CLADOENDESIS OF EPHEMEROPTERA

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Characters of Tenuibaetis of unclear phylogenetic status. 

(1) Labial palp: 2nd segment is not significantly projected medially, 3rd segment is nearly symmetric, pointed, with both margins slightly convex (Kluge & al. 2023: Figs 23, 46, 82, 158, 194, 252, 281). The same in the group lutheri of Baetis; similar in Baetiella.

(2) Larval legs are differentiated: Femur of fore leg is wider than others, widest in proximal part; if cuticle of femur is pigmented, fore femur has large proximal blank lacking stout setae, while middle and hind femora have no such blank (Kluge & al. 2023: Figs 25–27, 47–49, 70–75, 129–131, 160–162, 188–190, 232–234, 253–255). Among Baetis, the same in the group lutheri; among other mayflies, similar shape of femora and the proximal blank of fore femur occur in certain Leptophlebiidae (Kluge 2020: fig. 9; Kluge, Srinivasan et al. 2022: figs 64–66; Kluge, Vasanth et al. 2022: figs 27–29).

(3) Stout setae on larval femora (either two-channel, or pseudo-bifurcate) are located not only near outer and inner sides, but also on middle area of anterior (dorsal) side (Kluge & al. 2023: Figs 31, 47–49, 83–88, 129–132, 160–162, 202, 232–234, 253–255, 279). The same in many other mayflies, including certain species of Baetis groups vernus, buceratus, subgenus Rhodobaetis.

(4) Scales on larval abdominal terga: Wide, in semicircular sockets (Kluge & al. 2023: Figs 199–200). The same in many other Baetis, but not in the group lutheri.

(5) Tergalii: All seven pairs present, oval; tergalius I often much smaller than others (Kluge & al. 2023: Figs 73, 184, 239). The same in most other Baetis s. l. In Baetis (Tenuibaetis) sp. from Thailand, the first tergalius is relatively large (Kluge & al. 2023: Fig. 284).

(6) Larval pose. When swimming, larva does not stretch its legs along the body, but after short swimming passively falls down with its legs bent down and abdomen often bent on back (Kluge & al. 2023: Fig. 128); larva of B. (T.) hissaricus bents its legs, but abdomen retains straight (Kluge 1983, Novikova 1991). The same in Acentrella, Baetiella and some nonrelated mayflies (e.g. Ephemerellidae), in contrast to majority of Baetis, which retain ability to swim stretching legs backward along the body.

(7) Larval cuticular coloration: Varies from nearly uniformly brown (Kluge & al. 20.23: Fig. 184) or diffusively maculated, to brown with contrasting V-shaped or transverse blank on mesonotum (Kluge & al. 2023: Fig. 28) and contrasting blanks on abdomen; in this case abdominal tergum IV is lighter then neighbouring ones (Kluge & al. 2023: Fig. 24). The species name of the first described species, Baetis ursinus, was given as an allusion to the similarity of its dorsal V-shaped blank to the ventral white marking of the Asian bear (Ursus thibetanus Cuv.)

(8) Gonostyli. Male imaginal gonostylus with terminal (3rd) segment short (Kluge & al. 2023: Figs 42–43, 54–55, 98, 101, 112–114, 144–145, 181–183, 225, 227, 276–277).

(9) Sterno-styligeral muscle: Varies from vestigial with remnants of cross striation (Kluge & al. 2023: Figs 228–229) to completely absent (Kluge & al. 2023: Fig. 145).

(10) Hind wing. Hind wing, if present, with two longitudinal veins and short costal projection (Kluge & al. 2023: Fig. 13). The same in many other Baetis, in contrast to representatives having three veins or lacking costal projection.

Variable characters of Tenuibaetis. Hind wings present (see.. ) or absent (see Index of characters [2.2.59]). Most species of Tenuibaetis have hind wings as large as in most other species of Baetis s.l.; in this case larval protoptera are also well-developed (Kluge & al. 2023: Figs 13–14). In female, hind wings are often somewhat smaller or at least narrower than in male (Kluge & al. 2023: Figs 34–35, 56–57, 109–111, 138–139, 178–179). In parvipterus [Tenuibaetis] hind wings of female are greatly diminished (Fujitani et al. 2011: fig. 5b). In octomaculatus [Baetis (Tenuibaetis)] hind wings of both sexes are diminished (Kluge & al. 2023: Fig. 15), but larval protoptera are nearly as large as in the species with full-sized hind wings (Kluge & al. 2023: Figs 17, 19); in female, imaginal hind wing is as small as its larval protopteron, so that it develops under larval cuticle without crumpling (Kluge & al. 2023: Figs 18–19); in contrast to it, male hind wing is larger than protopteron and is crumpled before molt (Kluge & al. 2023: Figs 16–17), as in most other mayflies and most other insects. Thus, in contrast to most other mayflies, in Tenuibaetis size of hind protopteron of larva does not allow to make conclusion about size of its imaginal hind wing. In panhai [Tenuibaetis] and bialatus [Baetis (Tenuibaetis)], hind wings are completely absent in both sexes (Kluge & al. 2023: Fig. 20); larvae of these species have no any vestiges of hind protoptera (Kluge & al. 2023: Fig. 21).