They walk down the trunk to the sap and feed head downward.
The mourning cloak, like most caterpillars, has 5 stages or "instars," growing bigger and shedding their outer skin with each one. Mourning cloak butterfly pupae. The underside of their wings are dark with thin irregular lines. They will also feed on rotting fruit, and only occasionally on flower nectar. The border is duller than the upper side and the blue spots are more V … The Mourning Cloak is a species of large butterflies that are found in a wide range across North America and Eurasia. On the inside edge of this border are bright blue elongated spots. If you miss the chance to collect them as eggs, then just look for the caterpillars. They migrate over long distances, and hence, can often be located even in regions beyond their original range.

Adult Food: Mourning Cloaks prefer tree sap, especially that of oaks. The adult butterflies, known as the mourning cloak, are large attractive butterflies, with a yellow-bordered purplish-brown wing.
One species, the spiny elm caterpillar (larva of the mourning cloak butterfly), is reported to possess urticating spines.” According to the Auburn University webpage on Stinging Caterpillars : “Several species of nymphalid larvae bear conspicuous bristled or multi-branched spines similar to those found on some stinging caterpillars. The entire larval stage is about 2-3 weeks (depending on the weather and food availability.) Adults - The upper wings of the Mourning Cloak Butterfly are red-brown with a thick yellow border along the edge. It is one of the few butterflies that spend the winter in the adult stage, hidden behind bark flaps and other protected sites.