Arrowhead Flatworm: All You Need to Know

Arrowhead flatworms, also known as hammerhead or shovelhead worms, are fascinating creatures.

These invertebrates are land planarians, a type of flatworm, known for their distinctive hammerhead-shaped heads

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Source: Eduard SolàCC BY-SA 3.0, via Wikimedia Commons

Arrowhead Flatworm Overview

Species and Appearance

Arrowhead flatworms, also known as hammerhead flatworms or shovelhead worms, are invertebrates belonging to the phylum Platyhelminthes.

They have a distinct, flat body shape. Some key features of their appearance are:

  • Soft-bodied
  • Flat and elongated
  • Distinct “hammerhead” shape

There are multiple species of arrowhead flatworms, and they vary in color and size.

For example, the Blue garden flatworm (Caenoplana coerulea) has a dark grey to black upper surface with a creamy median line and a blue underside, while other species display different color patterns.

Habitat and Distribution

Arrowhead flatworms are predominantly found in moist and humid environments, like soil and leaf litter.

Some factors affecting their distribution include:

  • Region’s temperature and humidity
  • Availability of food sources (mainly earthworms and slugs)

These flatworms are native to Asia, but they have been introduced to various regions, such as North America and Europe.

They can adapt to new environments and spread if the conditions are suitable.

Biology and Behavior

Diet and Feeding Habits

The Arrowhead Flatworm, also known as the Hammerhead Worm or Bipalium kewense, is a species of land planarian that feeds on various prey found in the soil. Some examples of their diet include:

These flatworms are invasive species and can cause issues for native plants and animals. They are also known to occasionally feed on plants.

Reproduction and Lifecycle

Arrowhead Flatworms have a unique reproductive system that allows them to reproduce both sexually and asexually2.

Their lifecycle includes:

  1. Mating occurs during high-humidity periods, such as spring.
  2. Fertilized eggs are laid in soil.
  3. Young flatworms emerge from eggs and grow into adults.

They are hermaphroditic, meaning each individual possesses both male and female reproductive organs.

This allows them to mate with any other member of their species.

They can also reproduce through fragmentation, wherein a piece of the worm’s tail can grow into a new individual.

Reproductive CapabilityProsCons
Sexual ReproductionGenetic diversityRequires a mate
Asexual ReproductionRapid population growthLess genetic diversity

When handling these flatworms, it is important to wear gloves as their secretions may cause skin irritation.

They are commonly found in high-humidity environments and are more prevalent in the southern states of the US.

Control and Management

Natural Predators

Arrowhead Flatworms have some natural predators, which can help to control their population. For example:

  • Birds
  • Rodents
  • Frogs

These predators often target flatworms when they come across them in gardens and other habitats.

Chemical and Physical Methods

Various methods can be employed to control flatworms, including chemicals and physical techniques. For instance:

Chemical methods

  • Citrus oil: Deter flatworms by spraying a citrus oil solution around areas where they are commonly found.
  • Vinegar: Flatworms are sensitive to vinegar, so you can use it to eliminate them. However, be cautious not to harm other organisms or plants with vinegar.

Physical methods

  • Salt: Sprinkling salt on flatworms can cause them to dehydrate and die.
  • Freezing: Collect the flatworms with gloves and place them in a sealed container. Store the container in the freezer overnight to kill the flatworms.

Pros and Cons of Chemical vs. Physical Methods

MethodsProsCons
ChemicalEffective in killing flatwormsMay harm beneficial organisms or plants
PhysicalEnvironmentally friendly and more selectiveTime-consuming and labor-intensive

Arrowhead Flatworms are found in various locations, such as:

  • Southeast Asia
  • Southern states of the US, including Georgia, Louisiana, and Texas

While they can pose a threat to gardens and greenhouses, using a combination of natural predators and chemical or physical methods can help to control and manage their populations.

Common Misconceptions and Similar Species

Comparing Arrowhead Flatworms to Earthworms

Arrowhead flatworms, also known as planarians, differ significantly from earthworms. Here are some key differences:

  • Body shape: Arrowhead flatworms have a flat, arrow-shaped body whereas earthworms possess a cylindrical, elongated structure.
  • Habitat: Arrowhead flatworms live in moist environments, while earthworms reside in soil, providing essential nutrients for plants

Are Arrowhead Flatworms Dangerous?

Arrowhead flatworms are generally not considered dangerous to humans.

These flatworms are primarily scavengers or predators that feed on other small invertebrates such as insects, snails, and worms.

While some species of flatworms have been reported to release a sticky substance when handled, causing skin irritation in some cases, their effects on humans are usually mild and temporary.

However, it’s always a good practice to avoid handling any wildlife, including flatworms, without proper knowledge and precautions.

Some species of flatworms have toxins that can be harmful to other animals they prey upon, so it’s wise to avoid direct contact and wash your hands thoroughly if you do come into contact with them.

