Montana designated the lovely mourning cloak butterfly (Nymphalis antiopa) as the official state butterfly in 2001. Often the first butterflies to emerge in the spring (frequently before the winter snow has melted), mourning cloaks rest on tree trunks and turn their dark wings toward the sun to absorb heat for flight.

Mourning cloaks belong to one of the larger families of butterflies - Nymphalis - known as "brush-footed butterflies" because of their small, hairy, brushlike front legs. The common name "mourning cloak" refers to the wings resembling a traditional dark-colored cloak worn when one was in mourning.

They lead a long life for a butterfly, living as long as ten months as adults. Mourning cloak caterpillars are velvety black with raised white dots and rows of red spots and spines down the middle of their backs (they can grow up to two inches long ).

Source:

State Butterfly - Mourning Cloak: MontanaKids

Links:

State Insects - 50 States List