CLADOENDESIS OF EPHEMEROPTERA

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Autapomorphies of Posteritorna. The character (1) is present both in the extinct taxon Sharephemera/fg1 and in recent Baetisca and Prosopistoma; other characters listed below are known for recent Baetisca and Prosopistoma, but unknown for Sharephemera/fg1.  

(1) Tornus of fore wing is situated behind apex of CuP, usually behind apex of AA (Kluge 2004: Fig.16:B, 17:B) [for discussion – see Anteritorna (1) below]. In extinct Sharephemera/fg1, CuA retains furcation in form of triad; in Baetisca and prosopistoma veins CuA, CuP and AA [arising independently close to wing base – see (9)] are non-branched. 

(2) Nerve ganglia of mesothorax, metathorax and all abdominal segments are fused together, forming an integral synganglion in basisternum of mesothorax (Kluge 2004: Fig.16:H, 17:E); at the same time, imaginal and subimaginal furcasternal protuberances and bases of subalar-sternal muscles (located inside them) are contiguous medially (Kluge 2004: Fig.16:E, 17:E).

Unlike Posteritorna, in all Anteritorna at lest thoracic nerve ganglia are non-fused. In that Anteritorna, which metathoracic ganglion is more or less approximated with mesothoracic ganglion, it is shifted into furcasternum of mesothorax and separates bases of subalar-sternal muscles; thus furcasternal protuberances appear to be separated, with a median impression between them (Kluge 2004: Fig.57:A-B) (see Index of characters [2.2.23]). As in Posteritorna furcasternal protuberances retain their initial contiguous position, it seems that translocation of the metathoracic and abdominal ganglia anteriorly took place here not through that stages, which occur in certain Anteritorna, but as a saltation, i.e. in such a way, that ganglia initially situated behind mesothoracic furcasternum, appeared before it, never being inside mesothoracic furcasternum.

(3) Larva has unique structure (Kluge 2004: Fig.15:B-C): pronotum, mesonotum and fore protoptera are completely fused together and strongly enlarged, constituting an integral notal shield; this shield covers abdominal terga I–VI, forming a gill chamber, inside which all tergalii I–VI [see (4)] are located. In all larval instars notal shield has no any external or internal traces of division to pronotum and mesonotum, and no outline of margins of fore protoptera (Pescador & Peters 1974: Fig.10,11,14), thus when  subimaginal wing develops, it is crumpled in free space inside notal shield (unlike other mayflies, where protoptera are distinctly outlined even if they are fused with notum). Lateral margins of notal shield are bent to ventral side, being pressed to sterna and forming epipleura (this term is taken from coleopterology, where epipleura are lateral margins of elytra bent to ventral side; epipleura of Posteritorna are partly homoplastic with the epipleura of Coleoptera, being partly formed by fore protoptera). Thoracic sterna have a pair of longitudinal ridges. At least abdominal sternum I is not expressed, abdominal segments up to VI inclusively are connected nearly immobile (in Baetisca/f2=g1) or immobile (in Prosopis-to-ma/f1=g2). When adult fore wings develope inside lateral-posterior parts of notal shield, a membranous process can develope inside notal shield between them; thus adult can have an unpaired plumidium – membranous process on apex of scutellum (Kluge 2004: Fig.16:F-G); this plumidium is present in Prosopistoma/f1=g2, but not in Baetisca/f2=g1 (similar, but paired plumidia exist in some other mayflies with fused protoptera – see Index of characters and [2.2.13]). 

(4) Tergalii [hidden in gill chamber – see (3)] strongly differ on different segments, have unique and extremely specialized structure, nearly identical in Baetisca/f2=g1 (Pescador & Peters 1974: Fig.16,20) and Prosopistoma/f1=g2 (Kluge 2004: Fig.15:D-I). Tergalius I is the longest, with straight costal margin and dissected anal margin (as tergalii of Posteritorna are always directed posteriorly, the costal margin is directed laterally and the anal margin – medially). Tergalius II is modified as gill operculum, covering tergalii III–V: it is enlarged, nearly square, convex. Tergalii III-V have dissected margins. Tergalius VI is widened distally, rounded, with integral margin (in some species of Prosopistoma/f1=g2 lost). Tergalii VII are lost.

Tergalial structure of Posteritorna has some common features with Caenotergaliae and Leptohyphes/fg1: tergalii II are transformed to gill opercula, tergalii III–V are dissected, tergalii VII are lost. In connection with this, hypotheses about relationship of these groups were suggested. But in other respects tergalial structure of Caenotergaliae and Leptohyphes/fg1 differs from that of Posteritorna: their tergalii I are vestigial or lost, tergalii III–VI have different shape. Each of the named here common characters in tergalial structure of Posteritorna, Caenotergaliae and Leptohyphes/fg1 occurs also in some other groups of Ephemeroptera, where it evolved independently.

As in Posteritorna all tergalii are hidden under the notal shield and thus are protected better than in any other Ephemeroptera, the transformation of tergalii II to gill opercula is not justified functionally. This leads to an assumption, that ancestors of recent Posteritorna got specialization of tergalii and formation of gill opercula before their notal shield had appeared.

