Monsoma is a small genus of only four species globally distributed across the palaearctic and nearctic. In Britain and Ireland they are represented by a single species.
The genus is characterised by having the second antennal segment (pedicel) longer than broad. The head has a post-genal carina and the hind tarsus is about equal in length to the hind tibia.
Those species where the larvae are known appear to feed on alders.
Monsoma pulveratum (Retzius, 1783)
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