Wesmaelius nervosus

Wesmaelius nervosus

Of the 3 species of Wesmaelius that have been recorded in Norfolk this is the one I've seen most (although there have been considerably more records of subnebulosus across the county). Records span from April to September.

Like Hemerobius, Wesmaelius has a recurrent humeral vein but unlike Hemerobius the cross vein RMCV falls distal to the fork in vein M. Separating the various species of Wesmaelius from one another usually requires examination of the tip of the abdomen. This is hard to do on live specimens (though may be possible with males) so it will normally be necessary to retain specimens for identification.

The shape of the appendage at the tip of the abdomen is crucial for identifying males. As shown below it is hook-shaped, curving right back but lacking a fish hook structure at the tip which is present on subnebulosus (but note that Plant (1997) advises checking this from various angles, so presumably it can sometimes be tricky to see).

male Wesmaelius nervosus showing abdomen tip, North Elmham (Norfolk, UK), 18th July 2020

Females are trickier as it's not possible to see the whole genital plate on live individuals. The shape of this plate is critical for the ID and it is best viewed by clearing the abdomen in, for example, potassium hydroxide solution. In the penultimate photo below the dark rounded-triangular patch in the middle of the abdomen tip from below is the top half of the genital plate.

female Wesmaelius nervosus showing both forewings (one also close up), abdomen tip before clearing from side and below and cleared genital plate, Ling Plantation, North Elmham (Norfolk, UK), 15th April 2022

Another male:

male Wesmaelius nervosus showing abdomen tip and wings (including close-up of base of forewing), North Elmham (Norfolk, UK), 8th May 2023

Another female:

female Wesmaelius nervosus showing wings (with close up of forewing), abdomen tip from side and cleared abdomen tip from below with genital plate showing, North Elmham (Norfolk, UK), 4th September 2022

I don't recall noticing the peculiar hairy projections on the side of the thorax before so include a couple of photos of these in the next batch of photos. I haven't yet investigated what other species share this character but I think I can see them in several photos of other Wesmaelius species at least so I don't suppose they're helpful for ID.

female Wesmaelius nervosus showing thoracis projections, both forewings, cleared abdomen from below and genital plate, North Elmham (Norfolk, UK), 12th July 2022