Mysmenidae, Petrunkevitch, 1928

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 : 241-245

Mysmenidae

As currently circumscribed, Mysmenidae comprises 158 described species in 14 genera (World Spider Catalog, WSC, 2020, see Lopardo & Hormiga, 2015 for the removal of the genus Phricotelus from the family and its placement as Araneoidea incertae sedis). The respiratory system has been studied and described (or only reported) by a small number of arachnologists but for a relatively large number of species, encompassing six genera: Mysmena ( Marples, 1955; Levi, 1956, 1967; Forster, 1959; Lopardo & Michalik, 2013; Lopardo & Hormiga, 2015), Microdipoena ( Levi, 1956, 1967; Forster, 1959; Lopardo & Hormiga, 2015), Mysmenopsis ( Forster, 1959; Lopardo & Hormiga, 2015), Maymena ( Gertsch, 1960; Lopardo & Hormiga, 2015), Isela ( Griswold, 1985; Lopardo & Hormiga, 2015), and Trogloneta ( T. cantareira ; Brescovit & Lopardo, 2008; Lopardo & Hormiga, 2015). There is a large diversity of respiratory arrangements within Mysmenidae (see Table 1 and supplementary material). Contrary to other symphytognathoid families (see below), and except for a few species, a different and peculiar arrangement can consistently be found on each particular genus or subfamily, which are described below. The ancestral mysmenid respiratory system based on the total evidence phylogenetic hypothesis of Lopardo et al. (2011) is as follows: anterior tracheal system extending into the prosoma and connected by a transverse duct; posterior tracheal system comprising a single narrow spiracle adjacent to the spinnerets and internally consisting of a small atrium, median entapophyses and lateral tracheae restricted to the opisthosoma, and arranged separately, most likely resulting in four trunks usually arising independently from the posterior spiracle, via a small atrium (Figs. 5 and 6). No extant mysmenid species, however, is known to have this hypothetical ancestral tracheal system. The mysmenid ancestral reconstruction based on the phylogenetic hypothesis of Kulkarni et al. (2021) renders a similar anterior respiratory arrangement; but due to the placement of Synaphridae as sister to Mysmenidae , the reconstruction of the posterior respiratory system is rendered ambiguous (median tracheae as third entapophyses, but either a simple or a complex lateral tracheal system) (not shown, refer to Figs. 5 and 7). The respiratory system for Brasilionata , Chanea , Gaoligonga , Mosu , Mysmeniola , Phricotelus , Simaoa , and Yamaneta remains unknown.

Maymena The anterior respiratory system of Maymena has been studied in Maymena mayana by Gertsch (1960), and its respiratory arrangement (“Book lungs present and tracheal tubes arising from spiracle located immediately in front of spinnerets”; Gertsch, 1960:31) was assumed to be similar in the additional Maymena species described in that study (i.e., M. chica , M. misteca , M. calcarata , and M. ambita ). The respiratory system of Maymena was also briefly reported in Lopardo and Hormiga (2015:778). Anteriorly, M. mayana and M. ambita possess book lungs consisting of about 20 leaves and connected by a rigid transverse duct, in M. rica the book lungs are reduced ( Fig. 1a, c, d ). In addition, one tracheal tube arises from the reduced book lungs in M. rica ( Fig. 1d ; see discussion below). Posteriorly, all Maymena species studied have two long single lateral tracheal tubes (seemingly confined to the opisthosoma) and two short (seemingly) median entapophyses arising from a small atrium connected to a narrow posterior single spiracle ( Fig. 1b ), an arrangement identical to what can be regarded as the ancestral reconstruction of the mysmenid posterior respiratory system (Fig. 5). Concurrently, although characteristic of Maymena within Mysmenidae , the same respiratory arrangement of Maymena is also present in theridiosomatids as well as other members of Araneoidea (e.g., Steatoda ), suggesting that this arrangement may have evolved independently within the family.

Trogloneta The respiratory system of Trogloneta has been described in detail for Trogloneta cantareira ( Brescovit & Lopardo, 2008) and also reported in Lopardo and Hormiga (2015:779). Anteriorly, two reduced book lungs with seven leaves each, connected by a rigid intertracheal transverse duct ( Fig. 1e, g ). Posteriorly, a tracheal system with one narrow spiracular opening located adjacent to the spinnerets, small atrium connected to two unbranched single lateral tracheae confined to the abdomen and one single unbranched (i.e., fused) seemingly median entapophysis. This distinct respiratory arrangement is also found in T. granulum ( Fig. 1e, f ), and it may be widespread within the genus.