If you notice any adverse reactions after handling a flatworm, it’s recommended to seek medical attention.

What Do Arrowhead Flatworms Eat?

Arrowhead flatworms primarily feed on other small invertebrates.

Their diet typically consists of various insects, spiders, snails, worms, and other soft-bodied organisms.

They are predators that use their slimy, adhesive mucus to capture and subdue their prey.

Arrowhead flatworms have a unique hunting strategy.

Arrowhead Flatworm

They extend their flat bodies and glide along the ground, using chemoreceptors located on their heads to detect chemical signals from potential prey.

Once they locate prey, they capture it by immobilizing it with their mucus and then consuming it using their mouth located on the underside of their body.

The ability of arrowhead flatworms to consume a variety of invertebrates makes them important contributors to the ecosystem as decomposers and predators of pest species.

However, their predatory behavior can also impact populations of other small organisms in their habitat.

Conclusion

Intriguing and elusive, the Arrowhead Flatworm’s unique features and behaviors have illuminated our understanding of the natural world.

From their arrow-shaped heads to their role in ecosystems, these creatures continue to captivate researchers and enthusiasts.

This land planarian species is primarily found in damp and shaded habitats, such as leaf litter, decaying logs, and under rocks. Its arrowhead-shaped head and elongated body contribute to its unique appearance.

As a native of Southeast Asia, the Arrowhead Flatworm has become invasive in various regions around the world.

Its adaptability and intriguing biology continue to captivate researchers, offering insights into both its ecological role and the potential impacts of its presence in new environments.

Footnotes

  1. https://extension.psu.edu/hammerhead-flatworms-and-other-land-planaria-of-eastern-north-america 
  2. https://u.osu.edu/worldunseen/2022/04/26/flatworms-and-human-health/ 
  3. https://www.hawaii.edu/news/2022/08/29/13-species-rat-lungworm/ 

Reader Emails

Over the years, our website, whatsthatbug.com 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 – Arrow-headed Flatworm

Tape worm or slug?
Our dog either eliminated it or found it crawling in our carport in Hilo, HI a very long (about 1 foot), very skinny (like a guitar string) slimy worm like animal. The head is flat and fan shaped.

The body does not seem to be segmented and can elongate and shrink. Our dog was treated for tapeworm about a month ago, when we found a white segment on her tail; her vet confirmed it was a tapeworm by doing a fecal check.

Since then we haven’t found any more segments and we thought the tapeworm problem was finished. We’ve been using Frontline monthly to control fleas, but she is an outdoor dog who runs free. Please check out these 6 pictures. Thank you.
Rick and Karen

Hi Rick and Karen,
No problem here. Your dog did not eliminate the Arrow-headed Flatworm, a Land Planaria. It is probably Bipalium kewensis, a species with a wide distribution in warm climates with moist conditions.

It was originally discovered in the Kew Gardens greenhouses in London, hence the scientific name. It is a benign species.

Letter 2 – Arrowhead Flatworm

Dark Golden Slug with one Black Stripe
August 8, 2009
Hey Bugman!
My father found this outside our house here in Georgia. He asked me to take a look at it because he thought I would know. I took a look and I have no idea what it is.

It’s a dark golden color, slimy, has a wide head area, (Shaped like a hammerhead shark) legless, and has a black stripe running from it’s head down to it’s tail. It leaves a pretty nice slime trail so I’m guessing it’s a type of slug.

Can you end this weird mystery for my father? He’s dying to know. Thanks!
Sean M.
Albany, GA

Arrowhead Flatworm
Arrowhead Flatworm

Hi Sean,
You have found an Arrowhead Flatworm or Land Planarian,  Bipalium kewense.
Your specimen has a dark head and lacks the two distinct dorsal stripes normally associated with the species.  According to a Texas website:  “Now the good news . . . Land Panarians are effective predators as they will eat slugs and many types of harmful insect larvae.

The thought of having a beneficial that preys on slugs should be encouraging!  But now the not-so-good news . . . while all of this sounds rather benign, the land planarian is not necessarily without flaws (at least from a gardener’s perspective—but Mother Nature does not operate in such black-and-white perspectives).

Like an earthworm, it burrows in moist soil, but it can exhibit much more sinister epicurean habits. Although it will eat slugs and harmful insect larvae, the Land Planarian will also dine on earthworms!  It does so by lying atop the earthworm (the sticky mucus holds the earthworm to the soil.

The land planarian then protrudes its pharynx out of its mouth and into the earthworm to suck out bodily fluids of an earthworm.  Land Planarians are reported to be cannibalistic when food resources are low. This may help to keep the population down, since the mucus membrane of the worm deters most other pests.”

