If you have spiders in your mouth, then you're going to instigate fights between people. Mostly because the spider in your mouth makes you grumpy, but also because the spider keeps telling you to be an instigator with its little mouth inside your own mouth.
The spider in your mouth tells you to instigate a fight by sawing (さ) the other person's leg off. It even whispers where you can find a nice big saw.
Currently there are no similar looking kanji which exist in the WaniKani learning system