This is a variable species, the forewings ranging from pale buff to dark brown although the paler forms are generally more common. The forewings are usually marked with two dark stigmata and a black streak near the apex. The hindwings are whitish marked with a dark moon-shaped crescent on the hindwing is partially diagnostic and which gives the moth its English name.
The wingspan is 32–38 mm.Forewing varying from pale greyish ochreous to rufous and dark purple brown, the veins generally pale; upper stigmata distinct and filled up with dark, with distinct pale outlines; inner and outer lines double with paler centres, the inner obliquely outcurved, the outer simply sinuous; submarginal line pale, preceded by a row of dark spots, and on costa by a black blotch or bar; hindwing whitish, suffused with grey, especially in the female, with large grey cellspot and interrupted submarginal band. The type form is dull red without pale veins; the red forms with pale veins are ab. rufa Tutt;the palest form of all is obsoleta Tutt without pale nervures; a somewhat darker, more brownish form, with the veins pale is humilis H. & Wwd.; brunnea Tutt has the ground colour deep redbrown with pale veins; in subjecta Dup. it is dark grey brown, the extreme development of which, agrotoides Guen., is blackish grey; — olivacea Vasq., from Spain, has the forewing pale olive ochreous, and rubra Vasq., also from Spain, has it pale reddish ochreous. .
- Seitz, A. Ed., 1914 Die Großschmetterlinge der Erde, Verlag Alfred Kernen, Stuttgart Band 3: Abt. 1, Die Großschmetterlinge des palaearktischen Faunengebietes, Die palaearktischen eulenartigen Nachtfalter, 1914
- Chinery, Michael Collins Guide to the Insects of Britain and Western Europe 1986 (Reprinted 1991)
- Skinner, Bernard Colour Identification Guide to Moths of the British Isles 1984
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