According to Prous et al, 2019, there is currently no reliable key or species treatment available for this genus. The description follows Benson.
The head on the frons and temples above dull and with minute hair-warts. Cladius pilicornis is a black sawfly with almost white tibiae and tarsi on all legs. The sternopleural region is covered by an unbroken hair-patch. In the male, the third antennal segment has a dentate basal projection on the underside. In the female, the third segment is contracted medially and arched.
Larvae feed on hawthorn.
Size: approx. 5 - 7mm
Distribution: England, Scotland, Wales, Ireland
Flight period: Bivoltine, May to August
Plant associations: Crataegus spp. (hawthorns)
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
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.