Leeches are segmented worms found in various aquatic and terrestrial environments globally. Their physical characteristics provide insight into their diverse forms and behaviors.
Core Physical Features
Leeches possess an elongated, soft body, often flattened and tapering at both ends. Their size varies considerably, from a few millimeters to 20 centimeters, with some reaching half a meter when fully extended. Common coloration includes black, brown, and green, sometimes with mottled patterns or distinct stripes. Some leeches can even exhibit dramatic color changes.
Leeches are distinctly segmented, appearing as numerous ring-like structures (annuli) along their body. While externally segmented, their 32 to 34 internal body segments are not always clearly visible. They possess two prominent suckers: a smaller anterior (oral) sucker at the head end and a larger posterior (caudal) sucker at the rear. Both suckers are used for attachment and locomotion; the anterior sucker also contains the mouth.
Many leech species have one to five pairs of eyes, usually on the anterior portion of their body. These eyes can be arranged in various patterns, such as a horseshoe shape in some species.
Appearance During Movement and Attachment
A leech’s appearance changes significantly with its activity. When moving across solid surfaces like an inchworm, they employ a characteristic “looping” motion, alternately attaching their anterior and posterior suckers. This involves extending their body forward, attaching the anterior sucker, then releasing the posterior sucker and pulling the body up, causing their shape to contort. In water, most leeches move with an undulating, ribbon-like swimming motion, similar to an eel.
When a leech attaches to a surface or host, its body shape transforms. An unengorged leech appears flat and relatively thin. After feeding, a leech becomes engorged, appearing swollen, rounded, and plumper due to the ingested blood. This engorgement can also cause a change in their coloration, as the distended body wall may allow the color of the blood meal to show through.
Variations Among Common Types
Leeches vary in appearance, with distinct visual characteristics among species. Medicinal leeches, such as the European medicinal leech (Hirudo medicinalis), are dark brown to black, often displaying thin green, yellow, and red stripes or intricate patterns on their dorsal side. North American medicinal leeches (Macrobdella decora) have a greenish-brown back with orange or red dots and a reddish underside with black spots. These species can grow up to 20 centimeters and possess three jaws that leave a Y-shaped incision.
Horse leeches (Haemopis sanguisuga) are larger, up to 15 centimeters, with a plainer, often dull, uniformly dark brown to black appearance. Unlike blood-feeding relatives, they are predatory and do not attack humans or horses. Terrestrial leeches, found in humid environments like rainforests, are adapted for land movement and may burrow into soil during dry periods. Aquatic leeches, like Piscicola geometra, are slender and cylindrical. They swim with a strong whip-like motion, often resting rigidly extended while attached to plants.