Mysmenopsinae Each of the genera comprising the kleptoparasitic subfamily Mysmenopsinae is characterized by a different respiratory arrangement. The respiratory system of Isela was first described in detail by Griswold (1985) for I. okuncana . Posteriorly, one narrow spiracle located adjacent to the spinnerets, atrium connected to four long unbranching tracheal tubes (corresponding to a pair of single median tubes and a pair of single lateral tubes) confined to the abdomen. Anteriorly, the respiratory arrangement was described as “…atria giving rise to 13–14 short trachea-like lamellae…” ( Griswold, 1985:208) also restricted to the abdomen. The respiratory system of another Isela representative was examined ( Isela sp. – “ Kilifina- MYSM-002- KENYA ”; Lopardo & Hormiga, 2015:780). Posteriorly, it is identical to the respiratory system described for I. okuncana ( Fig. 2a, c ); anteriorly, it has reduced book lungs of about 14–15 leaves, but it also has a cluster of about six tracheae arising dorsally from the most dorsal leaf of the lungs ( Fig. 2a, b ; see discussion below). This could have been the confusing mixed condition observed in I. okuncana under light microscopy. The anterior system of Isela sp. is connected by a rigid intertracheal transverse duct, as it is presumably also in I. okuncana .

The respiratory system of Mysmenopsis was first reported for M. palpalis by Forster (1959), who discussed the details, diversity, and evolution of the respiratory system in many genera representing different families within symphytognathoids. Remarkably, the anterior system of this species consists of tracheae (not book lungs). The anterior tracheae extend to both prosoma and opisthosoma. Posteriorly, one wide spiracle (reported by Forster as two close spiracles) located adjacent to the spinnerets leads to two long tracheal tubes (most likely the lateral pair). The respiratory system of Mysmenopsis was described in Lopardo and Hormiga (2015:781, based on M. dipluramigo and M. penai ): anterior tracheal system composed of several tracheoles extending to the abdomen as well as the prosoma, connected by a membranous intertracheal transverse duct ( Fig. 2d ). In addition to sharing the same anterior tracheal arrangement, all Mysmenopsis species have a wide posterior spiracle adjacent to the spinnerets. However, the posterior tracheal system of the two species examined in Lopardo and Hormiga (2015) differs from the one described by Forster (1959) in that it consists of two bundles of lateral branching tracheae arising from the atrium at each end of the posterior spiracle, seemingly restricted to the abdomen ( Fig. 2d, f ). The posterior median structures are presumed absent, or are alternatively functional tracheae intercalated with and identical to the lateral tracheal bundles.

Mysmeninae Each of the four characteristic respiratory arrangements mentioned above are typical of four different but closely related genera ( Maymena , Trogloneta , Isela , and Mysmenopsis ). The subfamily Mysmeninae , comprising the remaining investigated mysmenid genera (i.e., Microdipoena and Mysmena , and the several undescribed taxa examined herein and in Lopardo and Hormiga (2015); see Table 1 and supplementary material), have a remarkably different, although quite conserved, fifth type of respiratory arrangement, which is typical and unique for this subfamily (but see below). This respiratory system has been previously described ( Marples, 1955; Levi, 1956, 1967; Forster, 1959), although these descriptions were understandably superficial in the absence of SEM examination (but see a detailed description of the respiratory system of Mysmena leichhardti in Lopardo and Michalik (2013) and of Mysmeninae in Lopardo and Hormiga (2015:783)). Anteriorly, several tracheal tubes seemingly restricted to the abdomen arise on each atrium, which are connected by a rigid intertracheal transverse duct ( Fig. 3c ). The posterior tracheal system opens through a wide and advanced tracheal spiracle located halfway between the spinnerets and the epigastric furrow. The shape of this wide spiracle is similar to that described for synaphrids (Lopardo & Hormiga, 2007; Lopardo et al., 2007; see discussion below), consisting of two distant openings connected by a thin furrow ( Fig. 3a ). The thin furrow is almost imperceptible under light microscopy, and therefore it has been described as two separate spiracles in previous mysmenid studies. Internally, mysmenines have a common M-shaped atrium, where two bundles of branching lateral tracheae arise on each side, most of them extending into the prosoma ( Fig. 3b–d ). Two-minute seemingly median entapophyses, each arising from the two most anterior locations of the M-shaped atrium, are each surrounded by a bundle of lateral tracheae ( Fig. 3e ). These apodemes are only visible by SEM and had been consequently overlooked in the original descriptions. Given that so far all examined Mysmeninae species seem to have this remarkable tracheal system (but see below), it seems reasonable to predict that this tracheal pattern is widespread in this subfamily.

