In the episode "Memories of Champions", Corona explains that the very first Spider Rider was called King Arachna the I. The reason the characters use the words "Arachna Power" when they call upon their armor? It's possible they incorporated his name into the mantra in his honor.
The reveal that Mantid is human shouldn't come as a shock if you pay close attention to when Hunter and Mantid first meet in "Afraid of the Dark". At one point, Mantid told Hunter that the Invectids have a world where the sun rises and sets. Only the Oracle Sun doesn't actually rise or set in the Inner World, nor have any of the humans of the Inner World ever used the phrase. But the one above the surface world does.
Though, some of the characters actually do use that phrase a few times in the show.
When you think about it, it's kind of frightening how quickly Hunter resorts to killing sentient beings. Remember that this is an 11-year-old (13 in Japan) child that's doing all this fighting. But what's really terrifying about all of this is the fact that he doesn't freak-out about all the killing he's done until right up to the end of the series.
While the plot element isn't played to its full potential. The battle bug in the episode "Wages of War" becoming more powerful and violent the more Katie resists is downright terrifying, if you think about it.
The scream Aqune lets out when her new mask attaches itself to her face in the dub of "Reunions" is chilling.
Mantid used to be a human, but was somehow able to pass himself off as an Insector/Invectid for a hundred years, all for the sake of vengeance.
Why did the Oracle create the Oracle Keys in the first place? They don't appear to be good for anything except giving people the ability to steal her own power from her.
According to Quake/Brade, the Oracle used the keys to bring sunlight to the cavern that eventually became the Inner World. Though he doesn't explain exactly why she needed to divide that power into the keys.
