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.
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, northern 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.|