Note the somewhat complex mix of painted aluminum alloy, painted composite (fiberglass) and polished Alclad that has distinguished the DC-9 / MD-8X / MD-9X / 717 family from the beginning. The triangular pattern of black lines is for icing inspection - ice, if present, will be visible on the high contrast markings. Note the safety line anchor just a bit further out, and the red fuel tank cap toward the end of the wing. Of course the whole fuel system can be filled from a single point pressure connection, or drained by the same. Its under the starboard wing leading edge. But if necessary, no pressure, gravity feed can fill it up too.