair of well-developed lateral lobes (see the review of Dupérré 2013). The uncertain phylogenetic relationship between Agyneta and other desmitracheate “micronetines” is largely due to a limited species-level sampling for such a diverse group. Clearly, more comprehensive sampling is needed for future studies.