The anterior respiratory system of Microdipoena guttata was described by Levi (1956; also 1967) as being an intermediate stage between tracheae and book lungs: “… structures between the seminal receptacles and the body wall which may be remains of book lungs but do not seem to be trachea” ( Levi, 1956:3). The internal respiratory system of M. guttata (examined here and in Lopardo & Hormiga, 2015), consists of a posterior system as described above (and also identical to that reported by Levi, 1956:3 “…trachea extending from the spiracles between spinnerets and genital groove, but these are bunched for a shorter distance and seem connected at their base”). Anteriorly, nevertheless, the extremely large, membranous and convoluted copulatory ducts of this species (also common within Mysmeninae ) might have been misinterpreted as the “intermediate” structures (see e.g., Fig. 3b ).

Two Mysmena species have each been reported as having a unique respiratory arrangement. Mysmena phyllicola has been described to have anterior book lungs of about five elongate lamellae ( Marples, 1955), which was considered similar to the “intermediate stage” reported for Microdipoena guttata (see above; Forster, 1959), and a posterior respiratory system consisting of one single relatively advanced tracheal spiracle (located halfway between the spinnerets and the epigastric furrow) connected to four tracheal tubes restricted to the abdomen. Mysmena vitiensis , on the other hand, has two anterior unconnected tracheae, and one single and advanced posterior spiracle also connected to four tracheal tubes restricted to the abdomen ( Forster, 1959). The holotype and only specimen of M. vitiensis was not available for study. The type material and only specimens of M. phyllicola were examined here, but no detailed observation was possible. Given the limited observations of the respiratory system of these two species and the peculiarity of their arrangements, more specimens are required to confirm the available data. Finally, we expect that the closely related genera Brasilionata and Mysmeniola as well as the three-dimensional spherical web builder genera Simaoa and Gaoligonga might also have a respiratory arrangement similar to those described here.

Figures

Fig. 1 The respiratory system of Mysmenidae — the genera Maymena and Trogloneta. Dorsal view. a M. ambita, ARS. b M. ambita, PRS with two median entapophyses and two single lateral tracheae. c M. mayana, ARS. d M. rica, ARS with reduced book lungs and tracheal tubes. e T. cantareira, overview of ARS and PRS. f T. cantareira, PRS with a single median entapophysis and two single lateral tracheae. g T. granulum, ARS with reduced book lungs. Abbreviations: BL, book lung; mE, median entapo- physis; plT, posterior lateral tra- chea; rBL, reduced book lung; Sp, spermatheca; TD, transverse duct; Tt, tracheal tube

Fig.2 The respiratory system of Mysmenidae — the genera Isela and Mysmenopsis. Dorsal view. a Isela sp., overview of ARS and PRS. b Isela sp., detail of left reduced book lungs show- ing the tracheal tubes arising from the most lateral and dorsal leaf. c Isela sp., detail of PRS showing the lack of median entapophyses. d–f Mysmenopsis dipluramigo. d Overview. e Detail of left anterior tracheae showing a book lung leaf arising ventrally. f Detail of PRS consisting of numerous tracheae. Abbreviations: BLl, book lung leaf; plT, posterior lateral trachea; pmT, posterior median trachea; rBL, reduced book lung; Sp, spermatheca; Tt, tracheal tube

Fig.3 The respiratory system of Mysmenidae — the subfamily Mysmeninae. a Microdipoena guttata, ventral view of opistho- soma showing the advanced and wide posterior spiracle with its two distinct openings (arrows). b Microdipoena gut- tata, dorsal view of ARS and PRS, which consist of numer- ous tracheae and especially the posterior tracheal tubes extending into the pedicel. c Mysmena-MYSM-015-MAD, dorsolateral view of ARS and PRS. d Mysmena-MYSM- 028-MAD, anterior view of the opisthosoma showing tracheal tubes extending into the pedicel. e Mysmena-MYSM-010-MEX, detail of PRS in dorsal view showing the small median entapophyses. Abbreviations: aTt, anterior tracheal tubes; GO, genital opening; mE, median entapophysis; pTt, posterior tracheal tubes; Sp, spermatheca; Spi, spiracle; TD, transverse duct; Tt, tracheal tube