Spider Poop: What It Looks Like, Where It Shows Up, and How to Clean It

What does spider poop look like?

Spider poop looks like tiny drops of paint. Each spot is a flat splat or dot, usually about the size of a pinhead and rarely over 1/8 inch (3 mm), and often white or cream with dark flecks. It lands wet and dries in place. Groups of spots build up directly below where a spider rests.

The mix of colors is the best clue. In a study of crab spiders, the biologist Douglass Morse described their waste as drops of whitish liquid with dark brown flecks. These dried in the air to a dirty light brown. Some spots are mostly white. Others look mostly black, especially on a light wall, because the dark part stands out.

The shape matters too. Spider droppings are never pellets. They are flat, because they leave the spider as a liquid. Fresh spots on a wall can run a little before they dry, so you may see a short drip line.

Size is the weak point in every guide, including this one. Pest control companies say “pinhead-size”. A biologist quoted by the keeper site Tarantula Heaven, Blaine Hebert, gives 1/16 to 1/8 inch (about 1.5 to 3 mm). However, no scientific paper we found gives a measurement, and tarantula keepers report bigger splats from their spiders.

Why is spider poop white?

Spider poop is white because of guanine, the same white compound found in fish scales and in DNA. Spiders turn their nitrogen waste into guanine instead of urine. Studies put it at 70 to 85% of the nitrogen a spider excretes. Guanine barely dissolves in water, so a spider can get rid of waste while losing very little water.

Spiders also use guanine for color. Some store it in special cells just under the skin, where the crystals reflect light. That is where the white markings on a garden spider come from. So the white spots on your wall and the white cross on a garden spider’s back are the same material.

Many websites say the white part is uric acid, and so does Wikipedia’s main spider article. Spiders do make a little uric acid, but guanine is the main waste. When researchers tested the waste of 34 spider species in the 1960s, guanine was the only nitrogen waste they detected.

Why some spider droppings look black or brown

The dark part is what is left of the spider’s meal after digestion. It mixes with the white guanine on the way out. So one spot can be mostly white, mostly dark or speckled, depending on how the two parts mix. On white paint or vinyl siding, the dark spots stand out more, and that is why many people describe spider poop as “black spots”.

Do spiders poop?

Yes, spiders poop. Anything that eats has waste to get rid of. A spider cannot chew, so it pumps digestive juices into its prey and sucks up the liquid. About 80% of known spider species feed this way. The waste leaves through one opening at the tip of the abdomen, as a single liquid drop.

Because a spider drinks its food, there is very little solid waste. Most of what comes out is not undigested food at all. Instead, it is guanine made by the spider’s Malpighian tubules, which work a bit like kidneys. The hard outer shell of the prey stays outside the spider. It drops the empty husk when it has finished.

How do spiders poop?

Spider poop follows a short path through the body. Our diagram above shows each step.

1. The spider sucks in its liquid meal through the mouth.
2. A muscular sucking stomach pumps the liquid into the midgut, which fills most of the abdomen.
3. The midgut takes up the food. Meanwhile, the Malpighian tubules collect nitrogen waste and turn it into guanine.
4. Both streams meet in a small sac called the stercoral pocket, where the waste waits.
5. The spider releases it through the anus, which sits on a small bump just above the spinnerets at the tip of the abdomen.

Do spiders pee?

No, spiders do not pee, at least not the way people do. They have no bladder and no separate opening for urine. The waste that would be urine in a mammal becomes solid guanine, and it leaves through the same opening as the rest of the waste. So every spider dropping is poop and “pee” in one drop.

This is a water-saving trick. Many spiders live in dry places and get most of their water from their prey. Guanine lets them get rid of nitrogen without losing water to make urine.

Do spiders poop out of their webs?

No. Silk and poop come out of two different openings. Silk is made in silk glands inside the abdomen and leaves through the spinnerets, small finger-like tubes at the back. The anus is a separate opening just above them. A web is never made from waste.

In fact, some spiders do the opposite: they eat their old webs. Orb weavers often take down and swallow their web, then reuse some of the silk protein. In one study with labelled silk, the spiders recycled up to 32% of it into new silk.

How often do spiders poop?

There is no simple answer, because it depends on how much the spider eats. A spider that has not caught anything for weeks has little to pass. What surprised researchers is how long spiders can hold it.

In a 2008 study, crab spiders kept in small vials almost never excreted, even over 50 days of regular meals. When the same spiders were let out onto plants, more than two-thirds of them excreted within 5 minutes. They walked to the edge of a leaf first, so the drop fell away from them. The researcher thought this might help them avoid giving away their position to predators or prey.

That habit may explain why many keepers say a pet spider often poops right after it is handled or moved.