According to the Featured Creatures website which has a photo of a specimen with the same coloration as your specimen:  “Habitat: Because land planarians are photo-negative during daylight hours and require high humidity, they are found in dark, cool, moist areas under objects such as rocks, logs, in debris, or under shrubs, and on the soil surface following heavy rains.

Land planarians are also found in caves, but are rare in rural sites. Movement and feeding occur at night. High humidity is essential to survival. They can survive desiccation only if water loss does not exceed 45 percent of their body weight. Land planarians are most abundant in spring and fall.”

Letter 3 – Arrowhead Flatworm


I saw this critter in Hawaiian Paradise Park on the east side of the Big Island (Hawai’i) south of Hilo. I first saw a larger one that was gray-bodied and it had the same fan-shaped head. At first I thought it was a kind of earthworm or other kind of worm.

But when I photographed this 2.5 in. one with the yellowish body and dark dorsal line, I noticed what appeared to be a slime trail. The larger gray one was about 6-7 in. long with a uniform thin body (not segmented like an earthworm), perhaps a bit thinner in aspect ratio that his one.

It seemed to move faster than the typical slugs in Hawaii (Veronicella cubensis?) but perhaps it was the more animated movement of the fan-shaped head that created that illusion. The points of the head articulated like a slug’s “antennae”. I didn’t want to turn it over to examine the ventral side. Might this be a juvenile form? Any idea what it is?
Les Chibana
Volcano, HI

Hi Les,
Thanks for sending this unusually colored Arrowhead Flatworm, a Planarium.

Update (04/26/2006): Arrowhead Flatworm in Maui
Aloha, I am impressed with the photo you just put up on your home page from the Big Island. We found this Arrowhead Flatworm in our outdoor shower a month ago, just after several days of rain. It was 4″ long and yellowish with black stripes down the entire length of it’s body.

We had never seen one before so I contacted our local Dept. of Ag to inquire if it is considered an invasive species. Apparently the answer depends on your point of view. If you want earthworms for your garden, then the flatworm is invasive.

If you have an abundance of the Giant African Snail, then you may appreciate the flatworm as it is a predator of said snail. I read on the internet that the flatworm expands to over twice it’s size after eating, but I have yet to find an earthworm to feed to it.

It would not touch my composting worms. Keep up the excellent and entertaining work on your website! It is much appreciated. Aloha,
Michelle
Kihei, HI

Letter 4 – Arrow-Headed Flatworm

Subject: What kind of worm is this?
Location: southern California
December 21, 2013 8:46 pm
Hi, I live in southern California, and was outside getting some leaves out of my pool. It was about 55 degrees, rather typical for this time of year. I live in 91381.

There was some leftover moisture from my sprinklers that had accumulated on the concrete hardscape. I noticed this weird looking, worm like bug. I have seen one like this last year. Sunny day, background is wet concrete.

The odd thing is that the color and markings resembled a baby snake, sort of. The head has an ever-changing shape that generally looks like a semi circle, or a fan shape. This bug moved like a worm. What is it? Is it dangerous? I have a Labrador Retriever that is outside, often and want to make sure there are no concerns for any of us.
Your help is greatly appreciated.
Signature: MPS

Arrowhead Flatworm
Arrow-Headed Flatworm

Dear MPS,
This oddity is an Arrow-Headed Flatworm, one of the Planaria, and we believe it is
Bipalium kewensis.  According to Charles Hogue in his landmark book Insects of the Los Angeles Basin:  “the species was discovered in 1878 in the greenhouses of Kew Gardens near London, hence its scientific name.  It has a wide distribution in warm climates. 

It needs a moist habitat and it is usually encountered near outdoor water faucets, where the soil often remains wet.  It original home is unknown but is possibly the Indo-Malayan region.  …

These are benign creatures — they do not damage plants or cause any medical problems.”  We suspect that populations of this species get established in new locations when plants are purchased from nurseries.

Arrow-Headed Flatworm
Arrow-Headed Flatworm

Update:  Benign or Not???
Thanks to a comment from Barbara, we decided to do a bit more research and we found some interesting information.  The Dirt Doctor states:  “Rather than helping control termite larvae, grubs and other pests, etc. it seems that it is only a destructive pest that needs to be gotten rid of.  It only eats earthworms.

The predatory land planarian is no friend of earthworms.  In fact, they are parasites that eat earthworms and can wipe out entire populations.”  Calling the Arrow-Headed Flatworm a parasite does not seem accurate to us.  A more correct term would be predator. 

The Red Worm Composting website states:  “Land planarians can be a serious earthworm predator in certain parts of the world – generally they are more of a threat in warmer regions, but certain species are found in more temperate zones as well. They are particularly dangerous because they can reproduce incredibly quickly, and have been reported to wipe out an entire worm population (in a worm farm) in a matter of days.” 

