In the episode "Memories of Champions", Corona explains that the very first Spider Rider was called King Arachna the I. The real 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 necessarily 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 showed 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.
Some of the characters actually due use that phrase a few times in the show.
When you think about it, it's actually kind of frightening how quickly Hunter willingly restorts to killing sentiant beings. Let's remember that this is a 11 year old (13 in japan) child that's doing all this fighting. But what's really creepy about all of this is the fact that he doesn't really freak-out about all the killing he's done until right up to the end of the series.
While the plot element is not played up to it's full potential, The battle bug in the episode "Wages of War" becoming more powerful and violent the more Katie resists and is applaud by the violence is downright terrifying if you think about it.
The scream Aqune lets out when her new mask attaches itself to her face in "Reunions" is chilling.
Mantid used to be a human, but was somehow able to pass himself off as an Insector for about a hundred years, all for the sake of personal vengance. Now that's frightening.