Polynomial monads and delooping of mapping spaces

We extend some classical results - such as Quillens Theorem A, the Grothendieck construction, Thomasons Theorem and the characterisation of homotopically cofinal functors - from the homotopy theory of small categories to polynomial monads and their algebras. As an application we give a categorical proof of the Dwyer-Hess and Turchin results concerning the explicit double delooping of spaces of long knots.