Poisonous little beasts

Write about a dangerous insect or animal in your world.
These creatures have stabby little teeth and don't like to let go once they latch on. They're relatively harmless by themselves, but they're rarely by themselves. They hunt in packs, generally moving through piles of refuse. These creatures make scrapping and scrounging particularly dangerous. Maybe someday they'll be named, but for now, they're generally referred to as land piranhas.