The two previous citations come from sites that recommend worm farming, and that is not necessarily a natural environment for the worms as they live in confinement.  The chances of a Land Planarian wiping out all the worms in a garden seem incredibly remote as the worms in a typical garden are not confined.  According to the Galveston County Master Gardeners Beneficials in the Garden page on Land Planaria: 

“Now the good news . . . Land Panarians are effective predators as they will eat slugs and many types of harmful insect larvae. The thought of having a beneficial that preys on slugs should be encouraging!  But now the not-so-good news . . . while all of this sounds rather benign, the land planarian is not necessarily without flaws (at least from a gardener’s perspective—but Mother Nature does not operate in such black-and-white perspectives).

Like an earthworm, it burrows in moist soil, but it can exhibit much more sinister epicurean habits. Although it will eat slugs and harmful insect larvae, the Land Planarian will also dine on earthworms!” 

The bottom line is that any species, however seemingly benign it might be, can negatively affect the natural ecosystem when it is introduced.  The Arrow-Headed Flatworm is an introduced species, so we will tag it as an Invasive Exotic species. 

The larger issue here is how human behavior has irrevocably changed the ecology of the planet by introducing foreign plants and animals, either intentionally for food and decoration, or accidentally, and then how those introduced species interface with native plants and animals. 

Once the factors of agriculture and animal husbandry are considered, the waters get very murky.  If a native meadow with native milkweed is destroyed to plant corn on many acres, and then some insect is introduced that decimates the corn crop, is the insect the invasive exotic or is the corn and the farmer who planted the corn to blame?  Sadly, that ship sailed long ago.

Letter 5 – Arrowheaded Flatworm

Arrow-headed Tapeworm photo
Hi Bugman!
Thanks for your website which helped me identify this arrow-headed tapeworm which I found in my backyard in Chattanooga, TN. You can’t tell the scale in the photo, but the worm was 12-14′ long.

I would have included something in the photo to show the scale, but the sucker was moving! Just had time to get this one shot before he moved off the concrete patio into the grass.
Thanks again!
John

Hi John,
Your Planaria is commonly called an Arrowheaded Flatworm, not a Tapeworm. That was sure a large individual.

Letter 6 – Arrowhead Flatworm

arrow headed flatworm
Hi Bugman
I came across an arrowheaded flatworm yesterday (14/05/2008) and was interested to see only one other reference that I could track down in Australia- which happens to be from last month from your site. I used your site to identify this worm.

I came across this one as I was walking through urban bushland at 6.45 am. I must have brushed a plant as it was stuck to my trouser leg. Half its length is missing because it got squished before I realised it was there.

However, in true flatworm form it continues to soldier on and the wound site has healed over after 24 hrs. I have not seen evidence of them in the Gold Coast area before. I think it might have beenout after a thunderstorm the previous evening.
Mandy Timmers

Hi Mandy,
Now we have two images of Arrowhead Flatworms from Australia on our site.

Letter 7 – Arrowheaded Flatworm from Australia

arrowheaded flat worm
Dear Bugman
Attached is a photo of an arrowheaded flat worm that we found in our garden tonight. It was about 10 inches long. We found your web site to identify the worm and found the information very interesting.

We have lived in this suburb of Sydney (Australia) for 36 years and have never seen this before. Have you ever had any other feed-back about this worm from anywhere else in Australia? Thank you,
Valerie.

Hi Valerie,
Thanks for sending us your documentation of the Arrowheaded Flatworm, Bipalium kewensis. Since the species was discovered in a greenhouse in the Kew Gardens near London in 1878, it is unknown where the actual country of origin is.

Probably because of the spread of greenhouse plants, and their use in warm damp gardens, the Arrowheaded Flatworm is now found in many parts of the world. We believe this is our first report from Australia.

Letter 8 – Arrowheaded Flatworm

Worm I found in my back yard
Dear Bugman,
I find these every once and again in my yard. This particular one was in some clover that’s by by house shaded from the morning sun and it usually stays cool and moist there all day. I’ve previously found them under things like plastic soil bags and things of the like.

They’re usually about 2 inches, but when you poke around at them too much they scrunch down to about half an inch and they have shovel-shaped heads. I live in South Louisiana, about 50 miles south west of New Orleans. Can you please identify it for me? Thanks,
Lindsay

Hi Lindsay,
Your letter is the second we received this week from Southwest Louisiana of an Arrowheaded Flatworm, a benign and harmless Planarium. WE have gotten reports of specimens from other parts of the world as long as 10 inches in length.

Letter 9 – Arrow-Headed Flatworm


Dear Bugman,
please tell me what this worm is, I found it under a log in my back yard. I live is southwest Louisiana. It’s head is flat and it is very slimy.
Zoey

Hi Zoey,
This is an Arrow-Headed Flatworm, a type of Planarium. They are benign creatures that like dampness.

