 The decision to center another episode around Skinner — following the third season episode " Avatar " — was made when Gillian Anderson took a week off to shoot the film The Mighty . Rather than have Duchovny carry the additional workload , the writing staff decided to focus the episode on supporting cast members . " Zero Sum " was the final appearance in the series by Morris Panych . In addition , the episode saw the return of the virus @-@ carrying bees from the season @-@ opener " Herrenvolk " and would later return in the 1998 feature film . 
