Mostly people call them "cuckoos"...wait, no. That's not right. Mostly, people don't call them anything at all, because mostly, no one realizes that they exist. The Johrlac are sneaky, telepathic ambush predator nest parasites who slip into your life like a knife slips into a wound. You never see them coming. By the time the walls start crumbling around you, it's already far too late.
They bleed clear, biological antifreeze that possesses antibiotic properties. Some people say this is proof that the cuckoos came from another dimension, that they didn't evolve in this world, under this set of evolutionary rules. Others say this is proof that the cuckoos are so mean, so dangerous, that they even kill bacteria. Medicinal properties fueled by spite. (The question of how the Johrlac can maintain sufficient intestinal flora to digest things has never been answered, and no one is entirely sure they want to know.)
They hate you. Don't take it personally; the cuckoos hate everyone, including each other. Baby cuckoos are conceived in hormonally-fueled hate, and dropped summarily on the doorsteps of the unwitting, because mother cuckoos even hate their children. Love is an alien weakness, and should never be tolerated. When cuckoos work together, it's because they've decided there's something more important than their hate. Be afraid.
They love math. No one knows why, except for maybe the cuckoos themselves, but it seems to be a biological compulsion for them. The Sudoku craze owes its success partially to the manipulations of the Johrlac. Look for them in math classes and in science museums, or even volunteering to help tutor children. The one time that cuckoos interact peacefully with humans is when math is involved. Don't trust it. Their attention span is short, and eventually, they'll forget that you were just helping them with their homework in favor of destroying your world.
They can read your mind. Johrlac are natural telepaths, and they're not above breaking past all your defenses to get what they want out of the recesses of your brain. They have no ethics. They have no qualms. They have no mercy.
Be afraid of the cuckoos. It might help you live a little longer.