Letter 10 – Arrow-Headed Flatworm

flatworm
Just when I thought I might have found something new… This critter was crawling in my carport this morning. I took the photo, then went to your website, just in case there was something there. Of course, the first two pictures under “worms” are of the same thing. Your site is awesome.
Rick
Doraville , GA

Hi Rick,
It is great you included that marker as an indication of the scale of the Arrow-Headed Flatworm.

Letter 11 – Arrowhead Flatworm

bug
Dear bugguy,
I love your website. Like everyone else, I need to know what the name of a bug is. It has a flat head. (I’m sure it was supposed to be like that.) It has a very long slug-like body that is about 4 inches long. I see it every once in a while underneath my dog’s water dish.

I would love to keep it in my insect collection, but I don’t want to kill it. I have never seen one like it before. My other pet slug that I had a few years ago turned into a liquid! Could you please tell me what this bug is? Thank you so much,
Frances

Hi Frances,
When we originally began What’s That Bug? years ago as a column in a small publication called American Homebody, we claimed it was a good idea because everybody wants to know “What’s That Bug?” and it seems we were right. This is an Arrowhead Flatworm, a species of Planaria.

Letter 12 – Arrowhead Flatworm

Parasite Worm?
I.m thinking some kind of parasitic worm. This was found in my “rain barrel” – a 55 gal plastic drum stocked with water plants fish, snails, tadpols, frogs & who knows what else.

The head looks as if it may have “suckers” on the underside – wasn’t able to get a picture of the underside though. This is about 10-15 mm long when streched out. Thanks
Bnuts

My apologies – I found the Arrow-Headed Flatworm on your worms page. The worm I have must have been young – As I stated 10-15 mm. Feel free to post the picture though – I think it shows good head detail. I loveyour site. It’s been a great help with “identifying bugs”

Hi Bnuts,
Glad you identified your Arrowheaded Flatworm, a species of Planarium, before we had a chance to answer.

Letter 13 – Arrow-Headed Flatworm

Something slimy for your slug page?
Hi, I found a few of these slug-like creatures while working in the yard, and the front edge ruffles as they (smell?) their way around. Thanks for an entertaining and informative website!
Jai Hari

Hello Jai,
We wouldn’t dream of posting your Arrow-Headed Flatworm on our slug page while we have a worm page to accomodate it. These is actually a Planarian, one of the Flatworms.

Letter 14 – Arrow-Headed Flatworm

Hammerhead worm
Hi Bugman,
Any ideas what this is. Found it in my yard in June 2003. Sorry, some of the pictures are fuzzy.
Regards,
Denis

Hi Denis,
Your Arrow-Headed Flatworm is actually a species of Planarium introduced, probably accidentally. It was first described in a greenhouse in Kew Gardens in London in 1878.

Letter 15 – Planarium

arrowheaded flatworm
i found a better picture of this creature thought u might like it . thank you for identifing.
Tammy

Hi Tammy,
Sorry to have lost your original letter, but we will post your photo.

Letter 16 – Arrow-Headed Flatworm

Dear Mr. Bugman,
I cannot say how much I love your site. I have three pics for you, I hope that’s ok? The first is a decent picture of a marbled orb weaver (I recognized it from your site), I just thought you might enjoy the picture. The other two are of a weird, weird worm that visited the concrete porch at my old house in Atlanta when it rained. I’m sorry about the picture quality.

Can you tell that it was kind of flat, slimy, not a snake and has a weird sort of hammer-head? We’ve moved so I’m no longer freaked out about them, but what were they? I even took a little one to the local university’s entomology department and they didn’t know.

We got tons of slugs and snails when it rained too, and that backyard had flooding problems.
Jennifer

Hi Jennifer,
Thank you for the compliment. We are very excited to get your worm photos. We have received several letters regarding the Arrow-Headed Flatworm, Bipalium kewensis, but we have never gotten a photo. According to Hogue: “This land planarian is slender and brown, with five dark longitudinal stripes; it can be large, up to 10 inches in length.

The species is ‘hammerheaded’: the head is shovel-shaped (wider than body) and there are numerous minute eyes along its border. The species was discovered in 1878 in the greenhouses of Kew Gardens near Londodn, hence its scientific name. It has a wide distribution in warm climates.

It needs a moist habitat and is usually encountered near outdoor water faucets, where the soil often remains wet. It original home is unknown but is possibly the Indo-Malayan region. Flatworms are hermaphroditic, and copulation involves mutual insemination; they may also reporduce asexually by fragmentation.

The eggs are encapsulated and affixed to objects in damp places. These are benign creatures–they do not damage plants or cause any medical problems.” Your Marbled Orb Weaver photo is awesome.

Letter 17 – Arrow-Headed Flatworm

Hi,
I am 24 but back when I was about 8 or so I remember seeing a worm or caterpillar with what seemed to be a hammerhead shark-shaped head. I have attached a really lame MS paint drawing of what I saw.

