Well, please revert to your H.S. Physics....When you add "anything" to a container, this time a wing cavity pf a given volume, you displace the air inside with that "anything".....

Given the geometry of the wing is fixed, you can either displace the air with spars/wing ribs/shear webs, or spars and foam....either way your filling the space with "something"...and yes, air does have a weight....

Cheers...