(Panephemeroptera Euephemeroptera Euplectoptera Anteritorna pm.Tridentiseta
- Metretopus/fg1)

Nomen hierarchicum: Metretopus/fg1 [f:1935; g:1901] (incl. Metreplecton, Siphloplecton)  

In circumscription fits:

— subfam. Metretopodinae: Edmunds & Traver 1954a: 237

— fam. Metretopodidae: Tshernova 1970: 135

— Metretopus/fg1: Kluge 2004: 86

References. Edmunds & Allen & Peters 1963: *; – Tshernova 1970: * * – Koss & Edmunds 1974: *; – Edmunds & Jensen & Berner 1976: * *; – Berner 1978: * *; – Kluge & Studemann & Landolt & Gonser 1995: * * * *; – Kluge 2004: * * *.

Autapomorphies of Metretopus/fg1.

(1) Larval claw of fore leg is bifurcate (unlike all other Ephemeroptera); lateral margins of claw and inner margins of both its branches bear a longitudinal row of very long slender denticles resembling setae (Kluge 2004: Fig.22:C). Claws of middle and hind legs have usual structure – non-bifurcate, slender, slightly curved, without any denticles.

(2) Mouth apparatus is somewhat modified: Labrum and hypopharynx are truncate. Maxilla has 2 canines only (instead of three initial ones – see Index of characters [1.1.33]); its apical-ventral row of setae (consisted of simple non-pectinate setae) is transferred to ventral side and directed parallel to inner margin (Kluge 2004: Fig.22:D). 2nd segment of 3-segmented maxillary palp is shortened. Glossae and paraglossae are curved ventrally; labial palp is 2-segmented, its distal segment (a result of complete fusion of initial 2nd and 3rd segments) is thickened and lacks muscle initially moving 3rd segment (Kluge 2004: Fig.22:G-H,I-J).

(3) All tergalii I–VII lack anal ribs; tergalii I–III lack costal ribs also (Kluge 2004: Fig.22:E-F). In connection with this, in some species of Siphloplecton costal margin of tergalii I–III is expanded and bent to ventral side.

Characters of Metretopus/fg1 of unclear phylogenetic status.

(4) Imaginal and subimaginal lateroparapsidal suture is elongate; subimaginal lateral pigmented area of mesonotum occupies most part of sublateroscutum, nearly up to posterior end of lateroparapsidal suture (Kluge 2004: Fig.21:D-E). Non-unique character (see Index of characters [2.2.14]); similar elongation of lateroparapsidal suture in Ameletus/fg1; possibly synapomorphy. 

(5) Cubital field of fore wing [see Anteritorna (1)] with intercalaries: usually there are two pairs of intercalaries in Siphloplecton (Berner 1978: Fig.1-5), one pair in Metretopus/fg2 (ibid.: Fig.6), single intercalary in Metreplecton (Kluge 2004: Fig.22:A). The same in Ameletus/fg1-Metreletus and many other mayflies (see Index of characters [2.2.52]); probably non-unique autapomorphy.

(6) Imaginal and subimaginal paracercus is vestigial. Non-unique apomorphy (see Index of characters [2.3.22]); particularly the same in Siphlonurus/fg1 and Ameletus/fg1.

(7) Larval paraproct with a spine on inner-apical margin. The same in Siphlonurus/fg2, Siphluriscus, Vetulata and Rallidens.

Plesiomorphies of Metretopus/fg1. Larva has primary swimming siphlonuroid specialization: legs are able to stretch posteriorly; abdomen is large, able to make undulate swimming movements; caudalii are not long, paracercus is well-developed, primary swimming setae are dense, secondary swimming setae are absent. Larval head is hypognathous. Maxilla [see (2)] with 3 dentisetae [see Anteritorna (2)] (Kluge 2004: Fig.22:D). Larval (and adult) patella-tibial suture is developed on middle and hind legs only. Larval claws of middle and hind legs are slightly curved, without denticles [about fore claws – see (1)]. Tergalii [see (3)] retain ability of rhythmical respiratory movements (see Index of characters [1.3.30]).

In imago and subimago: Mesonotal suture can be stretched backward medially (Kluge 2004: Fig.21:E) or nearly transverse (Kluge 2004: Fig.21:D). Anterior paracoxal suture is complete (unlike Rallidens and some Bidentiseta). Furcasternal protuberances are contiguous (unlike Amphinotic groups of Tridentiseta and some others – see Index of characters [2.2.23]). Hind wing is well-developed, as long as 0.35–0.4 of fore wing length (Kluge 2004: Fig.22:A-B). Imaginal and subimaginal tarsi are 5-segmented, 1st segment is the longest, fused with tibia. All claws of imago and subimago are ephemeropteroid. Gonostylus has 2 distal segments.

Size. Fore wing length 9–16 mm.

Distribution. Holarctic (North of Europe, whole Siberia, Nearctic).

The taxon Metretopus/fg1 is divided into:

1. Metretopus/fg2

2. Metreplecton

3. Siphloplecton