First NBC Nightly News anchor Brian Williams started making regular guest appearances on 30 Rock. Then CNN's Anderson Cooper had a bit part in a digital short for Saturday Night Live. Now CBS Evening News' Katie Couric is about to do a guest role on Glee. It seems we're in the midst of a highly specific micro-trend within the broader boom of star cameos on television series: serious newscasters appearing on comedy shows.

Here, an exploration of this new sub-genre: