Allomengea

Silva-Moreira, Thiago Da, Kulkarni, Siddharth & Hormiga, Gustavo, 2025, A total evidence phylogenetic analysis of the spider family Linyphiidae (Araneae, Araneoidea), Zootaxa 5685 (1), pp. 1-78 : 56

Position of Allomengea , Agyneta , and Microneta clades and Linyphiid tracheal system evolution

Allomengea Strand is a small Holarctic genus with six species. In his monographic work, van Helsdingen (1974) suggested that Allomengea would be closely related to Wubana based on the many similarities of their genitalia, but this hypothesis had not been tested in a phylogenetic context. Allomengea sp. is the sister group of a clade formed by the Agyneta clade + Microneta clade + Erigoninae , which has full support across our analyses.

The Agyneta and Microneta clades ( Fig. 8 ) are both parts of “Clade B” in Arnedo et al. (2009) and in Wang et al. (2015). In our hypothesis, the Agyneta clade is composed of Agyneta Hull , Parameioneta Locket , Nippononeta Eskov , and Helophora . The Microneta clade includes, besides Microneta Menge , at least the genera Asthenargus Simon and Fage , Parasisis Eskov , and Ryojius Saito and Ono. The Agyneta clade is recovered in all our analyses, most times with full support, while the Microneta clade is much more unstable (see discussion on Erigoninae ) and always recovered with very little nodal support. The Agyneta clade is supported by 11 synapomorphies, including a paracymbium with a complex tri-dimensional mesal section with pockets (char 51), suprategulum with distal suprategular apophysis extending ectally (char 79), and a marginal apophysis (char 80), epigynum with a sigmoid ventral plate scape (char 137) and anteriorly oriented fertilization ducts (char 158).

Both the anatomy and evolution of the tracheal system of linyphiids have received considerable attention (e.g., Lamy 1902; Blest 1976; Millidge 1984, 1986; Hormiga 1994b, 2000; Miller & Hormiga 2004). Blest (1976) and subsequently Millidge (1984) both recognized two basic tracheal conformations:the haplotracheate system, where the medial tracheal trunks are unbranched and restricted to the opisthosoma ( Fig. 15B ), and the desmitracheate system, where the medial trunks are thicker than the lateral, and branch into tracheoles that extend through the pedicel into the prosoma and the appendages ( Fig. 15D ). The desmitracheate condition is found mainly in Erigoninae and some “micronetines.” Blest (1976) and Millidge (1984) reported that Allomengea has a unique tracheal conformation within linyphiids, whereas the medial tracheal tubes are branched but, unlike classic desmitracheate condition, the medial trunks have less tracheoles and those are of the same thickness as the ectal branches and confined to the abdomen ( Fig. 15C ). The tracheal system of Helophora insignis (Blackwall) is very similar in its branching pattern to that of Allomengea ( Arnedo et al. 2009) , while Agyneta rurestris (Koch) and Agyneta nigra (Oi) have the same branching pattern observed in erigonines. Arnedo et al. (2009) hypothesized two independent origins of the desmitracheate conditions: one in a clade comprising Helophora and Agyneta (equivalent to our Agyneta clade) and a second in the “Distal Erigoninae ” ( Fig. 15A ). Considering our results and assuming that the placement of Labullinyphia is incorrect (as discussed earlier), it is possible that the desmitracheate condition might have a single origin. Allomengea marks a “transitional point” on the tree where we can observe the appearance of branching medial tracheal trunks in linyphiids. This condition would be shared by both the Agyneta clade and the “Distal erigonines” with two reversals on the Microneta clade and the Neomaso clade ( Fig. 14E ). This hypothesis, however, does require corroboration with more anatomical data on the tracheal system of other representatives of those lineages.

Figures

FIGURE 8. Partial tree from the preferred hypothesis for linyphioids (M4_MB). Clade B (part). Solenysa genus, Allomengea sp., Agyneta clade, and Microneta clade. Black-colored squares represent nodes with posterior probability =1; gray-colored squares represent nodes with posterior probability between 0.95 and 0.99.

FIGURE 14. Fickert’s gland evolution hypothesis A, Ancestral state reconstruction (parsimony, unordered) of character 103 over the pruned preferred hypothesis tree. B and C Schematic illustration of the two different morphologies recognized for Fickert’s gland in Linyphiidae: B. Megalepthyphantes nebulosus (Sundevall), embolic division (ventrolateral view). C: Agyneta subtilis (O. Pickard-Cambridge), embolic divison (ventrolateral view). Blue color indicates the sperm duct with Fickert’s gland (fg) within the embolus (e) area. B and C modified after Merret (1963).

FIGURE 15. Linyphiid, tracheal morphology evolution hypothesis. A, Tracheal morphology recorded over species on the preferred hypothesis tree. Letters and color frames indicate the three different types and their location on the tree: B (black), haplotracheate condition; C (gold), “intermediate” condition; C (brown), desmitracheate condition. Taxa with unknown conditions are colored gray. E: Ancestral state reconstruction (parsimony) over the pruned preferred hypothesis topology (char 233. Length = 5; ci = 0.2; ri = 0.5). Tracheae schematics in B–D after Blest (1979).