I was hoping you could let me know if anything like this exsists or if my mom put LSD in my Snackpac. Thanks
Jade Shiroma

Hi Jade,
You left out some crucial details, like the size of the worm, but I think I have a good idea what you saw. There is a species of planaria or flatworm known as the Arrow-headed Flatworm, Bipalium kewensis.

According to Hogue, “The species is “hammer-headed”; the head is shovel-shaped (wider than the body), and there are numerous minute eyes along its border.”

Thanks so much, I am going to go find a picture!

Letter 18 – Land Planarian from Japan

Damp terrestrial arrowhead flatworm
April 29, 2010
I came across this flatworm under a moist slab of wood in my backyard garden. The flatworm has a head that resembles a fan at times and at other times an arrowhead. The tail anchored the flatworm to the moist wood, although this flatworm was pretty much curled and very mucusy.

Along with this flatworm were orange winged beetles resembling ladybugs the size of peppercorn kernels and sow bugs under the wood slab. The dorsal side of the flatworm were 2 outer broad dark stripes (running from head to tail) with a thinner light dark stripe between and parallel to the 2 broad stripes. The underside was basically pale.

There was only one flatworm not a community of them. For April it has been raining a bit more than usual, and the garden ground is a bit moist making it easier to pluck out unwanted baby weeds. I was just curious to see what community lived under this particular slab of wood.
My question is what type of flatworm is this?
Lucy
Fukuoka City, Japan

Land Planarian

Hi Lucy,
Thank you for including the detailed information about the community you found under the slab of wood.  We do not have the necessary skills to identify what species of Planarian you have discovered.  It is very possible that it is a young

Arrowhead Flatworm, Bipalium kewense, which can grow to ten inches in length.  The markings are consistent with that species.  The Texas Master Gardener website has a nice page on the Land Planarian.  The Featured Creatures website has a great page on the Land Planarian.

Letter 19 – Terrestrial Planarian from Malaysia

Colorful Worm
April 29, 2010
It is look like worm, it’s head look like a hammer. and the length of body is approximate 40mm.
Goree Chong from Malaysia
Penang State of Malaysia

Terrestrial Planarian

Hi Goree,
Your photo is the second Terrestrial Planarian we received today, the other being a different species from Japan.  Your specimen most closely resembles Bipalium rauchi which is pictured on the Terrestrial Planaria website.  There is also a YouTube video that looks quite similar.

Letter 20 – Land Planarian

Trilobite Beetle!
Location:  Pulau Tioman, Malaysia
July 31, 2010 9:22 am
Just back from Pulau Tioman in Malaysia. Saw some reasonably strange critters, including this trilobite beetle.
You might also like this land planarian.
Bert

Land Planarian

HI again Bert,
We  really appreciate you sending us your excellent image of an Asian Land Planarian.

Letter 21 – Arrowhead Flatworm

What do ya think?
October 25, 2010 12:58 PM
Adam Schreck
Sent from my useless iphone

Arrowheaded Flatworm

Hi Adam,
What we think is that you did not use our form for submission, so much of the information we require in our form, which is of tremendous assistance to us for identification purposes as well as for the benefit of our readership, is absent from your submission which is curiously devoid of content. 

This is a Land Planarium or Arrowhead Flatworm. We also think your photo is a bit blurry and that it is a cold morning and that we should put on a pot of coffee and that we have a very busy day at work today, but somehow we don’t think you want to know all of what we are thinking.

Letter 22 – Land Planarian from Australia

My strange bug
Subject: My strange bug
Location: MElbourne, Australia
May 16, 2011 5:04 am
This strange bug was on my leg the other day, i thought it was a baby snake.. any ideas?
Signature: i dont understand this question?

Land Planarian

Dear idutq,
This is a Land Planarian, and after looking at Ask.Com, we believe it is the Blue Planarian,
Caenoplana coerulea.  According to the dedicated Ask.com page on the species, it is also called the Blue Garden Flatworm and

“This flatworm’s native range is eastern Australia and New Zealand. This species has however been accidentally introduced to the USA, including California, Florida, Georgia, Texas, South Carolina and Iowa.”

Letter 23 – Freshwater Planaria

Is this a Kansas Leech
Location: Ninescah River , Cheney Kansas
July 10, 2011 1:01 pm
I found these things feeding on partial dead fish..I think they might be leeches but not sure..The fish was submerged in a shallow pool of water and was covered with them.
Signature: Chris Harris

Planaria on Dead Fish

Hi Chris,
These are actually Flatworms known as Planaria.  Here is some information from the Planaria Homepage :  “They are a free-living, flat bodied, freshwater creatures that exhibit the remarkable ability to regenerate their lost body parts.

