Oh my god. This monster actually has origins in biology. I never expected to see it here.
The Ant Arachne monster girl is based on an actual species of spider that looks so much like an ant that it'll actually sneak into ant hills and kill the queen. Kinda like a spider ant assassin.
But really, they rarely kill the queen this way. Most spiders that look like ants just live nearby the ant colonies and sometimes sneak in to steal food, but avoid contact with the other ants for fear of discovery. The similarities to ants are really a defense to make them seem less appetizing. Most of the spiders in the species are actually pretty timid compared to the actual ants they resemble. Quite the opposite of the Ant Arachne monster girl.