62.056 Homoeosoma nimbella (Sheep's-bit Knot-horn)
ws: 16-21mm (Goater), fw: 8-11mm (Sterling & Parsons); May-Aug; sheeps-bit (Jasione montana); pRDB - sand-dunes, sea cliffs and stone walls on the coast in Norfolk, Dorset and Cornwall (according to the distribution map in Sterling & Parsons)
ID: H.nebulella, H nimbella and the three Phycitodes species all have a very similar forewing pattern. See Homoeosoma & Phycitodes for details on separating them. H.nimbella is similarly sized to P.maritima and P.saxicola; H.nebulella and P.binaevella being larger (though there is some overlap). H.nebulella is said to have a more prominent dark costal shadow especially in the distal half of the forewing than is shown by any of the other species, but this is a relative rather than absolute distinction. Examination of the genitalia will readily determine the genus - the only confusion species is then H.nebulella which has almost identical genitalia to H.nimbella. These two may be separated on the following features, H.nebulella given first:
Goater also mentions and illustrates the venation of H.nimbella (p129) - but uses this to separate it from the two smaller Phycitodes species (which are more easily excluded by the genitalia) leaving it unclear whether the venation can be used to exclude H.nebulella.
Discussion: If a forewing length of 7.8mm reliably excludes H.nebulella then the genitalia confirm §1 as H.nimbella. The specimen was taken at a known site for this species. Accepting this identification means that H.nimbella can have a fairly prominent dark costal shadow (as has been accepted by Norfolk Moths) and that Sterling and Parsons are incorrect in giving the apophysis ratio for H.nimbella as 1:3 (as appears to have been accepted by Dissection Group). Note, however, that the image shown at Norfolk Moths appears to have been accepted on the basis of the venation, the size of the specimen shown is not stated and it is not clear to me that the venation excludes H.nebulella.
- Size: ws 20-27mm vs 16-21 mm (Goater); fw 9-13mm vs 8-11mm (Sterling & Parson). So an almost reliable ability to separate on size according to Goater becomes a considerable overlap in size range according to Sterling & Parsons
- Black costal shadow prominent vs weak (however, the image of H.nimbella at Norfolk Moths shows a prominent costal shadow)
- Ventral margin of valva without a distinct central projecting lobe? vs with a small central projecting lobe
Goater also mentions and illustrates the venation of H.nimbella (p129) - but uses this to separate it from the two smaller Phycitodes species (which are more easily excluded by the genitalia) leaving it unclear whether the venation can be used to exclude H.nebulella.
Discussion: If a forewing length of 7.8mm reliably excludes H.nebulella then the genitalia confirm §1 as H.nimbella. The specimen was taken at a known site for this species. Accepting this identification means that H.nimbella can have a fairly prominent dark costal shadow (as has been accepted by Norfolk Moths) and that Sterling and Parsons are incorrect in giving the apophysis ratio for H.nimbella as 1:3 (as appears to have been accepted by Dissection Group). Note, however, that the image shown at Norfolk Moths appears to have been accepted on the basis of the venation, the size of the specimen shown is not stated and it is not clear to me that the venation excludes H.nebulella.
Dissection
Female genitalia
§1 Winterton, Norfolk; 09/07/2015; female; fw 7.8mm; to light
All images © Chris Lewis
All images © Chris Lewis
Page published 01/01/2016 (§1)