Genus Nematus

Nematus

Following recent changes to nomenclature there are thought to be around 127 species of Nematus globally. In Britain there are six species and in Ireland four (brischkei and umbratus are absent).

Difficult to separate morphologically but genetically distinct, the Nematus genus can probably be separated from the other Nematinae by the following features. In lateral view the left mandible narrows rapidly from a wide base. In the forewing the intercostal cross vein Sc is basal to where vein M meets vein R. The cross vein 2r-rs is absent from the marginal vein. The base of vein 2A+3A is incomplete and straight such that the basal loop of the anal cell is open. The apex of the costal vein C is not strongly swollen such that the width of cell c is at least half the width of vein C at the point where vein RS+M meets vein R. The tarsal claws have a subapical tooth but lack a basal lobe. In females, the apical tergite is less than twice as long as tergite eight when viewed from the side and in males the penis valve lacks a distinct filament. In most British and Irish species the hind tibia and first tarsomere are widely expanded and flattened.

Larvae feed on a broad range of tree species (Betulaceae, Oleaceae, Rosaceae, Sapindaceae, Ulmaceae).

Species list:

Nematus alniastri (Scharfenberg, 1805) = Craesus alniastri (Scharfenberg, 1805)

Nematus brischkei Zaddach, 1876 = Craesus brischkei (Zaddach, 1876)

Nematus latipes Villaret, 1832 = Craesus latipes (Villaret, 1832)

Nematus lucidus (Panzer, 1801)

Nematus septentrionalis (Linnaeus, 1758) = Craesus septentrionalis (Linnaeus, 1758)

Nematus umbratus Thomson, 1871

References:

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. 1977: Die Blattwespen Mitteleuropas. Die Gattungen Nematinus Rohw., Euura Newm. und Croesus Leach (Nematinae) sowie Heterarthrus Steph. (Heterarthrinae) (Hymenoptera, Nematinae et Heterarthrinae). - Entomologische Abhandlungen. Staatliches

Perkins, R. C. L. 1929: Notes on the black Croesus from Ireland. - The Entomologist's Monthly Magazine, Third Series, London 65(11): 18-19

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.

Togashi, I. 1997: Three New Species of the Genus Craesus Leach from Japan (Hymenoptera, Tenthredinidae). - Japanese Journal of Systematic Entomology, Matsuyama 3(1): 67-73

Togashi, I. 2007: A new species of the genus Craesus (Hymenoptera: Tenthredinidae) from Japan feeding on Juglans (Juglandaceae). - Proceedings of the entomological Society of Washington, Washington 109(4) : 909-913

Vasu, V. 2003: The sawfly genus Croesus Leach (Hymenoptera: Allantinae) in India. - Zoos' Print Journal, Coimbatore 19 (2004) (1): 1316-1320