CLADOENDESIS OF EPHEMEROPTERA

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Autapomorphies of Discoglossata. 

(1) Larva has a head shield and a peculiar modification of clypeus and labrum (Kluge 2004: Fig.44:E-F). Anterior and lateral margins of head are prominent and form a shield, covering mouthparts dorsally or anteriorly; at middle margin head shield is narrowly rounded or pointed. Clypeus is hidden under the head shield, dorsally (or anteriorly) is projected above base of labrum, and ventrally (or posteriorly) is reduced in such a manner, that labrum is articulated directly to its base – i.e. to a pair of sclerotized portions of head capsule located at bases of anterior tentorial arms and mandibles.

Besides Discoglossata, head shield is present in Heptagennota; but in Heptagennota labrum retains its articulation on apex of clypeus, and the head shield has another form (usually more flattened and widened anteriorly), that allows to assume its independent origin.

Probably initially for Discoglossata, fore margin of the head shield bears a row of long setae directed anteriorly-medially; such setae are present at least in Chromarcys and Oligoneuriella/g1.

(2) Mouth apparatus is more highly specialized for gathering dispersed particles, than in other Eusetisura, having unique structure of labium, maxillae and mandibles.

Glossae have unique structure (Kluge 2004: Fig.45): they are proximally fused together forming an integral movable semicircular glossal disc; this disc is so strongly widened ventrad of paraglossae that expands fare laterad of paraglossae, reaching 2^nd+3^rd segment of labial palp and covering all other mouthparts from ventral side. Medially, the glossal disc has a narrow longitudinal dorsal ridge with widened base, to which a pair of mentum-glossal muscles are attached. Dorsal side of the glossal disc bears numerous arched concentric rows of setae directed along these rows; between these rows there are present dense irregularly situated smaller setae of the same direction (Elpers & Tomka 1992: Fig.10-12; Kluge 2004: Fig.45:A). Paraglossae are hidden inside a pair of cavities on dorsal side of the glossal disc. As labial palps are partly covered from ventral side by glossal disc, their 1^st segments are always directed laterally (but not ventrally). 2^nd+3^rd segment of labial palp [see Eusetisura (2)] is integral, without muscle inside; its shape is banana-like with grooved lateral side; when its densely setose ventral side overlaps the densely setose dorsal side of glossal disc, its lateral margin matches lateral margin of glossal disc.

Labium is so strongly modified, that homology of its parts is not evident: glossae, being projected fare laterally, can be taken for paraglossae, while paraglossae, being brought together, can be taken for glossae. True homology of these parts can be understood if examine muscles attachment: mentum-glossal muscles occupy median position, and mentum-paraglossal muscles are located laterad of them (Kluge 2004: Fig.45:B).

Maxilla is strongly narrowed: it is widest in its proximal part and narrows toward apex; apically with 1 slender canine (unlike 3 initial canines and 2 canines in Isonychia/fg1); distalmost bristle of inner-ventral row is situated close to the canine and ventrad of it, enlarged and specialized, looks like a second canine, usually pectinate (Kluge 2004: Fig.44:D) (among taxa examined, only in Elassoneuria/g1 it is non-pectinate). 1^st segment of maxillary palp is strongly shortened, 5–6 times shorter than 2^nd+3^rd segment [see Eusetisura (2)] (Eaton 1883-1888: Pl.26).

Mandibles are strongly diminished, being hidden between head shield [see (1)] and glossal disc; incisor and kinetodontium are diminished, slender (Eaton 1883-1888: Pl.26:9-10).

(3) Larval fore protoptera are fused together by means of a wide plate, which fills an area between their hind margins and primary posterior margin of mesonotum; fore protoptera themselves diverge posteriorly (unlike Coloburiscus/fg1, where they are convergent); adult plumidia can be developed [see Eusetisura (5)]. Non-unique apomorphy (see Index of characters [1.2.6] and [2.2.13]).

(4) On fore wing costal brace [see Euephemeroptera (1)] is strongly modified: from place of its proximal connection with costal vein, it passes behind C nearer to Sc up to its crossing with Sc and falling into RA (Kluge 2004: Fig.42:A). Unique apomorphy: in all other Euplectoptera costal brace at most its length goes in front of costal vein.

(5) On fore wing apex of MP₂ is approximated to apex of CuA, thus distance between apices of MP₁ and MP₂ is strongly enlarged; more or less brought together are also apices of MP₁ and MA₂, of MA₁ and RSp (Kluge 2004: Fig.42:A); besides this, apices of iRS and RSa can be brought together (in Geminovenata only). 

Gemination of veins (i.e. uniting in pairs) appeared independently also in Behningia/fg3 and Palingenia/f3=g2 (see Index of characters [2.2.28]); unique feature of gemination in Discoglossata is that veins iMA and iMP do not take part in gemination; thus on the place where in Behningia/fg3 and Palingenia/f3=g2 two pairs iMA+MA₂ and MP₁+iMP are present, Geminovenata have only one pair MA₂+MP₁ and two single veins iMA and iMP (which can be reduced).

(6) On fore wing RS is modified in a following manner. RSa lacks primary furcation: in Chromarcys branch RSa₂ lost connection with RSa-RSa₁ and arises from iRS, and iRSa arises from RSa₂ (Kluge 2004: Fig.42:A); in Geminovenata both RSa₂ and iRSa are lost (Kluge 2004: Fig.49:A, 50:A, 51:A) (non-unique apomorphy). Vein iRS (which initially is intercalary) arises from RSa; in Chromarcys these veins widely diverge (Kluge 2004: Fig.42:A), in Geminovenata they are brought together all over their length (Kluge 2004: Fig.50:A, 51:A) (only in Elassoneuria/g1 and Homoeoneuria/g2 both are completely lost – Kluge 2004: Fig.49:A). In other mayflies iRS usually has free base, only in selected specimens being arisen from RSa.

(7) Imaginal and subimaginal fore leg is shorter than middle and hind legs (Kluge 2004: Fig.42:E-G); 1^st–4^th tarsal segments (i.e. all segments except for the last one bearing claws) are shortened, and their boundaries are strongly oblique; either these 4 segments are distinguishable (Kluge 2004: Fig.42:F), or only 3 of them, or only 2, or boundaries between segments are indistinct. Middle legs are the longest. These proportions are not connected with leg specialization in larva [see Eusetisura (1)], as larval legs are specialized in the same manner as in other Eusetisura. In all other Branchitergaliae and nearly in all other Ephemeroptera, tarsi of fore legs of male imago serve to fix female during mating, and are elongate, being longer than non-specialized tarsi of middle and hind legs; only in very rare cases fore tarsus is shorter than others (see Index of characters [2.2.74]).

(8) Patella-tibial suture (initially present on middle and hind legs) is lost on all legs of larva, subimago and imago. Non-unique apomorphy (see Index of characters [1.2.18]).

(9) In imago and subimago at least on fore legs of male both claws are blunt (Kluge 2004: Fig.42:F). This apomorphy is not unique (see Index of characters [2.2.77]), but does not occur in other Eusetisura (in Isonychia/fg1 only in imago both fore leg claws of male are blunt).

(10) Probably the initial form of lamellate lobe of tergalius [see Branchitergaliae (3) and Eusetisura (7)] is cup-shaped (Kluge 2004: Fig.44:A-C): small, roundish; dorsal side is sclerotized, convex, without prominent ribs; margins are also sclerotized, at anterior margin sclerotization is widely spread to ventral side; most part of ventral side non-sclerotized, concave; fibrillose portion can be partly inserted into this non-sclerotized concavity. Such tergalial structure is present in Chromarcys and Oligoneuriella/g1; in Oligoneuria/f4=g5 tergalii have similar convex-concave shape, but without ventral sclerotization; in other groups tergalii are secondarily enlarged up to size usual for Ephemeroptera, and have other form.

All tergalii I–VII are always present, but can be differently modified [see below, Geminovenata (1) and Homoeoneuria/g2 (3)]. 

(11) Each anchor on egg surface [initially consisting of a thread coiled under a knob – see Branchitergaliae (4)] has its thread reduced to a short nearly straight stem, thus anchor has mushroom-like shape (Koss & Edmunds 1974: Fig.72-78; Pescador & Peters 1980: Fig.67-70; Kluge 2004: Fig.43:H).

Characters of Discoglossata of unclear phylogenetic status.

(12) Tendency to secondary increasing of number of distal segments of gonostylus is expressed: in Chromarcys, in selected specimens of Oligoneuriella/g2 (particularly, in pallida [Oligoneuria]), in Oligoneuriopsis, in selected specimens of Elassoneuria/g1 instead of two initial segments, 3 or 4 segments can be present (Kluge 2004: Fig.42:C, 47:A-B, 49:D). Non-unique apomorphy (see Index of characters [2.3.12]). In some groups of Oligoneuria/f2-g3 number of distal segments of gonostylus is, on the contrary, reduced to one.

Plesiomorphies of Discoglossata. Imaginal and subimaginal paracercus is usually well-developed (unlike Coloburiscus/fg1 and Isonychia/fg1); only in that representatives of Discoglossata, whose paracercus is reduced in larva (Chromarcys, Lachlania, Spaniophlebia and some Oligoneuriella/g2) paracercus of imago and subimago is also reduced.