Let's say that the jar sits for a few weeks. Since there wasn't sterile conditions in the jar, what with all the flies and such, that eventually the flies will die and fall to the bottom of the jar, where over time, bacteria will multiply, due to the process of decay, and therefore will increase the mass of the inside of the jar, assuming that they will increase in mass more than the decrease in mass of the fly due to the fact that the bacteria are eating it. Also, bacteria and decay produce gasses, so the pressure inside the jar should increase, thus increasing the mass in the jar. So, regardless of how hard those little guys fly around, eventually they have to croak, and then for awhile anyway, they will weigh more. Sure, that's it...