Sheep's-bit Knot-horn Homoeosoma nimbella

This species was previously assigned the vernacular name Scarce Clouded Knot-horn. A rare species, in Norfolk confined to the coast between Winterton and Great Yarmouth (see Norfolk Moths).

These three were trapped at Winterton dunes. The similar Phycitodes species can be separated by the genitalia but if there are any genitalic differences from Clouded Knot-horn Homoeosoma nebulella then these are not clear to me. According to the late Barry Goater's British Pyralid Moths (Harley Books, 1986) there is a difference in forewing venation. Goater says the wing venation is easily seen by examining the underside of the forewing, however I did not find this to be the case. I had to painstakingly remove nearly all the scales from the wings in order to see the venation clearly, and of course with the membrane being so fragile, damaged the wing in the process of doing so. Enough of the wing remained intact to be able to see the critical vein M3. On Phycitodes (and Homoeosoma nebulella) vein M2 forks off from vein M3 well away from the end of the vein (less than 2/3 distance from the base) whereas in nimbella it forks beyond 2/3 or vein M2 is absent so M3 is not forked (note veins M2 and M3 are sometimes referred to as vein 4 and vein 5). Size can be helpful as nebulella averages larger then nimbella but there is overlaps (I probably did measure these but unfortunately failed to keep a note of the size). It has been suggested that nebulella shows a more prominent dark costal shadow but I am not convinced about this.

The first individual shown below was a female. It was my first year doing dissections and you can see I didn't do a good job of this one! The forewing venation shows that there was no vein M2 forking off from vein M3. My page for Clouded Knot-horn Phycitodes nebulella shows how vein M2 forks from vein M3 forks in that species.

female Sheep's-bit Knot-horn Homoeosoma nimbella showing genitalia (3 bits not to scale, the third being the signa detached from the corpus bursae) and tips of each forewing cleared of scales to show venation, Winterton (Norfolk, UK), 11th July 2015

I didn't photograph the venation of the two males but my notes imply that I checked the wing venation on all three. It looks like the first one had died by the time I photographed it.

male Sheep's-bit Knot-horn Homoeosoma nimbella showing genitalia, Winterton (Norfolk, UK), 11th July 2015

second male Sheep's-bit Knot-horn Homoeosoma nimbella, Winterton (Norfolk, UK), 11th July 2015