Forest Preserve District of Cook County, Illinois
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Leeches
Nature Bulletin No. 202-A   October 16, 1965
Forest Preserve District of Cook County
Seymour Simon, President
Roland F. Eisenbeis, Supt. of Conservation

LEECHES
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 operations.

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 "shiner".


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