Lio's featured the cartoon Pastis at least once before (Pearls Before Lio). There are all kind of subtle things going on here. First, no one in Lio talks -- or uses word balloons of any kind. The fact that Pastis does so marks the character as one coming from outside the confines of the strip. Second, he final gag has Pastis's head stuck on a duck's body. A reference, perhaps, to Lil' Guard Duck, another character from Pears? Third (and the point of this post) the unhappy Pastis threatening to complain to the strip's syndicate.
What makes this doubly interesting is that in Pastis' own strip on the same day, the cartoon Pastis also threatens to complain to the strip's syndicate. Coincidence? Or is there some kind of subtext here?