Leeches are carnivorous invertebrates in the class Hirudinea. A few specific leeches feed on blood, which has made this large and rather diverse group of creatures famous with humans. Bloodsucking leeches have been used in medical treatment for thousands of years, and they continue to be used in certain circumstances today. Humans also encounter leeches in the wild, where they are irritating but not usually harmful.
These animals are annelids, which means that their bodies are divided into segments. When a leech is viewed under a microscope, the segments can be clearly seen. Leeches are also hermaphroditic, like many invertebrates. They tend to be aquatic, living in freshwater, swamps, and marshes. Some leeches are also comfortable on land, especially in humid areas like jungles.
Many leeches are carnivorous, feeding on smaller invertebrates. Others scavenge for various organic materials, while some leeches attach themselves parasitically to other animals to feed on their blood. Leeches will feed on fish, reptiles, waterfowl, amphibians, and mammals, depending on available sources of potential food in their areas. Bloodsucking leeches secrete special chemicals which open blood vessels, inhibit clotting, and numb the wound so that their hosts are not aware of their presence until they are already gone.
Historically, bloodletting was an important aspect of medical practice, prescribed for a wide range of conditions. One form of bloodletting involved the use of leeches, and “leech” was actually a common slang term for “doctor” at one point. In modern medicine, bloodletting for therapeutic purposes is vary rare, but the use of leeches is actually not uncommon. They may be used at surgical sites to promote circulation and the flow of blood, for example, and they are also used in the treatment of frostbite and other circulatory conditions.
These interesting creatures do have a few tricks up their sleeves. Some leeches will actually care for their young, for example, which is a fairly rare trait in invertebrates. They can also be harmful in some cases; parasitic leeches can harbor infections like hepatitis, for example, and pass these infections on to new victims. If you notice a leech on your skin, avoid the temptation to yank it off, as this can cause damage to the wound and it will encourage the leech to vomit, which could cause an infection. If you can stand it, allow the leech to finish feeding and drop off on its own. If this idea is distasteful, use your fingernail to gently scrape the leech away from your skin.
After a leech has been removed, it is a good idea to wash the wound and treat it with a light antiseptic, if one is available. Bandaging can also promote clotting; since the leech leaves anticoagulants behind, the wound may bleed more than you expect it to, but this is normal.