This was a new one on me. I caught it wandering across the bottom of our front door after a rainstorm, and quickly grabbed some photos of the “black worm”. Imagine my surprise when I saw these beady lil eyeballs! Meet Platydemus manokwari, the New Guinea flatworm.
Googling it turned up terrifying articles with headlines like “Toxic Worms Invade Florida“. The New Guinea flatworm, apparently, is originally native to New Guinea but has traveled around the world in potted plants (and even been introduced in some places to control invasive snails), and came to Florida a few years ago. It’s not really dangerous to humans in and of itself — it’s just a three-inch-long worm that eats primarily snails and other invertebrates — but it is a potential host for rat lungworm (Angiostrongylus cantonensis), which can be transmitted to humans. Hence the scary articles, which conveniently forget to mention that native snails, frogs, and other creatures can also carry rat lungworm, and that you can only catch lungworm by eating the critter carrying the parasite, so, unless you’re the type to eat three-inch-long black worms, you’re probably going to be okay.
New Guinea flatworms do exude an allergenic slime, so it’s best not to handle them bare-handed, and, since they are hermaphroditic, even one in your yard has the potential to make many more. For these reasons, a lot of the articles I found end with various ways to kill the flatworms, including pouring boiling water on them (which kills the worms but not other animals/plants in your garden) and by putting them in the freezer overnight. (That’s what I did with this one. Rat lungworm can infect pets, and there’s almost nothing my dog won’t try to put in her mouth.)
A lot of the (circa 2016) articles suggest that you contact the FWS or a local university to report the finding; this is no longer relevant, unless you live outside of the flatworm’s known range in the US.