It lives in lakes, streams, ponds, and other freshwater bodies The planarian is non-parasitic, and eats decaying meat.”  The website also includes details on keeping Planaria in captivity.  A single Planaria is known as a Planarian.

Planaria on Dead Fish

Letter 24 – Land Planarian from Malaysia

Wonders from Malaysian Borneo!
Location: Malaysian Borneo
August 12, 2011 9:09 pm
Hey Bug-people!
A challenge for you!
I took myself backpacking through Southeast Asia a while ago, and came back with some amazing pictures of bugs.


I’ve included three of what were to me the most fascinating and baffling varieties. Can you help me identify them?
Cheers!
Signature: Doug

Land Planarian

Hi Doug,
We really only like posting identification requests with a single species, or at most, members of the same family so that they would be archived together.  The orange and black wormlike creature is actually a Land Planarian or Flatworm. 

We will try to research the species because that coloration is quite distinctive.  We quickly found a match on Flickriver that was identified as a species in the genus Bipalium.

The website also contains this fascinating information:  “Terrestrial flatworms, land planarians or hammerhead worms whatever you call them they are simple yet interesting creatures. During the day they are hidden away from the sun and heat.

But at night, at night when the temperature lowers, and the humidity spikes, these predators voraciously feed on annelids. Their diet comprises mainly of earthworms, following their slimy trails until they fall upon and devour them. However they also eat insect larvae, slugs and are not above a little cannibalism.

They feed by entwining themselves around their prey and entrapping it in a sticky mucous. It then proceeds to evert its pharynx onto the prey and secrete digestive enzymes, taking up the partially digested food particles into the gastrovascular cavity. Where they can then be digested and properly absorbed in the intestinal epithelia.

Land planarians move on a film of mucous secreted onto a ventral strip of closely spaced, powerful cilia (creeping sole). They usually follow plants for ease of movement and are able to lower themselves down to the earth on a string of mucous.

The flatworm body can reproduce asexually, reproducing an entirely new organism by breaking off from the parent organism. Only the posterior end has this capability as the front end houses nerve ganglia and organs vital to survival.

However this would lead to a genetically homogenous population making it prone to disease and unstable. Therefore flatworms also reproduce sexually. Like the earthworms on which they feed, they are hermaphroditic, having both male and female sexual organs. In fact the entire body is very metabolically flexible since during lean times they are able to digest reproductive organs and most other cells to keep themselves alive.

Flatworms come in a variety of beautiful colours to display their aposematism. In addition to their mucosal secretions, they may have toxins to deter predators. In this way, their most fearsome predators are other flatworms.”

Wow!  I guess everything’s bigger on Borneo, because that larval Firefly was  nearly three inches long!
Thanks for that. The info about the flatworm was particularly fascinating.
Doug

Letter 25 – Planarium from Peru

Subject: Flatworm from Peru
Location: Peru; near Iquitos
March 13, 2014 5:52 pm
I know this is not exactly a “bug”. However I d be very glad if u d be able to help me to identify this.. flatworm. Thanks for any suggestion 🙂
Signature: Jiri Hodecek

Planarium
Planarium

Hi again Jiri,
When we were contemplating the subtitle of Daniel’s Book, The Curious World of Bugs, we settled upon “the mysterious and remarkable lives of things that crawl” because “Bug” is a generic term, despite the fact that True Bugs are in the suborder Heteroptera.  If it crawls, we have room for it on our site. 

Flatworms, including Planaria, are in the class Turbellaria, and when we attempted to research this identification for you, we discovered a nearly identical image on Stock Photography that interestingly was also taken at Iquitos, Peru. 

Alas, it is not identified further than the class Turbellaria.  Another unidentified individual from the Andes in Peru is pictured on Age PHotostock.

Hello, yeah I guess its quiet impossible to ID it better, thank you! 🙂
Jirka

Letter 26 – Planarian from Borneo

Subject: Unknown Slug from Mount Santubong, Sarawak, Borneo
Location: Santubong NP, Sarawak, Borneo
April 14, 2015 11:47 am
Hi. Recently i went hiking at Mount Santubong National Park at Sarawak, Borneo, using the Summit Trail. I encounter this pretty blue black with white strip/bands worm on a tree trunk. It has a split in the middle too.

As I’m a zoology student, I’ve ask around (lecturers etc) and they could only ensure me that it was not a platyhelmintes but some slug. I’m not sure about the elevation, but it was found after some 2280 m along the trail.

And since I’ve no known experts to ask and my curiosity is giving me sleepless nights, I would like to try my luck here. It would be great if you know what this slug is. Thanks.
Signature: Tan, C.F.

Hi. I’ve missed to input some details. Santubong NP is a tropical rainforest and the slug is about 5cm long,1 cm width. Thanks.

