Aprosthema melanurum - Vetchling Twinhorn

One of three very similar Aprosthema species found in the British Isles. Only two confirmed records to date, in 1941 and 2016 (Musgrove, 2022). The separation of the three species has been subject to much study and conjecture over the years. Aprosthema melanurum is bivoltine and shows seasonal dimorphism adding to the confusion. Head shape differs in that the summer generation has a short head with strongly narrowing temples and longer flagellae, whereas the overwintering spring generation has a longer head with parallel or posteriorly broadening temples and shorter flagellae. Vikberg postulates that the head shape corresponds to the muscle requirements needed for emergence from the cocoon. The summer spun cocoon is weakly formed from soft plant material, whereas the overwintering cocoon (spring generation) is stronger and constructed underground.

Aprosthema melanurum males can be distinguished from Aprosthema fusicorne by the penis valve shape. Females of melanurum have hind tibiae and tarsae with extensive infuscate colouring.

Larvae feed on Lathyrus pratensis L. and in full grown larvae the anal tergum has long, sharp medial projections. For key to species see Vikberg, 2004.

Jump to other Argidae

Size: 5-8mm

IUCN Status: Vulnerable
GB Rarity Status: Nationally Rare

Distribution: England

Flight period: Bivoltine, May to August

Plant associations: Lathyrus spp.. Vetchlings, including Lathyrus pratensis, Meadow Vetchling (Vikberg, 2004) and Lathyrus tuberosus, Tuberous Pea (Lorenz and Kraus, 1957).


1Benson, 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

Lorenz, H. and Kraus M., 1957. Die Larvalsystematik der Blattwespen (Tenthredinoidea und Megalodontoidea). - Abhandlungen zur Larvalsystematik der Insekten No. 1. Berlin: Akadamie - Verlag

Musgrove, A.J. 2022. A review of the status of sawflies of Great Britain - Phase 1: families other than Tenthredinidae. Natural England, unpublished

Vikberg, V. (2004). Seasonal head dimorphism and taxonomy of some European species of Aprosthema (Hymenoptera: Symphyta: Argidae). Beiträge zur Entomologie= Contributions to Entomology, 54(1), 107-125