The description here is taken from Muche's description of Pachynematus xanthocarpus which is considered as a synonym of fallax. The former Pachynematus species require further study and any descriptions are currently tenuous.
The female is black with yellow-brown below and between the antennal bases, the orbits, the posterior margin of the pronotum, usually a spot on the mesepisternum, a double spot at the base of the scutellum, the tegulae, more or less the edges of the mesonotum. Sometimes the mesonotum or the mesopleura are translucent brown, the sternites narrowly edged with yellow and the tergites yellowish brown on the sides. The hypopygium is yellowish brown. Legs yellowish with the coxae, more or less trochanters, base of the femora, apices of the tibia and tarsi black.
The male is black with only the labrum and genital plate yellowish, rarely also the posterior orbits and the tegulae pale. On the legs only the knees, tibiae and tarsi yellowish.
Euura fallax larvae are not described but are associated with grasses.
Size: 6.0 - 8.5mm.
Distribution: England, Ireland
Flight period: April to June
Plant associations: Grasses
Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
Liston A, Knight G, Sheppard D, Broad G, Livermore L (2014) Checklist of British and Irish Hymenoptera - Sawflies, ‘Symphyta’. Biodiversity Data Journal 2: e1168. https://doi.org/10.3897/BDJ.2.e1168
Muche, W.H., 1974. Die Nematinengattungen Pristiphora Latreille, Pachynematus Konow und Nematus Panzer (Hym., Temthredinidae). Deutsche Entomologische Zeitschrift, 21(1/3), pp.1-137.
Prous, M., Liston, A., Kramp, K., Savina, H., Vårdal, H. and Taeger, A., 2019. The West Palaearctic genera of Nematinae (Hymenoptera, Tenthredinidae). ZooKeys, 875, p.63-127
Taeger, A. and Blank, S.M., 1998. Beitrag zur Kenntnis einiger Nematinae (Hymenoptera: Tenthredinidae). Pflanzenwespen Deutschlands (Hymenoptera, Symphyta). Kommentierte Bestandsaufnahme. Goecke & Evers, Keltern, pp.247-277.