CLADOENDESIS OF EPHEMEROPTERA

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(1) In imago and subimago pterothorax is extremely specialized (Kluge 2004: Fig.87): mesothorax is greatly enlarged, has only cross wing musculature and modified sclerites; metathorax is extremely reduced. Mesothorax constitutes about 1/2 of total body length: length of mesonotum is subequal to 0.4–0.5 of fore wing length (which is subequal to body length, as in other mayflies); this is larger than in all other mayflies: in Neoephemera/fg1, Tricorythodes/fg1 and Prosopistoma/f1=g2 mesonotum is subequal to 0.3–0.4 of fore wing length, in majority of other mayflies – to 0.2–0.3 of fore wing length. Some features of pterothorax structure are common with some Tricoryptera [see (8)], and the following features are unique.

Anterior transverse impression of mesonotum is completely lost (Kluge 2004: Fig.87:A).

Mesonotal suture (initially curved backward by sides of median line) is transformed to a pair of arched sutures converging anteriorly and posteriorly by acute angles and bordering a small closed medionotal membrane [non-unique apomorphy – see Fimbriatotergaliae (2)]; (sometimes this membrane forms a small projection; in vermifera [Caenis] this membrane is produces into a vermiform process, which exists in imago, but not in subimago).

Subalar-sternal muscle (SA.Sm) is completely lost (at the same time there are retained strongly diminished and widely separated furcasternal protuberances, which initially serve as places of attachment of the paired subalar-sternal muscles). In other Ephemeroptera subalar-sternal muscles are always present, while in some Leptohyphes/fg1 (particularly, in Tricorythodes/fg1) can be diminished.

In male (but not in female) left and right halves of phragma between mesothorax and metathorax are fused by mean of two newly appeared walls – anterior and posterior ones; thus, phragma of male is unpaired and has three cavities – a pair of opened cavities in the left and the right halves of the phragma (as well as in other mayflies), and a median unpaired cavity, closed from all sides.

(2) Fore wing [of single pair – see (9)] has unique features: iMP and MP₂ arise from extreme base of wing independently one from another and from neighbouring veins – thus they seem not as intercalaries or branches, but as two independent non-branched veins (Kluge 2004: Fig.86:F). Shape of wing strongly differs from that of all other Fimbriatotergaliae and is uniform in all Caenoptera – fan-like widened proximally, without curvation of MP₂ and CuA [see Fimbriatotergaliae (5)]; it has much in common with Tricorythodes/fg1 and some other Tricoryptera [see (10)–(12)].

(3) Ventral side of gill operculum [tergalius II – see Caenotergaliae (1)] near its lateral (i.e. costal) and posterior (i.e. apical) margins bears microtrichia – small immobile outgrowths of cuticle; each microtrichion corresponds to one hypodermal cell and is not homologous to seta (Eastham 1936). In Brachycercus/f1=g3 these microtrichia are spine-like (Kluge 2004: Fig.84:E) (as microtrichia on some other parts of body), while in Caenis/f3=g1 microtrichia have peculiar structure (Kluge 2004: Fig.85:A, 88:D,G) (see below).

(4) Each tergalius lacks any vestige of ventral lamella [see Caenotergaliae (1)] (Kluge 2004: Fig.88:A,C).

 Oscillating tergalii III-VI can make composite movements, thanks to which produce an asymmetric water flow across the body and can change its direction (unlike Neoephemera/fg1, whose tergalii produce symmetric flows, as in other mayflies) (Eastham 1934, Notestine 1994).

(5) Genitals have following unique structure (Kluge 2004: Fig.84:A–D). Base of styliger is strongly transferred anteriorly and W-shapely speaks into sternite IX; boundary between styliger and sternite can be lost, but is indicated by muscle bases (Malzacher 1997); thus styliger is immobile connected with sternite being unable to turn down. Dorsal wall of styliger, which is invariably pressed to penis, has sclerite with a pair of projections directed proximally; ventral wall of styliger is membranous and translucent, thus the sclerite of dorsal wall is visible from ventral side.