Where do spiders poop?

Spiders let their waste drop straight down, so spider poop lands directly below where a spider rests or hunts. Most of it turns up in five places: under webs in ceiling corners, on windowsills, under ledges and furniture, under porch lights, and on outside siding near doors and windows.

A good tip from pest control companies: find the droppings first, then look straight up. The web or the spider is usually right above them.

Spider poop on walls and ceilings

Indoors, the classic sign is a cluster of spots on a wall just below a ceiling corner. Cellar spiders, often called daddy long legs, are the usual source. The entomologist Michael Raupp of the University of Maryland describes the “telltale white droplets” of their waste below their loose webs. House spiders, which build tangled cobwebs in corners and windows, leave the same kind of marks.

A few spots do not mean many spiders. One spider in one corner can leave dozens of spots over a few months. Fresh spots after you have cleaned are the real sign that a spider is still there.

Spider poop outside: siding, porches and boats

Outside, spider poop collects under porch lights, eaves and window frames. Insects fly to the light at night, and spiders build their webs where the food comes to them. On white vinyl siding, the dark spots are easy to see. Some orb weavers even leave their web at night to excrete, yet the spots still end up below the web.

Boat owners know this problem well. Pontoon and boat owners’ groups trade tips on removing the black spots every summer.

Wolf spider, black widow and house spider poop

All spiders produce the same kind of droppings. What changes is where you find them, because each spider lives a different way. You cannot name the spider from its poop alone. To do that, look at the spider or its web, or try our spider identifier.

If the spider over the spots has a shiny black body and a red hourglass, leave it alone and read our black widow guide before you clean.

Spider poop or something else?

Many small pests leave dark spots, and spider poop is easy to mix up with them. The surest test is the shape. Spider droppings are flat splats, often with white in them, below a web or a ledge. Bed bug spots soak into fabric, cockroach droppings look like ground pepper, and mouse droppings are pellets 1/8 to 1/4 inch (3 to 6 mm) long.

Spider poop vs bed bug poop

This is the mix-up that worries people most. The quickest test is where the spots are. Bed bug spots gather along mattress seams, sheet edges and bed frame joints. Spider spots gather on walls, sills and floors below a web.

The second test is the fabric. The EPA says bed bug droppings can bleed on fabric the way a marker does, because they are digested blood. Press a damp white cloth onto a speck. A bed bug speck smears rusty brown. Spider poop sits on top of a hard surface and has white in it. If you find dark dots on bedding, our guide to the early signs of bed bugs walks you through the full check.

Spider poop vs fly specks

Fly specks are tiny dark dots, and they usually come in large numbers. They show up on a window, a wall or a light fixture, often with no web nearby. Cluster flies, which gather in houses in fall, are a common cause. Penn State Extension notes their spots stain windows and walls, but that they are not known to spread disease.

Spider poop, on the other hand, usually has white in it, comes in smaller groups, and sits under a web.

Spider poop vs cockroach droppings

German cockroach droppings look like specks of ground pepper, in brown to black clusters. They gather in kitchen cabinets, drawer corners and behind the fridge, where roaches hide. Spider poop is rarely in those places, and it is never grainy. If the specks are in your kitchen, our German cockroach guide explains what to do next.

Spider poop vs mouse droppings

Mouse droppings are solid pellets, 1/8 to 1/4 inch (3 to 6 mm) long, with pointed ends. Spider poop is never a pellet. This difference matters for your health, because mouse droppings need a different clean-up. Do not sweep or vacuum them (see the warning in the cleaning section below).

Gecko droppings are the other pellet people find. Texas A&M describes them as elongated and brownish, and often tipped with white.

Is spider poop dangerous?

No. Spider poop has no documented disease risk for people or pets. The one published spider allergy case we found, from 2011, tested spider bodies, not droppings. University of Kentucky entomologists describe spider spotting as an “unsightly nuisance”, not a hazard. The main problem is the stain, which is hard to remove once it dries.

A-Z Animals says spider poop “needs to be treated like it’s toxic”, and some pest control blogs say it triggers allergies. We found no evidence for either claim. That 2011 case was a farmer who developed asthma while working in a warehouse full of cellar spiders. The doctors tested extracts of whole spider bodies, and it was a single patient.

There is even a small test on germs. A Washington State University graduate student swabbed droppings from western black widows and looked for bacteria by their DNA. She found none she could identify. It was a preliminary test of one species, reported in the trade magazine PCT in 2013, so treat it as a hint, not proof.

That said, all animal waste deserves basic hygiene. Wear gloves when you clean, and wash your hands afterwards. If a pet spider poops on your hand, wash it off with soap and water.

