Are Leeches Insects? Fact Checking Myths

Leeches, often misunderstood and sometimes feared, are not insects. They belong to a group of animals known as annelids or segmented worms.

This article aims to provide a clear understanding of leeches, their classification, and their relationship with other organisms.

By the end of this article, we hope that the reader will have a comprehensive understanding of why leeches are not classified as insects and will be familiar with their unique characteristics.

Are Leeches Insects

Are Leeches Insects? Classification and Anatomy of Leeches

Leeches are classified as annelids, a group that also includes earthworms.

However, unlike the common earthworm, leeches have undergone some specific anatomical and behavioral specializations.

Segmentation and Symmetry

Leeches exhibit bilateral symmetry, meaning their left and right sides are mirror images.

Their bodies are muscular and segmented, but these segments might not always be visible to the naked eye.

Physical Characteristics

Most leeches have a dorso-ventrally flattened appearance. This means they are flattened from their back to their belly.

They possess two suckers: a smaller one surrounding the mouth (oral sucker) and a larger one at the rear (caudal sucker).

Some marine leeches, specifically the Pisciolidae, have a larger oral sucker.

Anatomical Differences

Unlike other annelids, leeches lack parapodia (tiny protrusions or ‘feet’) and chaetae (bristles).

This absence is a distinguishing feature, setting them apart from many other segmented worms.

Subclasses of Leeches

There are two primary subclasses of leeches. The Euhirudinea, often referred to as ‘true’ leeches, have 32 internal segments when mature.

The Acanthobdellida, a smaller group primarily parasitic on fish, has 29 segments.

Counting these segments can be challenging due to the structure of the leech’s suckers and the secondary annulation (ring-like structures) on their bodies.

Overall, while leeches share some similarities with other segmented worms like earthworms, they have distinct characteristics that set them apart.

Their classification as annelids and not insects is based on these unique anatomical features.

File:A leech on the leaf..jpg
Source: Thejaswi I from Bangalore, IndiaCC BY-SA 2.0, via Wikimedia Commons

Habitat and Distribution

Leeches are versatile creatures, inhabiting a range of environments, from freshwater bodies to terrestrial regions and even the sea. 

They are found globally, with around 500 species identified worldwide.

Freshwater Habitats

The majority of leech species thrive in freshwater environments. They have a preference for still or slowly flowing waters, such as ponds, lakes, and slow streams.

However, some can also be found in faster-flowing waters.

Terrestrial Leeches

These leeches are commonly found on the ground or in low foliage in wet rainforests. In drier forests, they might be located in areas moistened by seepage.

Interestingly, most terrestrial leeches do not swim and can only survive short periods of immersion.

During dry spells, some species have the ability to burrow into the soil, where they can endure for many months, even in the absence of water.

Marine Leeches

While less common, there are marine leech species that feed on the blood of fish and other marine life.

Humans generally do not encounter these leeches, as they do not feed on human blood.

File:Leech in the point.jpg
Source: GlebKCC BY-SA 3.0, via Wikimedia Commons

Feeding Habits and Diet

Leeches have diverse feeding habits, with many species being sanguivorous, meaning they feed on the blood of other animals.


Many leeches are parasites that feed on the blood of specific hosts. If their preferred host is unavailable, they might feed on other animals.

Some leeches feed on humans and mammals, while others target fish, frogs, turtles, or birds. There are even instances of leeches feeding on other blood-sucking leeches.

Feeding Mechanisms

Leeches have developed various methods to feed on their hosts. Jawed leeches, or Gnathobdellida, possess jaws with teeth that allow them to bite their host.

They release a substance called hirudin, which prevents the blood from clotting.

Jawless leeches, or Rhyncobdellida, use a needle-like protrusion called a proboscis to pierce their host and release an enzyme that dissolves blood clots.

Another group, the Pharyngobdellida, lacks jaws or teeth and consumes small invertebrates whole.


After feeding, leeches often retreat to a dark location to digest their meal. This process is slow, allowing leeches to survive extended periods without food.

Some leeches can consume several times their body weight in a single feeding.

File:Leech (Theromyzon tessulatum?) (22997456469).jpg
Source: AnemoneProjectorsCC BY-SA 2.0, via Wikimedia Commons

Behavior and Adaptations

Leeches have evolved a range of behaviors and physiological adaptations that allow them to thrive in their respective environments.


Unlike many animals, leeches respire through their body wall. Some aquatic leeches will move to the water’s surface when oxygen levels are low.

This behavior was historically observed and used as a rudimentary method to predict weather changes, as a decrease in atmospheric pressure can lead to reduced dissolved oxygen in water.

Sensory Organs

Leeches are equipped with sensory organs that detect changes in light, temperature, and vibrations.

They also possess chemical receptors on their heads, functioning as a sense of smell. Many leeches have eyes, the number and arrangement of which can vary and aid in their identification.


Leeches move in two primary ways. They can swim with an undulating, eel-like motion or crawl in an ‘inch-worm’ manner using their anterior and posterior suckers.

Color Changes

Some leech species, particularly rhyncobdellids, can undergo significant color changes. The exact purpose of this adaptation remains unclear.