Gonostylus is non-segmented – i.e. without distal segments and without traces of suture between 1^st and 2^nd segments (non-unique apomorphy – see Index of characters [2.3.12] and [2.3.10]).

Sensory globular papillae (which in Neoephemera/fg1 and other mayflies are present of inner surface of gonostylus – see Morphology) are lost. Possibly, initially for Caenoptera, imaginal gonostylus has smooth sclerotized surface and a deep longitudinal groove opened medially, while subimaginal gonostylus is covered by microtrichia (as in other mayfly subimagoes) and retains normal roundish cross section; such structure is present in all Brachycercus/f1=g3 (where the groove can be so deep, that gonostylus has a form of leaf rolled in a tube – Kluge 2004: Fig.84:A–D), in Madecocercus and Tillyardocaenis (Kluge 2004: Fig.86:C–E); in Caenis/f5=g3 imaginal gonostyli resemble subimaginal ones: they are roundish in cross section and usually covered by microtrichia.

Larval protogonostyli are extremely shortened or lost, but developing inside them gonostyli of adults are never crumpled, as their bases are shifted proximally (Kluge 2004: Fig.84:B, 86:C) (unlike many other mayflies, particularly Neoephemera/fg1, where larval protogonostyli are also very short, and subimaginal gonostyli are crumpled inside them – Kluge 2004: Fig.81:D, 82:H).

In connection with immobility of styliger, penial arms are functionless, often weak, desclerotized, or non-expressed. Penis [developing without larval protopenis – see Caenotergaliae (3)] is integral, poorly sclerotized or non-sclerotized.

(6) Imaginal and subimaginal caudalii [cerci and paracercus – see Plesiomorphy] are modified in a unique manner, differently in male and female. In male subimago caudalii are short (as in larva), segments are short and thick, with whorls of long setae on joinings only; in male imago caudalii are very long, segments are strongly elongated and slender, completely lack setae (unlike majority of mayflies whose caudalii of subimago and imago bear irregularly situated setae). In female caudalii are short and slender (as in larva), with usual irregularly situated setae; unlike majority of mayflies, caudalii do not moult when subimago moults to imago (while all other parts of body do moult, unlike females of some short-living mayflies). About sexual dimorphism of larval caudalii – see (14).

Apomorphies of Caenoptera common with Tricoryptera (see Classifications of Furcatergaliae II).

(7) Adults are shortly-moulting: in mature larva ready to moult to subimago, imaginal cuticle is already developed; shortly (in a few minutes) after the moult from larva to subimago, the next moult from subimago to imago follows. At the same time, imagoes are not short-living, and have functional legs. Subimaginal cuticle has no pigmented and sclerotized areas characteristic for Neoephemera/fg1 and many other mayflies [see Fimbriatotergaliae (2)–(4)]. The same in Tricoryptera (at least in Tricorygnatha and Leptohyphes/fg1).

(8) Imaginal and subimaginal mesothorax, besides unique apomorphies [see (1)] has following modifications common with all or some Tricoryptera (Kluge 2004: Fig.87):

On sublateroscutum transverse interscutal suture is present (the same in Tricorygnatha and Leptohyphes/fg1).

Anterior paracoxal suture is transferred posteriorly, close to anterior margin of coxal conjunctiva (the same in Tricorygnatha and Leptohyphes/fg1).

Posterior scutal protuberances are enlarged and convergent posteriorly (the same in Leptohyphes/fg1-Tricorythodes/fg1).

Lateropostnotum is enlarged and stretches ventrally nearly up to furcasternum, reaches coxal conjunctiva, and ventrally is fused with epimeron (the same in Tricorythodes/fg1). Fusion of lateropostnotum with epimeron (but not enlargement of lateropostnotum) takes place also in Fossoriae and some others, but not in Neoephemera/fg1.

Scutellum is large, its lateral impressions are enlarged; infrascutellum is interrupted medially (the same in all Tricoryptera and Teloganodes/f3=g1).

At rest, subimago and imago keep wings spread by sides or somewhat turned up (unlike most mayflies, whose wings are raised up). Among maylies with functional legs, the same in Tricorygnatha, Leptohyphes/fg1 and a few others; the same in short-living mayflies.

