Marsh Marigold Moth

Pupae are brown with dark oculi and paler abdominal segments. Adults are small, with a forewing length of up to 4.6 millimetres (0.18 in) and a wingspan of 8–10 millimetres (0.31–0.39 in). The species is more plainly marked than many other European species of Micropterix, and has dark bronze-coloured (or pale shining bronzy-golden) forewings with a purple sheen in places and distinctive purple bases. The hindwings are rather dark bronzy-grey. The head is deep ferruginous-yellow.

Eggs are laid amongst vegetation on the soil surface. They are translucent white turning grey, prior to hatching. The larvae feed on tender, young growth of herbacaeous plants and pupate in a tough, silken cocoon. Like other members of the family, Micropterix calthella has functional jaws in its imaginal stage. As adult, it feeds on pollen grains from a wide variety of plants such as maple (Acer species), bugle (Ajuga species), Caltha, Cardamine, sedge (Carex species), hawthorn (Crataegus species), mercuries (Mercurialis species) and buttercup (Ranunculus species). The adults possess sociable behavior.