File:Land Leech (Haemadipsa zeylanica) - Khao Yai NP, Thailand.jpg
Source: Bernard DUPONT from FRANCECC BY-SA 2.0, via Wikimedia Commons

Economic and Medical Impacts

Leeches have played a role in human society for centuries, both economically and medically.

  • Medicinal Use: Historically, leeches were used in various medical treatments, especially bloodletting. Their peak usage in Europe was between 1830 and 1850. Today, they have a legitimate clinical application, aiding plastic surgeons in cases of venous congestion in skin and muscle flaps. Leeches are treated similarly to blood products, ensuring they are reused only on the same patient.
  • Hirudin: This substance, produced by leeches to prevent blood clotting during feeding, is being researched for potential medical applications, including as an anticoagulant.
  • Leeches in Urban Areas: As urban sprawl continues, leeches are increasingly found in suburban areas, especially in regions that were previously damp valleys or forests. This proximity can lead to increased human-leech interactions.

Are Leeches Dangerous? Management and Interaction with Humans

As humans and leeches come into closer contact, management and understanding of these creatures become essential.

While a leech bite can lead to prolonged bleeding due to hirudin, the blood loss is generally not significant.

However, bacteria in the leech’s gut can cause wound infections. Some individuals may also experience delayed irritation or itching after a bite.

  • Leech Repellents: One of the most common inquiries about leeches is how to repel them. While no specific commercial preparation is universally recognized, various methods have been suggested, from eucalyptus oil to bath soap lathers.
  • Disease Transmission: There’s no concrete evidence suggesting that leeches transmit diseases to humans. However, they can carry trypanosomes, which cause diseases in other animals but are not harmful to humans.

Frequently Asked Questions

Is leech a worm or insect?

Leeches are segmented worms and not insects. They belong to the annelid family, closely related to earthworms.

Are leeches harmful?

While leeches can bite and cause discomfort, they are generally not harmful to humans and don’t transmit diseases.

Are leeches fish?

Leeches are not fish. They are annelids, a distinct group of invertebrates.

What do leeches live?

Leeches primarily inhabit freshwater environments, but some species can be found in terrestrial regions or marine settings.


To summarize, leeches are not insects but belong to the annelid group, closely related to earthworms.

They inhabit diverse environments, from freshwater to terrestrial regions and even the sea.

While many leeches are blood-feeders, their feeding mechanisms vary. Historically used in medicine, they still have clinical applications today, especially in surgeries

Despite their occasional presence in urban areas and the discomfort they might cause, there’s no evidence to suggest they transmit diseases to humans.

Understanding their role and characteristics is essential for both appreciating their ecological significance and managing human-leech interactions.

Reader Emails

Over the years, our website, has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Probably Land Planarian rather than Leech from Colombia


Leech? June 9, 2010 Here is a leech (I think) that I see fairly frequently at my property in “cloud forest” at 2100m on the eastern slope of the Western Cordilla in Valle De Cauca. It´s about 9cm long. Can you identify it for me? Pitter Valle De Cauca, Colombia
Leech or PLanarian?
Dear Pitter, We are very excited to have received your letter, because it represents the first posting to our site for a Leech.  We did some research, and Leeches are actually considered to be Worms, and according to the Australian Museum website, they belong to the same class as earthworms, Clitellata.  You may also find information on Wikipedia.  Alas, we do not have the necessary knowledge to further classify your particular Leech.
Leech or Planarian?

Letter 2 – Mystery: Possibly a Leech


Leech June 30, 2010 Hi Bugman, I found this crawling across the yard and thought it was a worm. The nearest water source was at least 75 feet away. I’m thinking a bird or something dropped it. My husband informed me it was a leech. Did not know leeches were here in Chesapeake, VA. I did not kill but placed it in a ditch the farthest from the house. I was looking up a bug the other day on WTB and noticed you only had one pic of a leech posted and was reminded of mine. The pics are of it “stuck” to a rock when I wet it to keep it moist while I took pics. You could not pull off. Can you tell me what type it is? Thanks! ps I have more pics! Dawn in SE VA Southeast VA
Leech? or other Annelid Worm???
Hi Dawn, Though we are inclined to agree that this is a Leech, we are not certain.  We hope that by posting your letter and image, we can get a confirmation from an expert in Leeches and Annelid Worms.


  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

  • Piyushi Dhir

    Piyushi is a nature lover, blogger and traveler at heart. She lives in beautiful Canada with her family. Piyushi is an animal lover and loves to write about all creatures.

2 thoughts on “Are Leeches Insects? Fact Checking Myths”

  1. I am not an expert on these groups, but I am as sure as I can reasonably be that this is not a leech, but a terrestrial planarian. If Pitter were to turn one over it would not show a sucker at the front end and another on the back end like a leech; it would have a smooth underside instead. There are a lot of these land flatworms in wet tropic areas and some species are quite large. They are predatory and eat other critters that live in the soil. Take a look at the Wikipedia article on the family Geoplanidae.

    Best wishes,

    Susan J. Hewitt

  2. Ì am no expert but to me it looks like some sort of medicinal leech (although there are several different spiecies) they can apear black and are indigenous to America. Leeches can move very far from their water source.


Leave a Comment