Cross-Cast Role: Averted. Rather than have one person play Marty's male and female forms, producers wanted two actors. Kate Twa was hired first, so casting looked for a man who could reasonably resemble her.
Crop Circles: After the aliens leave at the end of the episode, Mulder and Scully come across a large crop circle in the field. It's implied it's a print of their spaceship.
Death by Sex: Sleeping with the aliens is almost inevitably fatal to the human.
Monster of the Week: Even though the episode featured aliens (apparently), this was the only time when they made an appearance and they were not a part of the mythology.
Ninja Pirate Zombie Robot: There's so much going on with the Monsters of the Week that it gets a bit overwhelming. They're human-looking aliens who have no resemblance to any other aliens in the show, who switch genders after they have sex, and also kill their sex partners. And they're not very good-looking, but their touch can seduce. And there's something going on with the clay they use. Oh, and they're immortal. And as if all that wasn't enough, they have a No Communities Were Harmed Amish schtick going on as well.