CLADOENDESIS OF EPHEMEROPTERA

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Autapomorphies of Monilistylus.

(1) In male imago, sterno-styligeral muscle (initially unpaired) has enlarged paired lateral portions, which are attached to the bases of unistyligers (Kluge 2020: Figs 51, 53), so that each lateral portion can serve as adductor of the unistyliger.

In our previous paper (Kluge et al. 2014) the lateral portions of the sterno-styligeral muscle were wrongly interpreted as «penial muscles» (Kluge et al. 2014: legend to Fig. 25) or as parts of the «penial muscles» (ibid: legend to Fig. 31). This error was caused by the fact that in monilistylus [Procloleon] the lateral portions of the sterno-styligeral muscle run parallel to the gonovectal (penial) muscles and completely overlap them in ventral view, so that these two pairs of muscles look as one pair (compare left and right halves of Fig. 52). In ornatipennis [Procloedon] the lateral portions of the sterno-styligeral muscle are divergent caudally (Kluge 2020: Fig. 51), while the gonovectal (penial) muscles are convergent caudally (ibid. Fig. 50), so that in ventral view these two pairs of muscles cross one another, being well distinguishable (ibid. Fig. 49).

Initially for Ephemeroptera, the sterno-styligeral muscle is unpaired and serves for rotation of the unpaired styliger as a whole to the ventral direction; when this muscle is contracted, the styliger becomes directed perpendicular to the body and the penis becomes protracted (Kluge 2003: Figs 3–7; 2004: Fig.11). In Baetidae, the styliger is divided to a pair of unistyligers separated by a deep incision, and the sterno-styligeral muscle is attached to the integument of this incision (Kluge & Novikova 2011: Fig.3); as a result of this, contraction of the sterno-styligeral muscle leads not to a rotation of the styliger ventrally, but to inclining the unistyligers toward one another. Primitively for Baetidae, the sterno-styligeral muscle is wide and strong, the place of its attachment between unistyligers is sclerotized, and its lateral parts touch the bases of unistyligers (Kluge 2018: Fig.46). In many baetid taxa, the median sclerite between unistyligers is lost; the sterno-styligeral muscle is often weak and unable to move the unistyligers; in selected taxa this muscle is completely lost (e.g., in Labiobaetis/f1=Pseudopannota/g1 and Rhodobaetis). In Monilistylus, the sterno-styligeral muscle evolved in opposite direction, so that its lateral portions become strong and look as paired muscles.

(2) The second segment of the gonostylus has a composite shape: its proximal 2/3 has a groove on its concave median side; the distal 1/3 has no groove and forms a blunt angle with the grooved proximal portion (Kluge 2020: Figs 49, 53).

Characters of Monilistylus of unclear phylogenetic status. 

(3) Mandibles of the «Centroptilum-type»: the kinetodontium is deeply separated from the incisor and rotated perpendicular to the plane of the mandible, so that its denticles hide one another in dorsal or ventral view; the left prostheca is bifurcate and rotated in the same manner (Kluge 2020: Fig.4, 18–25). Among Procloeon/g1, such «Centroptilum-type» of mandibles, besides Monilistylus, occurs in Pseudocentroptiloides and Securiops as well; it is also present in some other baetid taxa (see Index of characters [1.1.22], [1.1.24]). In other taxa of Procloeon/g1, the mandibles are of the «Baetis-type»: the kinetodontium is fused with the incisor; the left prostheca is wide, terminated by 3–6 blunt, more distal denticles and 2–4 pointed, more proximal denticles; all denticles of incisor, kinetodontium, prostheca and mola are visible in one view. 

(4) Larval femur without stout apical setae on outer margin (Kluge 2020: Figs 30–31) (in contrast to Oculogaster and various taxa not belonging to Cloeon/fg1, which have 2 stout apical setae). 

(5) Hind wings [of the «Centroptilum-type» – see Cloeon/fg1 (9)] are absent in both species (see Index of characters [2.2.59]).

Plesiomorphies of Monilistylus. In contrast to Oculogaster, pterostigma with several cross veins (Kluge 2020: Fig. 44), eggs with sculptured chorion (known for monilistylus [Procloeon] only). In contrast to Pseudocentroptiloides, the maxillae are narrowed distally (ibid. Fig. 17), the labrum is not deeply emarginated (ibid. Fig. 16). In contrast to Pseudocentroptiloides and Securiops, the glossae are not much longer than the paraglossae (Figs 28–29).