The real risk is a mistake in identification. If the “spider poop” is actually mouse droppings, clean-up needs extra care. If it is bed bug spotting, you need to act fast. So check the look-alike table above before you clean.

How to clean spider poop

Fresh spider poop wipes off with a damp cloth in seconds. Dried spots take longer, because the guanine crusts and sticks. The 7 steps below soften dried spots first, and test any cleaner on a hidden spot. Then they remove the web above, or new spots will appear.

No university or government guide gives a tested method for spider droppings, so treat these steps as plain household cleaning.

Spider poop on painted walls and ceilings

Painted walls need a gentle touch. Use as little water as possible, especially on ceilings and flat paint, where water can leave a ring. Blot rather than rub. An old, dried spot may leave a faint yellow or brown shadow even after the crust is gone. If so, stop before you damage the paint. A dab of touch-up paint often looks better than a scrubbed patch.

Avoid rubbing alcohol and solvents on painted or finished surfaces. They can soften or strip the finish.

Spider poop on siding, decks and boats

Outside, follow the siding or boat maker’s care instructions first. A soft brush, a bucket of soapy water and a hose rinse handle most fresh spots on vinyl siding. Old, baked-on spots are much harder. Many boat owners use cleaners made for boat finishes. If you buy one, check that it is safe for your surface, and use it as its label says.

Treat a pressure washer with care. Too much pressure can damage siding, paint and boat finishes, and can force water behind siding.

Spider poop on fabric and carpet

Blot fresh spots on fabric with a damp cloth. Do not rub, which spreads the stain. Then wash the item as its care label says. On carpet, blot with a little soapy water and rinse by blotting with clean water.

Spider poop in a pet spider’s enclosure

Pet spiders poop on the walls of their enclosure, on decorations, and often in or near their water dish. Experienced tarantula keepers treat it as an eyesore rather than a danger. Most spot-clean the glass with a damp paper towel or cotton swab. A full clean is needed only when waste builds up.

Avoid bleach and strong cleaners inside an enclosure. Keepers warn that the residue can harm the spider. Move the spider to a safe container before any deep clean, and do not tear down the main web a jumping spider uses as a home.

How to stop spider poop from coming back

Spider poop stops when the spider above it leaves. So the fix is fewer spiders indoors, and fewer insects for them to eat. The 6 steps in the table below cost little or nothing. For most homes they are enough, and you do not need a spray.

If spiders keep coming back after you remove webs and seal gaps, the reason is usually a supply of insects. Fix that first. If exclusion and cleaning are successful, insecticide treatments are not required. If you do use a spray, choose an EPA-registered product labelled for spiders and the place you plan to treat, and follow the label exactly. The label is the law.

Do spider droppings mean you have an infestation?

Not on their own. One spider can leave many spots, and spiders are normal in most homes. A spider infestation is better judged by what you see: many webs that come back quickly, lots of live spiders, or several egg sacs. Also, keep in mind that dark spots can come from flies, cockroaches, bed bugs or stink bugs too. Check the look-alike table before you decide you have a spider problem.

Spider poop myths

Most spider poop myths come from how little people know about how spiders eat. Here are seven common ones, each with what is actually true. The biggest is that the white part is uric acid, when it is mostly guanine.

• Myth 1

  Myth: “The white part of spider poop is uric acid.”

  Truth: Mostly not. Spiders make a little uric acid, but guanine is their main waste, at 70 to 85% of the nitrogen they excrete.

• Myth 2

  Myth: “Spiders make their webs from poop.”

  Truth: No. Silk comes from silk glands through the spinnerets. Waste leaves through a separate opening just above them.

• Myth 3

  Myth: “Spiders do not poop, because they only drink.”

  Truth: They do. A liquid diet still leaves waste, and spiders also have to get rid of nitrogen as guanine.

• Myth 4

  Myth: “Spider poop is toxic and must be treated like a hazard.”

  Truth: No evidence supports this. No disease or allergy has been traced to spider droppings.

• Myth 5

  Myth: “Spiders swallow the insect shell and throw it up later.”

  Truth: No. A spider never swallows the shell. It sucks out the liquid and drops the empty husk.

• Myth 6

  Myth: “Dark spots on the wall always mean spiders.”

  Truth: Flies, cockroaches, bed bugs and stink bugs leave spots too. Spider spots sit below a web and usually have some white in them.

• Myth 7

  Myth: “Lots of spider poop means a big infestation.”

  Truth: Not necessarily. One spider in one corner can leave dozens of spots over a few months.

When to call someone

Most spider poop is a cleaning job, not a reason to call anyone. But 5 findings point to something else: pellets instead of splats, dark dots on a mattress, a black widow above the spots, stains that will not come off, or a reaction after cleaning. Those are worth an expert’s help.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.