The Kids' Page: Learn about Leeches

June, 2015


 A small leech attached to a man’s thumb.  Photo: Charles Haynes, CC.  Did you know that leeches have 32 brains?

Leeches are a type of segmented worm, like an earthworm.  They have 34 segments, and 32 of those contain a brain.  

There are over 700 species of leeches in the world.  Most of them live in slow moving freshwater, like ponds and marshes.  There are about 100 species that live in the ocean and even some that live on land.  Some leeches eat decaying leaves and logs, or insects and worms, but most eat blood.  
They have several sets of eyes, depending on species, but they do not use these eyes to hunt prey.  They can sense light but not see clear pictures, similar to if you close your eyes.  Leeches use their sense of smell, and are also able to sense vibrations, which help them find their prey.  Once their prey is located they attach their suckers.  They have suckers on both ends of their bodies.  

A leeches bite contain a painkiller so it is not painful, and most of the time it isn’t noticed.  Once attached, the leech releases a chemical that prevents blood from clotting so they can continue feeding until they’re full.  Leeches feed on mammals, fish, reptiles, amphibians, and birds.  Leeches do not carry diseases, but the host can run the risk of infection from the attachment site.  

Leeches are able to eat up to 10 times their weight! When they are done feeding, the leech simply falls off the host.  They can go up to a year without feeding again.  

They range in size from the width of a pencil eraser up to the height of a construction cone.  The largest leech is the Amazonian giant leech, which can grow up to 18 inches and live up to 20 years.  That leech feeds by injecting a long (up to 6 inches!) needle-type mouthpart into its host in order to suck its blood.  

Leeches are also used in medicine.  Doctors use leeches to stimulate blood flow to parts of the body after surgery, in particular after finger reattachment.  Medical leeches are grown and kept in a clean laboratory environment; they are not caught from the wild.

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