Planarian
Planarian

Dear Tan, C.F.,
This is not a Slug which is a shell-less mollusc.  This is a Planarian or Flatworm.  We located a very similar looking image on Photographers Direct, but alas, it is not identified as to its species.
  A video on Siam Answer does not provide an identification either.  A similar looking image on Project Noah is identified as a Hammerhead Worm, possibly in the genus Bipalium.

Authors

    by
  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. whatsthatbug.com 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.

35 thoughts on “Arrowhead Flatworm: All You Need to Know”

  1. I too saw one of these in a flat of herbs we were transplanting to a greenhouse. It was about 12″ long as well. He had tied himself into a know. I have been gardening for 10 years and have never seen one.

    I live in Carrollton, Georgia.

    Reply
  2. Sorry to be persnickety, but in case it’s useful for someone to know, the name should be spelled “planarian”. 🙂

    Reply
  3. Just found one of the brown worms in Alabama, Looks to be about 10 inches long. First one I have ever seen. Should I be contacting my Ag. Agent?

    Reply
  4. I found your site, while searching for the ID of the worm that crawled into my house this evening!

    The pictures and description of the “mystery” worm was dead-on!….It is a Flat Headed Worm!

    You didn’t have any pictures until the post from Jennifer in Atlanta (I’m located just outside of Atlanta)….give me mailing info and I will send you the actual worm!

    Mike!

    Reply
    • We are happy to hear you identified your Arrow-Headed Flatworm. Thanks for the offer but we do not deal in the mail order distribution of “bugs”.

      Reply
  5. I found your site, while searching for the ID of the worm that crawled into my house this evening!

    The pictures and description of the “mystery” worm was dead-on!….It is a Flat Headed Worm!

    You didn’t have any pictures until the post from Jennifer in Atlanta (I’m located just outside of Atlanta)….give me mailing info and I will send you the actual worm!

    Mike!

    Reply
  6. These Arrow-headed worms keep coming into my house, at first I thought they were just slugs but they were extremely long. My son found one last night by accidently stepping on it. My question is: are they harmful to humans and animals?

    Michelle

    Reply
  7. These Arrow-headed worms keep coming into my house, at first I thought they were just slugs but they were extremely long. My son found one last night by accidently stepping on it. My question is: are they harmful to humans and animals?

    Michelle

    Reply
  8. Hello, just found one of these last night on my porch. Took several pics of it.I was curious to what he was. Should I be contacting my local Ag. Agent?

    Reply
  9. Hello I’m have a worm farm and found what looked like a flat brown worm with a flat head in my tote that I’m have te worm farm in unsure of what it was in grabbed a stick and threw it out. But all te info is have seen online does not mention them being in michigan. And I’m live in southern michigan

    Reply
    • I found one in charlotte n,c.
      it seems like michigan wouldnt be humid enough for the flat worm .
      but being summer I bet you guys do have them

      Reply
    • I found one in charlotte n,c.
      it seems like michigan wouldnt be humid enough for the flat worm .
      but being summer I bet you guys do have them

      Reply
  10. UUUGGGHHH. Thanks goodness. I was out in my backyard today in Hong Kong and I saw one of these guys. I totally thought it was a leech. I have HUGE fear of leeches, so I am VERY glad it wasn’t one. I just stumbled across your site, and it is very helpful, especially living overseas with so many unknown to myself creatures. Thanks.

    -Carter

    Reply
  11. UUUGGGHHH. Thanks goodness. I was out in my backyard today in Hong Kong and I saw one of these guys. I totally thought it was a leech. I have HUGE fear of leeches, so I am VERY glad it wasn’t one. I just stumbled across your site, and it is very helpful, especially living overseas with so many unknown to myself creatures. Thanks.

    -Carter

    Reply
  12. i found one this morning after a very hard rain lasts night. it was about 6 inches long curled up under my carport.

    Reply
  13. found two bipalium nobile today. We live in Rabun County in North Georgia temperate forest landscape. Gave them the orange oil eternal treat.

    Reply
  14. Thank you for the identification. We just found two crawling across our front walkway. I’m an avid gardener and have been digging in the dirt in Los Alamitos, CA for 47 years, this is the first time I’ve ever seen one! I wonder how they got here as I don’t think this species is “normal” around here.

    Reply
    • The Arrowhead Flatworm is mentioned in our 1993 edition of Insects of the Los Angeles Basin so they have been in California at least that long.

      Reply
  15. Flat head worms are not harmless. They eat the good worms. Earthworms are fearful of them with good reason. The flathead worms can be caught by leaving a dead earthworm in a jar at night and checking on it often. Do not mash the flathead worm as each piece will multiply. The answers on this worm are wrong!

    Reply
  16. We just found one here in Hawaii Big Island Mtn.View. Do they bite ? This one was light yellow with a dark strip down it’s back . It had an arrow head . It moved fairly quickly and it was about 6 inches .
    Our area is very wet in the winter. It’s winter here now.

    Reply

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