“What is this?” asks Cyndi in her submission regarding the long, black worm with a gray striped down its back pictured below. “It’s fascinating! I found it crawling on my window and along the window sill. It lifts its head moving forward. It almost appears to have eyes. It only moves forward (front is left to right). I appreciate the help!” Based on the fantastic photo Cyndi sent us, we think she has found a New Guinea flatworm. As their name suggests, this species of flatworm is endemic to New Guinea (the island off the coast of Australia), and is considered an invasive species in other countries. Besides concerns about how their presence will disrupt the biodiversity in the habitats they are introduced to, New Guinea flatworms do not pose much of a threat to humans, despite popular anti-flatworm rhetoric.
New Guinea flatworms get an especially really bad rep on the internet. We even wrote an article on how “online publications use flatworms as clickbait” to needlessly spread fear about these creatures. It is true that they are an invasive species, and significant numbers of them could negatively affect populations of endangered snails, and other at-risk invertebrates, but are they some dangerous, lethal monster like so many publications want to claim? No. The most they can do is secrete a toxin which, while being lethal to the small prey they hunt, would only cause a mild allergic reaction in humans (like skin irritation or stinging). Another problem many bring up is the New Guinea flatworm’s capability to be a host for the rat lungworm parasite, which is a real threat. However, the only way to contract this parasite from a New Guinea flatworm is to eat it. So, unless anyone has a taste for worms, this is not a concern for humans.
That said, pet owners would be understandably concerned about their dog, cat, or other animal eating the worm and contracting the parasite. So, if Cyndi does have a pet she is concerned about, then we do recommend moving the flatworm away from her property. We also recommend doing this without making direct, skin-to-skin contact, given the possibility of an allergic reaction. She can achieve this by scooping the worm onto a dustpan with an oblong object, or by handling the worm with thick gloves (like gardening gloves). If Cyndi does not have a pet and has no other concerns about this worm, we suggest just leaving it be!
In conclusion, we think Cyndi found a New Guinea flatworm. They are land predators that are concerned solely with finding their next meal, and have no reason to want to bother humans or pets, as they are much too big for prey! We hope this helps, and we wish Cyndi the very best.
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Where I live, rat lung worm can be a serious problem. A huge percentage of the flat worms, slugs, semi-slugs, and snails carry the parasite. No one is inclined to eat them intentionally, however, they can be hidden in garden produce. The semi-slugs can be incredibly small and missed when washing lettuce and other greens normally eaten raw. We’ve even had problems with food prepared in restaurants and stores.