This species occurs in a huge range of colour types and is one of the most variable species in the family. Forewing colour ranges from very pale straw-coloured to very dark brown, with the pale forms predominant in the south of the range and the darker forms prevalent further north.
- See glossary for terms used
The wingspan is 28–36 mm. "Forewing with basal area grey, outer area red-brown; the lines and stigmata grey; hindwing dull grey with a dark lunule and transverse line and the fringe reddish; as a rule the male is paler than the female ; a very variable species alike in colour and plainness of markings; ab. congener Hbn. is wholly red-brown with the stigmata yellowish; -ab. subrufa Haw. is distinguished by having the cell dark at base and between the stigmata; — ab. conflua H.-S. nec Tr. has the ground-colour reddish ochreous mottled with darker; — ab. mendica F. is like the last but shows black or dark brown spaces in the cell; — ab. ignicola H.-Sch. has the ground-colour yellowish ochreous, with the markings distinct; — primulae Esp. is the same form but with the cell filled in with blackish; — ab. ochrea-virgata Tutt is a banded form, the whole space between the median and subterminal line being darker; — ab. grisea Tutt is a small dark grey form, the stigmata clear; — ab. coerulea Tutt is of a clear slaty or lilac colour with pale stigmata; — lastly, ab. lamentanda Alph. is a pale uniformly ochreous form from Siberia and Kamschatka. subsp. conflua Tr. (= thulei Stgr.)is smaller, with narrower, more pointed forewing, reddish brown varied with yellowish; the stigmata large with dark marks in cell; described originally from Iceland and occurring in the Shetlands and Lapland; — ab. borealis Zett. is a grey brown form without any red but with the cell blackish; obsoleta Tutt resembles borealis, but the cell is unicolorous and the markings obscure; - ab. diducta Zett. has the forewing grey with a broad rusty-testaceous fascia, and the markings obscured..
The larva is brown with black triangular markings along the side. It feeds on a wide range of plants (see list below). The species overwinters as a larva.
This moth flies at night from June to August  and is attracted to light.
Recorded food plants
- Calluna - Heather
- Campanula - Harebell
- Crataegus - Hawthorn
- Lactuca - Lettuce
- Rubus - Bramble
- Rumex - Dock
- Salix - Willow
See Robinson, G. S. et al.
- D. m. borealis - Scandinavia, north and central Russia, Iceland.
- D. m. mendica - Central and Northern Europe, except in northernmost Fennoscandia.
- D. m. orkneyensis - Orkney
- D. m. thulei - Shetland
- D. m. lamentunda- Siberia, Kamchatka, Mongolia, China and Tibet.
- D. m. monochroma - Alborz and other ragions of Iran.
- undescribed subspecies in North oTurkey, Armenia, the Caucasus and other regions of Transcaucasia.
- 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
- "Robinson, G. S., P. R. Ackery, I. J. Kitching, G. W. Beccaloni & L. M. Hernández, 2010. HOSTS - A Database of the World's Lepidopteran Hostplants. Natural History Museum, London.".
- 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
|Wikimedia Commons has media related to Diarsia mendica.|