Then they go down to the ground, spin a cocoon and emerge from it to become fully developed moths in the spring.

I spin a cocoon around myself; let others do the same.

When full-grown, the larvae lower themselves to the ground by a thread and spin a cocoon among the debris where pupation takes place.

When the larvae mature, they spin a cocoon and pupate within the host gallery.

One then spun a cocoon (a paper bag) and came out a butterfly.