(9) Hind wings are always completely lost (non-unique apomorphy – see Index of characters [2.2.59]); metathorax is vestigial; at least in female wing muscles of metathorax, including scuto-episternal muscle (S.ESm) and median tergal muscle (MTm) are lost (in male at least S.ESm can be retained, but lost in meridies [Caenopsella]). The same in Tricorythodes/fg1; in other mayflies, even if hind wings are completely lost, wing muscles S.ESm and MTm are retained in both sexes.

(10) On fore wing [see (2)] CuP is not connected with CuA and looks as arising from AA; cubital field has only a bifurcate vein [see Caenotergaliae (9)], which is usually connected basally both with CuP and CuA (Kluge 2004: Fig.86:F: x₁, x₂). The same in Tricoryptera and some other mayflies (selected Leptophlebia/fg1, Tetramerotarsata and others).

(11) On fore wing [see (2)] marginal intercalaries are absent (non-unique character – see Index of characters [2.2.55]); number of cross veins is strongly diminished (Kluge 2004: Fig.86:F).

(12) In imago amphitornal margin of wing [see (2)] with setae (as in subimago of all mayflies). Non-unique apomorphy (see Index of characters [2.2.27]); particularly, the same in Tricorygnatha and Leptohyphes/fg1.

(13) In imago and subimago of both sexes tarsi of all legs are distinctly 5-segemented, because 1^st tarsal segment of middle and hind legs [initially fused with tibia and shortened – see Furcatergaliae (2)] on its outer side is secondarily distinctly separated from tibia; joining of tibia and tarsus is strongly inclined, thus outer side of 1^st tarsal segment is elongate (Kluge 2004: Fig.86:G). The same in Tricorythodes/fg1 (Kluge 2004: Fug.105:A); in Tricorygnatha 1^st tarsal segment is separated from tibia in the same manner, but is fused with 2^nd (see Index of characters [2.2.84]).

(14) In male larva caudalii [cerci and paracercus – see Plesiomorphy] are thickened in proximal part (Kluge 2004: Fig.88:A), unlike caudalii of female, which have usual form. The same in Tricorygnatha, Ephemerythus and Leptohyphes/fg1. About modification of adult caudalii – see (6).

Non-unique apomorphies of Caenoptera.

(15) Eyes of males lost division into dorsal and ventral portions, spherical, usually not large, have the same size and shape as in female. Non-unique apomorphy (see Index of characters [2.1.3]); particularly, the same in many Tricoryptera, which have other characters in common with Caenoptera (see above).

Among Caenoptera eyes of male are larger than in female only in Caenopsella/g1 and undescribed species of Brachycercus/f1=g3; in these cases male eyes have the same spherical shape as in other Caenoptera, without division into dorsal and ventral portions; most probably, this is a secondarily enlargement.

(16) On fore leg of male imago and subimago both claws are blunt. Non-unique apomorphy (see Index of characters [2.2.77]).

(17) Patella-tibial suture (initially present on middle and hind legs) is lost on adult legs (Kluge 2004: Fig.86:G) and at least on larval middle leg; it can be retained on larval hind leg only (in many Caenis/f3=g1), or lost on all legs (in Brachycercus/f1=g3 and selected species and specimens of Caenis/f3=g1). Non-unique apomorphy (see Index of characters [1.2.18]).

(18) Gill operculum (tergalius II) usually has an integral Y-shaped rib convex on dorsal side: three apices of this rib are directed, correspondingly, to base of the gill operculum, to its anal-proximal angle (situated medially-anteriorly), and to its distal margin [see Caenotergaliae (1)] (Kluge 2004: Fig.88:A). Unlike Neoephemera/fg1, median margins of gill opercula are non-modified, overlap one another (Kluge 2004: Fig.88:F). Among Caenoptera, the Y-shaped rib is modified in Clypeocaenis/g1 (Kluge 2004: Fig.88:J–K) and absent in Caenopsella/g2 (see below).

Plesiomorphy of Caenoptera. In all stages paracercus is always well-developed, subequal to cerci [see (6) and (14)].