(1) Mouth apparatus has unique structure, being adopted to catch and filtrate food particles dispersed in water (Kluge 2004: Fig.53:E): Maxillary palp is unusually elongate, 2-segmented. Its 1^st segment is elongate so greatly, that being turned posteriorly, reaches mesonotum; trachea, which goes to muscles of this segment (i.e. to flexor and extensor of 2^nd segment), is thicker than tracheae in other appendages. Distal (2^nd+3^rd) segment of maxillary palp is unusually long, about twice as long as the 1^st segment, slender, arched, bears two regular longitudinal rows of long filtering setae. Working by its maxillary palps, larva can gather particles dispersed in water, and also can swim back to front. Maxilla has a single canine (instead of 3 initial ones), lacks apical-ventral row of pectinate setae, bears long simple setae on apical side. Glossae are crescent-shaped, paraglossae are much larger, crescent-shaped. Distal (2^nd+3^rd) segment of labial palp [see Heptagennota (8)] is large, crescent-shaped, pointed, with numerous long filtering setae, among which are present: a regular longitudinal row of setae on ventral-outer side, a regular longitudinal row of setae on inner side, a field of setae on dorsal side and other setae.

(2) Mandibles are inverted: mandible with distally projected mola is right one, and mandible with proximally projected mola is left one. Non-unique apomorphy, the same in selected species among Radulapalpata.

(3) Each tergalius I–VII lost fibrillose portion, costal and anal ribs [see Branchitergaliae (3)], has a large anterior-proximal lobe and pointed apex (Edmunds & al. 1986: Fig.54; Studemann & al. 1987: Fig.13).

(4) On fore wing RSa₂ lost its connection with RSa and has a form of intercalary, which begins more distally than iRSa (Kluge 2004: Fig.53:C). This apomorphy is not unique, occurs in many Furcatergaliae and Tridentiseta, but never occurs in other Branchitergaliae.

(5) On hind wing MA lacks furcation; thus only two triads are present – RS and MP, with a single MA between them [see Euplectoptera (1)] (Kluge 2004: Fig.53:D). Non-unique apomorphy (see Index of characters [2.2.67]); among Pentamerotarsata the same in dimicki [Cinygma].

(6) Imaginal and subimaginal gonostylus with 3 distal segments (instead of two ones initial for Ephemeroptera). Non-unique apomorphy (see Index of characters [2.3.12]).

(7) Superlinguae are curved laterally (Studemann & al. 1987: Fig.15–16). Probably apomorphy of Pentamerotarsata, as the same in Heptagenia/f5=g4 (but not in other Radulapalpata).

(8) Penis, besides the pair of median titillators [see Pentamerotarsata (5)], with a pair of ventral titillators – immovable spines, situated on ventral side proximad of the median titillators (Studemann & al. 1987: Fig.27–32).

Plesiomorphies of Arthroplea. Labium and labial palps completely lack that specialization, which is peculiar for Radulapalpata. Unlike Radulapalpata, posterolateral spines are present on abdominal segments II–IX and are enlarged toward segment IX (Studemann & al. 1987: Fig.11). Larval primary swimming setae [vestigial – see Pentamerotarsata (6)] are present. Larval and adult patella-tibial suture is developed on middle and hind legs only.