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Colias croceus, clouded yellow, is a small butterfly of the family Pieridae, the yellows and whites.
Colias croceus has a wingspan of 46–54 millimetres (1.8–2.1 in). The upperside of the wings is golden to orange yellow with a broad black margin on all four wings and a black spot near the centre forewing. Usually these butterflies settles with its wings closed, consequently the black margin of the uppersides of the wings is difficultly visible.
The underside lacks the black borders and is lighter, with a more greenish tint, particularly on the forewings. In the forewing underside is the same dark spot as on the upperside, but often with a light centre; the hindwing underside has a white centre spot, often with a smaller white or dark dot immediately above it. Sometimes, a row of black dots occurs on the underwings' outer margins, corresponding to where the black border ends on the upperside. Females differ from the males in having yellow spots along the black borders on the upperside.
In flight, Colias croceus is easily identifiable by the intense yellow colouring, much brighter than that of the lemon-yellow male common brimstone which also lacks black markings. Like all Colias species they never open their wings at rest.
In about 5% of females, the golden upperside colouration is replaced by a pale cream colour. These females have been distinguished as form helice. The pale form helice does not seem to be distinct, as intermediates exist, and the variation is to some extent related to humidity during development, with dryer conditions producing paler colouration. These pale forms helice can be confused with Berger's clouded yellow (Colias alfacariensis) and the rarer pale clouded yellow (Colias hyale). Even the palest C. croceus tends to have more black on the upperside, however, in particular on the hindwings.
Young caterpillars are yellow-green with a black head. Later they become completely dark green, with a white red-spotted lateral line after the third moult. The pupae are green and have a yellow side stripe.
This species is similar to Colias myrmidone, Colias chrysotheme, Colias erate, Colias hyale, Colias alfacariensis, Colias caucasica, Colias aurorina.
