The mother leech forms a tough, gelatinous cocoon around the egg mass and attaches it to a hard object or buries it in the mud. The cocoon contains all the nutrient that the young leeches need to survive, and the young emerge several weeks later. Some species nurture their young, which is unusual among annelids. However, most leeches only reproduce once or twice in their lifetimes. Leeches are not capable of regeneration and can only reproduce sexually. Although leeches are mainly nocturnal, they are attracted to disturbances in the water which might indicate food sources.
They live mostly in shallow water with many cover objects or hiding places. The Rhyncobdellids are capable of dramatic colour changes but this is apparently not an attempt at camouflage, and the significance of this behaviour is unknown.
Leeches move by either an undulating swimming motion eel-like or by an 'inch-worm' like crawling motion using the anterior and posterior suckers. The posterior sucker is attached to a substrate and the leech stretches out and attaches to the substrate with the anterior sucker, the posterior sucker is then detached and pulled up to the anterior sucker. As hermaphrodites, leeches have both male and female sex organs.
Like the earthworms they also have a clitellum, a region of thickened skin which is only obvious during the reproductive period. Mating involves the intertwining of bodies where each deposits sperm in the others' clitellar area. Rhyncobdellids have no penis but produce sharp packages of sperm which are forced through the body wall.
The sperm then make their way to the ovaries where fertilisation takes place. The clitellum secretes a tough gelatinous cocoon which contains nutrients, and it is in this that the eggs are deposited. The cocoon is either buried or attached to a rock, log or leaf and dries to a foamy crust. After several weeks or months, the young emerge as miniature adults. Studies show that the cocoons are capable of surviving the digestive system of a duck.
For over years, leeches were needlessly applied for many ailments as an adjunct to blood letting. Their use in Europe peaked between and , but subsequent shortages led to a decline in their use. Today there is a real clinical application in that they are of great value to plastic surgeons when venous congestion of skin and muscle flaps is a problem.
Medical use of leeches also includes treatment of black eyes, and hirudin is used in the treatment of inflammation of the middle ear. Hirudin is also being developed for experimental use as a systemic anticoagulant, and may prove useful in invitro blood sampling.
The Sydney suburban sprawl is resulting in houses extending into areas such as wet valleys that leeches normally prefer. It is therefore less distance for these leeches to travel in wet weather before they end up in backyards and can get a meal from domestic animals or humans living there. Similarly the domestic animals and humans themselves unwittingly bring many leeches home with them from bush walks, holidays, and other travels. If these leeches are adult they will invariably find a suitable damp spot in the garden to lay their eggs and suddenly you have a colony of leeches in your backyard.
The most common enquiry regarding leeches concerns repellents. It is unknown whether a specific preparation is commercially available but there is a plethora of tried and tested, but unproven leech-protection ideas. These include a lather of bath soap smeared on exposed parts and left to dry, applications of eucalyptus oil, tropical strength insect repellent, lemon juice and impenetrable barriers of socks and pantyhose.
The presence of hirudin in the wound following a leech bite may cause oozing to continue for several hours. Although inconvenient, blood loss is not significant. Gut bacteria can cause wound infection. In the post-operative use of leeches this is closely monitored and dealt with by use of the appropriate antibiotic. There may also be a delayed irritation and itching after a bite. There appears to be no support for the theory that mouthparts left behind after forced removal of the leech causes this reaction.
There is no evidence to suggest that they do. Leeches are found all over the world, and there are hundreds of species. In Michigan there are at least 40 species, and probably more. Most leech species are found in shallow, slow-moving freshwater, but some live in the oceans, and a few live in moist soil on land.
Leeches lay eggs in cocoons, and the babies that hatch out look and behave like little adults. They don't change much as they grow, they just get bigger. Leeches that live in habitats that freeze or dry out during part of the year bury themselves in mud and stay dormant until the habitat improves. Leech growth rate is strongly affected by temperature and food supply. Most species can mature in a few weeks or months if conditions are good.
Some leeches complete their life cycle in a few months, but many can live for several years. All leeches can crawl, and some are good swimmers. They search for prey by following the scent or touch of the animals they want to eat. When they first detect food, they extend their bodies and hold very still, probably to carefully sense their prey. Leeches have very poor vision often they can only tell the strength of light , but are very sensitive to touch.
They also have a strong sense of taste. They cannot hear, but are sometimes very sensitive to vibrations. Although they can't walk or run, they are perfectly able to move around, namely by using their swimming and crawling skills. Leeches spend most of their time within still or sluggish freshwater pools. However, they also are relatively common sights in saltwater and on terra firma within damp dirt. Their approaches to motion generally differ depending on which of these habitats they're in.
The majority of leeches can hold their own swimming. When they swim, they make seamless, wavelike movements.
The overall impression of their swimming is an elegant one -- far from awkward.
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