The Amazonian Giant Leech, otherwise known as Haementeria ghilianii, is the biggest leech in the world by an impressive margin — it can grow up to a whopping 18 inches (46 cm) long. Leeches live in wet, humid places all over the world, and while the majority of them are no bigger than a normal person’s index finger, some of them can grow to massive sizes.
The Haementeria ghilianii species in particular is so large it’ll probably give you nightmares. This elusive leech only resides in some parts of Brazil and in French Guyana, so unless you live in or are visiting one of those countries, you’ll probably never see one in the flesh. Still, just looking at photographs of this slimy beast is enough to give you goosebumps. Although the majority of these critters are 30-25 centimeters long, the unusually big ones can reach up to 46 centimeters.
Italian naturalist Vittore Ghilliani discovered the Amazonian Giant Leech in 1849, and ever since then, it’s been regarded as a near-mythical creature due to its abnormal size and appearance. While there isn’t recorded scientific proof that backs this up, there are plenty of legends and folklore about these oversized leeches drinking all the blood out of an adult human in just hours.