Nature Bulletin No. 202-A October 16, 1965
Forest Preserve District of Cook County
Seymour Simon, President
Roland F. Eisenbeis, Supt. of Conservation
Leeches, or "bloodsuckers", have such conspicuous characteristics and
habits that if you only know one kind it is easy to recognize others.
They are predatory or parasitic segmented worms related to the
earthworms, and should not be confused with the land slugs which
have no suckers, live entirely on plants, and are snails without shells.
All leeches live in water except a few which are found in moist soil in
warm climates. There are dozens of kinds in the ponds, ditches, lakes
and streams of this region, but they are most numerous in quiet
shallow water with a soft mud bottom.
Adults of the different species range in size from scarcely a half-inch
(some of those that live on fish) up to 4 or 6 inches in the spotted
"horse" leech and the American Medicinal Leech. In early days, the
European Medicinal Leech was shipped to America in quantities. A
few escaped to natural waters and have become established in New
York state. Blood-letting by leeches has been a common medical
practice in many parts of the world since ancient times. They were
used, in millions, so universally that physicians and surgeons are still
called "leeches" in the dictionaries. The saliva which a leech injects
into the wound it makes, contains a substance called "hirudin" which
prevents clotting of the blood, and this is used occasionally in surgical
A leech has a large muscular sucker on the tail end and a smaller one
on the head, around the sucking mouth which, in some kinds, is armed
with jaws. There are one or more pairs of eyes -- tiny black spots -- on
the head and, believe it or not, a few kinds also have a pair of eyes on
the tail sucker. They are very sensitive to shadows passing over them,
to the slightest vibrations in the water, and to small changes in the
flavor of the water about them.
All leeches are contortionists. Some can stretch out as long and
slender as a toothpick one moment, and be contracted into a tight ball
the next. Their usual method of travel is by "looping": the worm
stretches out and takes hold with its head sucker; then it lets go with
the tail sucker, contracts, and attaches the tail sucker near the head.
Many kinds swim with a serpentine movement like miniature eels.
In leeches, as in earthworms, pairs of animals mate even though each
one has a complete set of male and female organs. The eggs of one are
fertilized by the sperm cells of the other. They lay their eggs in spring
and summer, usually in a horny capsule or cocoon which is glued to
stones, plants or trash. However, one family of leeches fastens these
egg capsules to their undersides, where the young hatch out and cling
to the parent with their heads hanging free.
Their feeding habits are curiously varied. A leech may feast on a snail
at one meal, and suck the blood of a turtle at its next one. Their staple
foods are small mollusks, insect larvae, earthworms, and the blood and
body juices of larger animals. Some are scavengers.
While leeches are only a minor annoyance to bathers in this country,
they are a serious problem in some parts of the tropics. Our soldiers in
World War II came to fear and abhor them in New Guinea, Burma and
the Philippines. One kind lives in the springs and wells of the Near
East, and there its young enter the mouths and nostrils of men and
beasts to grow and seriously interfere with breathing.
At Maple Lake, in the forest preserves, a large brown leech commonly
obtained by minnow-seining or by turning over logs and trash, has
proved excellent bait for game fish.
Leeches can be used to reduce the swelling and discoloration of a
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Update: June 2012