Is it true that I must subtract the weight of my ultralight wing from the gross weight of my airplane inorder to determine the wing loading of my ultralight airplane? If yes,why is this so? I read a pdf sometime ago stating that the weight of the wing is negligible when determining the wing loading.
The wing weight may or may not be negligible (usually not), but "wing loading", as a term, is simply defined with respect to the total weight. Wings have to carry all the weight, including their own, anyway.
Wing loading is a simple integral figure for the whole aircraft, and we use whole weight and whole lift for it.
Wing weight matters when we calculate its bending moment, but then we don't use the integral wing loading figure: we need the actual spanwise lift (and weight) distribution.