Myrmecarchaea Wunderlich, 2004

Wood, Hannah M., Singh, Hukam & Grimaldi, David A., 2021, Another Laurasian connection in the Early Eocene of India: Myrmecarchaea spiders (Araneae, Archaeidae), ZooKeys 1071, pp. 49-61 : 49

Genus Myrmecarchaea Wunderlich, 2004

Remarks.

The presence of a cheliceral gland mound, peg teeth running along the inner cheliceral margin, cuticle texture with scales and/or tubercles (in this case, having both), and the lack of leg spines indicate Palpimanoidea . The following characters indicate Archaeidae : setal bases on tubercles on the sternum, the shape of the sternum (narrow throughout, not shield shaped), the elongated chelicerae, the shape of the gland mound (pointed, positioned close to fang tip), the blunt setae on the abdomen (rather than tapering), the presence of a bump on the dorsal, basal surface of the femora, and the presence of a curve in femur IV. The specimen is referred to as Myrmecarchaea based on having a slightly elongated pedicel and greatly elongated legs ( Wunderlich 2004). Specifically, elongated legs are defined here as femur I being at least four times as long as the carapace length. Another diagnostic character for the genus may be the presence of a spur on each lateral side of the pedicel, adjacent to the anterior of the abdomen (Fig. 1F ). The presence of lateral spurs is also observed in M. petiolus Wunderlich, 2004, and M. pediculus Wunderlich, 2004 (Fig. 2 ; pedicel is obscured in the single known specimen of M. antecessor Carbuccia et al., 2020). There are other palpimanoid genera with elongated legs, including both extinct (e.g., Planarchaea Wunderlich, 2015) and extant members (e.g., Eriauchenius workmani Pickard-Cambridge, 1881, although with only leg I elongated). However, these other taxa do not also have an elongated pedicel, nor a pedicel with lateral spurs.

Myrmecarchaea is comprised of three species: M. petiolus , M. pediculus , and M. antecessor . The exuvium shows similarities to M. antecessor in having a diastema between coxae III and IV (compare Fig. 1D, F with fig. 2 from Carbuccia et al. 2020). The pedicel seems slightly longer than in non- Myrmecarchaea archaeids, but not as extreme as the pedicel of M. petiolus and M. pediculus . The ratio of cephalothorax length to pedicel length can be used to compare these shape differences: M. pediculus = 1.2; M. petiolus = 1.4; M. antecessor = 2.3 (estimated from figures in Carbuccia et al. 2020); E. workmani = 4.3. This ratio should be treated with caution because measurements were taken from different views for the different species out of necessity due to inconsistencies in fossil preservation. The exuvium from Cambay amber has a ratio of 4.0, and does not present a remarkably long pedicel. The adult ratio may be closer to that of M. antecessor , but because this exuvium is from a juvenile, it cannot be determined whether this is M. antecessor or a new species.

Figures

Figure 1. Images of Myrmecarchaea sp. (BSIP 41985) from Cambay amber A exuvium, ventral; arrow pointing to dorsal of abdomen B cephalothorax, dorsal; asterisks denote the coxal openings on the right side where the legs were pulled out of the ventral portion of the exuvium; arrow pointing to some silk threads that is part of a mesh that covers the dorsum of the exuvium C abdomen, lateral; arrow pointing at spinnerets; ' ds' showing the anterior dorsal abdominal sclerite, which is folded back as part of the molting process when the spider freed its body from the exuvium; ' bl' marking the booklung cover that is attached to the anterior ventral abdominal sclerite (labeled ' vs') D anterior portion of cephalothorax, ventral; for reference, the coxae on the right side are numbered and labeled (c = coxa) E distal portion of chelicerae, posterior; dashed line outlines the cheliceral gland mound on the right chelicera; arrow points to one peg tooth F posterior portion of cephalothorax, ventral; for reference, the coxae on the left side are numbered and labeled (c = coxa); arrows show the lateral spurs on the pedicel; black line shows the diastema between coxa III and coxa IV. Scale bars: 1 mm (A, B); 0.25 mm (C-F).

Figure 2. Pedicel of different Myrmecarchaea species from Baltic amber, arrows marking lateral spurs A M. pediculus Wunderlich, 2004, pedicel, ventral, holotype specimen, No. S 3907 / 4338, from Geologisch Palaeontologisches Institute und Museum (GPIH) B M. petiolus Wunderlich, 2004, pedicel, dorsal, holotype specimen, No. S 3999 / 4337, from GPIH. Scale bars: 0.5 mm.