High wings and structural strength

When I look at pictures of high-wing aircraft, I don't see anything
between the wings that would provide structural strength--in some
aircraft there's even a window in the top of the cockpit between the
wings. What provides rigidity and structural strength in high-wing
aircraft? I somehow expect a sturdy metal beam across the wings on
top to withstand the stresses put upon them, but instead they seem to
be glued onto the skin of the fuselage on either side. How do such
planes hold together?

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