Invertebrates
Leech
Hirudinea · also called leeches
A leech can drain several times its own body weight in blood from a host that never even feels the bite, thanks to an anaesthetic and an anticoagulant delivered together in its saliva. That same trick has kept leeches in genuine use in modern operating rooms, applied after delicate reconstructive surgery to keep blood flowing through reattached tissue until the patient’s own veins can take over.
Overview
Leeches, class Hirudinea, are segmented worms with a sucker at each end, found in fresh water, damp forest floors, and some marine habitats on every continent except Antarctica. Many species feed on blood, while others hunt small invertebrates or scavenge dead animal matter.
A painless, well-engineered bite
Blood-feeding leeches inject a local anaesthetic and an anticoagulant through their saliva, letting them feed undetected for as long as needed and taking in several times their own body weight in a single meal that can sustain them for months.
Still useful in the operating room
Medicinal leeches remain in real clinical use today, applied after reattachment surgeries to relieve dangerous blood pooling in reconnected tissue until the patient’s own small veins heal — a genuinely modern application of an ancient medical practice.
Status
Least Concern — common and widespread across a huge range of wet and humid habitats worldwide, with only isolated local declines tied to wetland loss or pollution.
Leech: common questions
Why don't you feel a leech bite when it happens?
A blood-feeding leech's saliva contains a local anaesthetic compound that numbs the skin around the bite site, along with an anticoagulant that prevents the blood from clotting so the leech can feed steadily for as long as it needs. Together these substances mean a person or animal bitten by a leech very often does not notice the bite occurring at all, and may only discover the leech, or the small wound it left behind, sometime after it has already finished feeding and detached on its own.
Why are leeches still used in modern medicine?
After certain reconstructive surgeries, such as reattaching a severed finger, ear, or other tissue, the tiny veins responsible for draining blood away from the area are often much harder to reconnect successfully than the arteries bringing blood in, which can cause dangerous blood pooling and tissue death in the reattached part. Medicinal leeches applied to the site continue to draw blood and, through their anticoagulant saliva, keep local circulation flowing, relieving that pressure and buying time for the patient's own small veins to heal and reconnect, a genuinely effective and still-used application of a very old medical practice.
How much blood can a leech actually drink?
A blood-feeding leech can take in several times its own body weight in a single meal, swelling noticeably as it fills, and a feeding session can last anywhere from several minutes to a few hours depending on the species and how undisturbed the leech is while attached. Because leeches digest such a large blood meal very slowly, aided by symbiotic bacteria living in their gut, a single big meal can sustain a leech for months before it needs to feed again, which is markedly different from most blood-feeding parasites that need to feed far more frequently.
Do all leeches feed on blood?
No, while blood-feeding leeches are the best known and most commonly discussed, a substantial number of leech species are instead predators that actively hunt and eat small invertebrates such as worms, snail, and insect larvae, and some species will also scavenge on dead animal matter when the opportunity arises. Leeches as a whole are a diverse group with a range of feeding strategies, and blood-feeding, though prominent in popular perception, is only one of several ways different species in the class make a living.