Abstract: A structure for forming an open-air enclosed space is described. The structure includes an umbrella, having a plurality of ribs supporting a cover and a tightly woven netting arranged to hang down from the perimeter of the umbrella when the umbrella is in an open position, to form the open-air enclosed space. At least some of the ribs are further provided with rib extensions arranged to protrude beyond the perimeter of the cover and to support the netting in order to create an extended footprint of the open-air enclosed space. An add-on assembly for umbrellas for forming an insect-protected open-air enclosed space is also described.