Synaphridae, Wunderlich, 1986

Lopardo, Lara, Michalik, Peter & Hormiga, Gustavo, 2022, Take a deep breath … The evolution of the respiratory system of symphytognathoid spiders (Araneae, Araneoidea), Organisms Diversity & Evolution (New York, N. Y.) 22 (1), pp. 231-263 : 247-249

Synaphridae

Although its first species was described in 1881 ( Simon, 1881:132), this relatively obscure and small family has lately benefited with very detailed morphological studies (Lopardo & Hormiga, 2007; Lopardo et al., 2007; Miller, 2007; Marusik & Zonstein, 2011). Synaphridae comprise to date only 13 described species in three genera, two of them ( Africepheia and Cepheia ) monotypic. However, the respiratory system has been described and depicted in detail for four species representing all three genera, and seems uniform within the family: Africepheia madagascariensis ( Miller, 2007: fig. 8), Cepheia longiseta (Lopardo & Hormiga, 2007: figs. 30–31, 58–63; see also Lopardo & Hormiga, 2015: fig. 107F), Synaphris saphrynis (Lopardo et al., 2007: figs. 30, 33–39; see also Lopardo & Hormiga, 2015: fig. 110F), and Synaphris schlingeri ( Miller, 2007: fig. 61). The respiratory system of synaphrids consists exclusively of tracheae. The anterior spiracles are connected to the epigastric furrow. Interiorly, the anterior tracheae are connected by a rigid transverse duct, with five tracheal tubes arising from each anterior spiracle, four oriented anteriorly toward the cephalothorax ( Fig. 4d ), one oriented laterally. The posterior tracheal system consists externally of two distant spiracular openings exteriorly connected by a thin ridge (i.e., one wide spiracular opening). The thin ridge leads to a deep, flat, membranous atrium, anteriorly ending in a rigid U-shaped canal that connects to the tracheal ducts. Two main tracheal bundles arise from the junction of the tracheal ducts and the U-shaped atrial canal, one on each side, directing tracheoles mainly anteriorly. Both tracheal systems seem to extend into the prosoma (Fig. 5).

◂ Fig. 5 Schematic reconstructions depicting the diversity of respiratory arrangements of symphytognathoids, sorted by family. See bottom of figure for the sketches used to represent the different respiratory structures

The bigger picture: the evolution of the respiratory system in symphytognathoids

General remarks

Given the diversity of symphytognathoid anterior respiratory structures, some workers have hypothesized a trend of the “primitive” well-developed book lungs towards a reduction in leaf number and later its complete replacement by anterior tracheae (e.g., Forster, 1959, 1980; Gertsch, 1960; Levi, 1967; Levi & Kirber, 1976). A comparable “simplification” event has also been suggested for the evolution of the posterior respiratory system of Araneomorphae (see Huckstorf et al., 2015 and Ramírez et al., 2021; and references therein).

Forster (1959:272) hypothesized that two separate posterior spiracles (i.e., the wide spiracle of Mysmeninae , Anapistula , and Synaphridae ) leading into tracheae located midway between the spinnerets and the epigastric furrow was the ancestral condition of his Symphytognathidae s. l. ( Anapidae + Mysmenidae + Symphytognathidae ). To explain the diversity of the respiratory system in symphytogna - thoids, Forster (1959) proposed a pattern of changes related to the position and number of posterior tracheal spiracles, which involved the fusion of the two advanced spiracles into a single spiracle adjacent to the base of the spinnerets, to ultimately being lost (i.e., from complex to simple to absent).

We inferred the evolutionary transformations of the respiratory system of Mysmenidae and other symphytognathoid families based on the optimization of such characters onto the preferred optimal trees from Lopardo et al. (2011) and Kulkarni et al. (2021). Our optimizations offer different evolutionary hypotheses for both the anterior and posterior respiratory systems compared to the aforementioned hypotheses for symphytognathoids (Figs. 6 and 7). The reconstruction of the ancestral symphytognathoid respiratory system based on the total evidence analysis (Figs. 6 and 7) is as follows: anteriorly, fully developed book lungs connected by a transverse duct; posteriorly, tracheal system arising from one single narrow spiracle located at the spinnerets and internally consisting of two median apodemes and two lateral simple tracheae restricted to the opisthosoma, arranged separately in four tubes arising independently from the spiracle. This plesiomorphic respiratory arrangement is found in one species of the basal Theridiosomatidae ( Theridiosoma gemmosum ) and optimizes as homologous to that of the theridiid representative in this dataset, which is placed as either sister taxa of symphytognathoids (Fig. 6) or sister to a clade that includes all other araneoids (Fig. 7).

The plesiomorphic symphytognathoid arrangement seems to have re-evolved independently in the mysmenid Maymena . Furthermore, the reconstruction of the ancestral respiratory system of the ANTS (Anterior Tracheal System) clade on both the phylogenetic hypotheses of Lopardo et al. (2011) and Kulkarni et al. (2021) results in a similar arrangement to that reported as the mysmenid ancestral reconstruction (i.e., anterior tracheae extending into prosoma and connected by a transverse duct; posterior tracheae comprising a single narrow spiracle adjacent to spinnerets and internally a small atrium, median entapophyses and lateral tracheae restricted to the opisthosoma; Figs. 5-7; see above).

Figures

Fig. 4 The respiratory system of symphytognathoid spiders. Dor- sal view. a Tasmanapis strahan (Anapidae), ARS with uncon- nected reduced book lungs and tracheal tubes arising from the leaves. b Patu-SYMP-001-DR (Symphytognathidae), overview of ARS and PRS, which con- sists of numerous tracheae. Note the single median entapophysis. c Coddingtonia euryopoides (Theridiosomatidae), overview of ARS and PRS. d Cepheia longiseta (Synaphridae), ARS with few tracheae and promi- nent transverse duct. Abbre- viations: aTt, anterior tracheal tubes; mE, median entapophy- sis; plT, posterior lateral tra- chea; rBL, reduced book lung; Sp, spermatheca; TD, transverse duct; Tt, tracheal tube