(Panephemeroptera Euephemeroptera Euplectoptera Anteritorna Bidentiseta Branchitergaliae  
Eusetisura - Isonychia/fg1)

Nomen hierarchicum: Isonychia/fg1 [f:1953; g:1871] (incl. Prionoides)

In circumscription fits:

— gen. Isonychia Eaton 1871: 134

— gen. Jolia Eaton 1881: 192 (partim: larva, non imago)

— gen. Chirotonetes Eaton 1881: 21 (partim: imago, non larva)

— subfam. Isonychiinae Burks 1953: 108

— fam. Isonychiidae: Landa 1973: 156

— Isonychia/fg1: Kluge 2000: 251

References. Needham & Traver & Hsu 1935: * *; – Edmunds & Allen & Peters 1963: *; – Koss & Edmunds 1974: *; – Edmunds & Jensen & Berner 1976: * *; – Kondratieff & Voshell 1984: ; – Kluge 2004: * * * *; – Tiunova & Kluge & Ishiwata 2004: * * * *. 

Autapomorphies of Isonychia/fg1.

(1) Larval frons between antennae bases forms a projection of unique form, which narrows toward clypeus and terminates by an edge flattened from sides and hanging over clypeus (Kluge 2004: Fig.41:D).

(2) Mouth apparatus has uniform structure in all representatives, with following unique features. 

On outer surface of labrum (densely covered with long setae, as in other Eusetisura and many other mayflies), besides slender long pointed setae, are present more stout long stick-shape setae with truncate apices (Tiunova & al., 2004: Fig.57)

On maxilla [see (10) and Bidentiseta (1)] distal dentiseta is strongly diminished (Kluge 2004: Fig.41:C) or lost, and proximal dentiseta is long, straight, spine-like (unlike other Eusetisura, where both dentisetae are equally developed). 

Labium has characteristic shape; 2nd+3rd segment of labial palp (integral, but with muscle retained), besides numerous irregular long setae [see Eusetisura (2)] bears a regular row of very long setae directed laterally-dorsally (Kluge 2004: Fig.41:B,D).

(3) Subimaginal lateral pigmented area of mesonotum occupies entire submedioscutum up to medioparapsidal suture (Kluge 2004: Fig.40:D). Similar form of this area is in Ameletus/fg1 only.

(4) Basisternum of mesothorax posteriorly with a bifurcate projection directed posteriorly, basisternum of metathorax with a pair of projections directed posteriorly; these projection are distinctly developed in larva, being smaller or non-expressed in imago (Kluge 2004: Fig.40:E-F).

(5) Larval fore legs [specialized as filtering – see Eusetisura (1) and bearing gills – see (12)] have unique features (Kluge 2004: Fig.41:E): apex of tibia on its inner side with a long jointed spine-like appendage (in some other mayflies on this place only non-jointed projection can be present); in addition to the filtering setae on femur and tibia, there are 2 longitudinal rows of smaller filtering setae on inner side of tarsus.

(6) Imaginal and subimaginal claws (initially ephemeropteroid) on all legs except for fore legs of male imago [see (11)], are similar and pointed (with this the name "Isonychia" is connected). This apomorphy is not unique (see Index of characters [2.2.85]), but does not occur in other Eusetisura.

(7) On hind wing bifurcation of PM is transferred to distal part of wing, distad of bifurcation of MA (Kluge 2004: Fig.40:B) (only for selected specimens of formosus [Chirotonetes] MP bifurcation proximad of MA bifurcation is reported). Non-unique apomorphy (see Index of characters [2.2.69]).

(8) Each tergalius I–VII [unable to rhythmical movement – see Eusetisura (7)] has following structure: anal rib [arising behind base of fibrillose ventral portion – see Branchitergaliae (3)] bifurcates near base, its anterior branch passes by dorsal side near middle of tergalius up to its apex, and posterior branch borders the anal margin of tergalius; thus there are three ribs (including well-developed costal rib on costal margin) (Kluge 2004: Fig.41:A). Similar three ribs independently appeared in Nesameletus/f2=Metamonius/g2 and Rallidens. Margin of tergalius in apical part of costal rib and in area between apices of anterior and posterior branches of anal rib, bears denticles. Sometimes smaller denticles are present also on tergalius apex between these two areas (their absence was described as a character of Prionoides, but they are absent also in group japonica of Isonychia/fg2).

(9) Imaginal and subimaginal paracercus is vestigial, consists of several segments. Non-unique apomorphy (see Index of characters [2.3.22]).

Characters of Isonychia/fg1 of unclear phylogenetic status.

(10) Maxilla [see (2)] with acute apical angle (unlike nearly rectangular in Coloburiscus/fg1), bearing 2 canines only (instead of three initial ones) (Kluge 2004: Fig.41:C). Non-unique apomorphy (see Index of characters [1.1.33]); possibly synapomorphy with Discoglossata (which has more acute angle with one canine only).

(11) On fore legs of male imago [but not subimago – see (6)] both claws are similar and blunt: each claw lacks its point and is provided with a soft plate. The same in Discoglossata; probably synapomorphy.

(12) Larval fore leg [see (5)] with a gill in joining of coxa with thorax; this gill has a form of a tuft (in majority of representatives – Kluge 2004: Fig.41:D-E) or a filament (in majority of Prionoides); in imago on this place a dark soft protuberance is retained. The same in Murphyella; possibly this is initial for Eusetisura [see Eusetisura (3)].

Plesiomorphies of Isonychia/fg1. Larva has most typical siphlonuroid form among Eusetisura: legs are able to stretch posteriorly (unlike at least Oligoneuriella/g2, while other Eusetisura were not observed at life); abdomen is able to make undulate swimming movements; paracercus is well-developed, cerci and paracercus have dense primary swimming setae (only ends of cerci consist of elongate cylindrical segments without setation). Mouth apparatus has no autapomorphies peculiar for Discoglossata, 1st segment of maxillary palp is only 2 times shorter than distal segment [2nd+3rd – see Eusetisura (2)]. Unlike Discoglossata, larval and adult middle and hind legs retain patella-tibial suture. Tarsus of fore leg of male imago is long (unlike Discoglossata), 1st segment is non-shortened (unlike Coloburiscus/fg1), equal or slightly longer than 2nd segment. In cubital field of fore wing several (4–7) veins go from CuA to basitornal and tornoapical margins: usually first 2 of these veins are non-branched, and next 2–4 veins are branched (Kluge 2004: Fig.40:A) [see Anteritorna (1)]. Gonostylus has 2 distal segments.

Size. Fore wing length 8–16 mm.

Distribution. Holarctic (dominate in Nearctic). 

The taxon Isonychia/fg1 is divided into:

1. Isonychia/fg2

2. Prionoides

A taxon Borisonychia has uncertain systematic position