Leeches are annelids or segmented worms, and although closely related to the earthworms, are anatomically and behaviourally more specialised.

The bodies of all leeches are divided into 34 segments, with a powerful clinging sucker at each end (although the front sucker can be very small). Body shape is variable, but to some extent depends on the degree to which their highly muscular bodies are contracted. The mouth is in the anterior sucker and the anus is on the dorsal surface just in front of the rear sucker.

Leeches usually have three jaws and make a Y-shaped incision. The Australian land leech varying in size from 7mm to as long as 200mm when extended has only two jaws and makes a V-shaped incision.

Leeches are grouped according to the different ways they feed. Some have jaws armed with teeth with which they bite the host. The blood is prevented from clotting by production of a non-enzymatic secretion called hirudin. The leech commonly encountered by bushwalkers is included in this group.

Leeches are common on the ground or in low foliage in wet rain forests. In drier forests they may be found on the ground in seepage moistened places. Most do not enter water and cannot swim, but can survive periods of immersion.

In dry weather, some species burrow in the soil where they can survive for many months even in a total lack of environmental water. In these conditions the body is contracted dry and rigid, the suckers not distinguishable, and the skin completely dry. If sprinkled with a few drops of water, within 10 minutes these leeches emerge fully active.

Respiration takes place through the body wall, and a slow undulating movement observed in some leeches is said to assist gaseous exchange.

Sensory organs on the head and body surface enable a leech to detect changes in light intensity, temperature, and vibration. Chemical receptors on the head provide a sense of smell and there may be one or more pairs of eyes.

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. Leeches die after one or two bouts of reproduction.

As if we didn’t know, leeches are blood sucking parasites. Some feed on the blood of humans and other mammals, while others parasitise fish, frogs, turtles or birds and if the preferred food is not available most leeches will feed on other classes of host. Some leeches will even cannibalise other leeches which may die after the attack.

Leeches can ingest several times their own weight in blood at one meal and after feeding the leech retires to a dark spot to digest its meal. Digestion is slow enabling the leech to survive during very long fasting periods (up to several months).

How does a leech go about searching for a meal of blood? A hungry leech is very responsive to light and mechanical stimuli. It tends to change position frequently and explore by head movement and body waving. It also assumes an alert posture, extending to full length and remaining motionless. This is thought to maximise the function of the sensory structures in the skin.

In response to disturbances by an approaching host, the leech will commence "inchworm crawling", continuing in a trial and error way until the anterior sucker touches the host and attaches.

When a leech bites it holds the sucker in place by making its body rigid. Using its semi circular and many toothed jaws, it then makes an incision in the skin and secretes a mucous which helps the sucker to adhere. A salivary secretion containing the anticoagulant and a histamine floods the wound and the leech relaxes its body allowing the blood to be ingested. This mixture allows the blood to flow and also prevents clotting once inside the leech. A bacterium in the gut of the leech assists the digestion of the blood, and it has been shown that the type of bacterium varies with the type of host on which the leech feeds.

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.

Bacteria from the leech’s gut may cause wound infection and there may be a delayed irritation and itching after the bite. There appears to be no support for the theory that mouthparts left behind after forced removal of the leech causes this reaction.

Can leeches transmit disease? There is no evidence to suggest that they do. Allergy to leech bite has been reported. Medical opinion should be sought, depending on the severity of the reaction.

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. Otherwise do a regular inspection of your boots etc. and pick them off before they come any closer to you.

With information obtained from the Australian Museum