Escape without breaking the cocoon

Wood found an emperor moth in its cage, though the cocoon looked unbroken. He thought it had come from elsewhere and waited for the cocoon to open. A year later, he cut it open: inside were an empty pupal shell and the caterpillar’s shrivelled skin. At the neck, stiff crossed fibres yielded to pressure from within but pressed together under pressure from outside. The moth had escaped without leaving an obvious break.