(5) Larval abdominal segment VI is enlarged, middle of posterior margin of its tergum is elevated, tergalial bases are shifted anteriorly (Kluge 2004: Fig.15:B). Such specialization of segment VI can be functionally connected with gill opercula [see (4)] or with notal shield [see (3)], because in both these cases tergum VI closes gill clamber posteriorly. Imago partly retains this larval character: abdominal segment VI is longer than others, in imaginal Baetisca/f3=g2 tergum VI retains convex form. Youngest larvae, which have no tergalii, have segment IV non-enlarged (Pescador & Peters 1974: Fig.10). 

(6) Imaginal and subimaginal scutellum is elongate (Kluge 2004: Fig.16:F-H, 17:F) [probably in connection with presence of notal shield in larva - see (3)].

Characters of Posteritorna of unclear phylogenetic status.

(7) Some common characters are present in mouthpart structure (while mouth apparatus of Baetisca/f2=g1 is not specialized, and that of Prosopistoma/f1=g2 is highly specialized). On maxilla apical-ventral row of pectinate setae is absent (Kluge 2004: Fig.15:A, 17:G) (the same in some other mayflies – see Index of characters [1.1.31]). Submentum has lateral lobes produced anteriorly (Eaton 1883-1888: Pl.43:7, Pl.52:9) (about particular structure of these lobes in Prosopistoma, see Prosopistoma/f1=g2 (1) below). Paraglossae are fused with mentum (the same in Ephemerella/fg1). Probably, all these are autapomorphies. 

(8) Anterior paracoxal suture is transferred to anterior part of episternum, so anepisternum is much smaller than katepisternum (Kluge 2004: Fig.16:E,G; 17:E-F). Non-unique character (see Index of characters [2.2.19]).

(9) While wing venation of Baetisca/f2=g1 and Prosopistoma/f1=g2 is quite different, their vein bases on fore wing are similar (Kluge 2004: Fig.16:A, 17:A): common stem of MA+RS is reduced, so RS, MA, MP₁ and CuA arise from the same point; base of MP₂ lost its connection with MP₁ and is transferred proximally; CuA and MP₁ diverge more strongly than MP₁ and MA; base of CuP lost connection with CuA (but at the same time, unlike Tetramerotarsata and Leptophlebia/fg1, its base is nearer to CuA than to AA). Other Ephemeroptera have different combinations of these characters.

(10) On hind wing MA lacks furcation; this is clearly visible in Baetisca/f2=g1, which has primitive wing venation (Kluge 2004: Fig.17:C), but is unclear in Prosopistoma/f1=g2, where nearly all veins of hind wing are transformed to intercalaries and can not be homologized (Kluge 2004: Fig.16:C). Non-unique apomorphy (see Index of characters [2.2.67]).

(11) Patella-tibial suture (initially present on middle and hind legs) is lost on all legs of larva, but its vestige can be retained on middle and hind legs of subimago and imago. Non-unique apomorphy (see Index of characters [1.2.18]).

(12) Imaginal and subimaginal gonostylus has 1 distal segment only (instead of two primary segments). Non-unique apomorphy (see Index of characters [2.3.12]).

(13) Larval cerci have secondary swimming setae on lateral sides; these setae form less regular row, than primary swimming setae on median sides of cerci and lateral sides of paracercus. Non-unique apomorphy (see Index of characters [1.3.67]).

Plesiomorphies of Posteritorna. 

In larva: On maxilla number of dentisetae can be indeterminate and more than 3 (Kluge 2004: Fig.17:G) (unlike Anteritorna). Maxillary palp is 3-segmented. Labial palp is 3-segmented. Claws have no denticles. Caudalii have dense primary swimming setae [which differ from secondary swimming setae – see (13)]; distal margins of segments of cerci can be oblique (in Prosopistoma/f1=g2).

In imago and subimago: Anterior paracoxal suture [see (8)] is complete, i.e. completely crosses ventral side of episternum (Kluge 2004: Fig.16:E, 17:E) (unlike Furcatergaliae and some others – see Index of characters [2.2.19]). Hind wing is well-developed, as long as 0.3–0.4 of fore wing length (Kluge 2004: Fig.16:C, 17:C) (in spite of the fact, that metathorax and its musculature are reduced in greater degree than in majority of mayflies, and in greater degree than in many mayflies lacking hind wings – Kluge 2004: Fig.16:G-H, 17:F).

Systematic position of Posteritorna. A relationship between Posteritorna, Caenotergaliae and Ephemerella/fg1 was assumed; in connection with this different authors established following taxa: Pannota McCafferty & Edmunds 1979 (uniting Posteritorna, Caenotergaliae and Ephemerella/fg1); Furcatergalia Kluge 1989 (uniting Posteritorna and Furcatergaliae); Rectracheata McCafferty 1991 (uniting Posteritorna, Furcatergaliae and Vetulata). This assumption is not grounded, and the taxa Pannota, Furcatergalia and Rectracheata are actually polyphyletic (Kluge 1998).