This ant.

(The scale is in mm)
I found this ant in my house and I was really, really, really concerned that there was a nest nearby. I didn't find one, so I can only hope it's a stray that came in on someone's load of wood.
This is the bulldog ant. Its sting will cause anaphylactic shock in people who are susceptible. They are really aggressive and have good eyesight. The jaws are like being pinched by a pair of scissors. And the sting in the tail...
Oh boy. That sting.
It rates 4/4 on the Schmidt sting pain index which is described as "...stings induced immediate, excruciating pain and numbness to pencil-point pressure, as well as trembling in the form of a totally uncontrollable urge to shake the affected part.". By my sale it hits 10, with a ramp up to 11 with every heart beat. And it lasts for hours.
I live in Australia, where we embiggen the dangers in the bush. Crocodiles and box jellies are a bit of a worry, but I can avoid them. I'm wary of venomous snakes. I don't like spiders much.
But this ant truly frightens me.