Spiders produce waste, but they handle it differently from mammals. Instead of separate solid and liquid streams, most spiders release a combined excretion from a single opening, and the main nitrogen-rich compound in that waste is not urea or uric acid but guanine, the same molecule found in DNA. The process that leads to this unusual waste product is just as distinctive, starting with digestion that happens largely outside the spider’s body.
Digestion Starts Outside the Body
To understand what comes out of a spider, you first need to appreciate what goes in and how. Spiders are liquid feeders. They cannot chew solid food the way insects with mandibles can. After subduing prey with venom, a spider floods the carcass with a cocktail of digestive enzymes that dissolve the prey’s internal tissues from the outside. This process, called extra-oral digestion, means the prey’s own body essentially becomes an external stomach. The spider then pumps the liquefied slurry back and forth between the prey and its own gut, mixing and further breaking down the material with each cycle.1PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestion
The enzymes involved include proteases for breaking down proteins, lipases for fats, and nucleases for genetic material. By the time a spider actually ingests the meal, most of the hard digestive work is already done. What enters the gut is a nutrient-rich liquid with very little solid material. This is why spider droppings look nothing like the pellets or feces you see from animals that eat solid food. There simply isn’t much indigestible bulk to expel.
What Spider Waste Actually Looks Like
If you have ever noticed small whitish or dark splotches on a wall, windowsill, or underneath a web, you were probably looking at spider droppings. The appearance varies depending on the spider’s recent diet. A well-fed spider that has consumed a juicy insect tends to produce more liquid waste with a darker stain, while the solid component is typically a chalky white or cream-colored substance. The dark liquid portion is the closest thing to “pee” a spider produces, and it often dries in a splatter pattern around the solid white center.
That white material is primarily guanine. Across a study of 34 spider species spanning 17 families, guanine was the only purine detected in excreta, and it accounted for roughly a third to three-quarters of the waste by weight. In terms of excreted nitrogen, guanine made up about three-quarters to nearly nine-tenths of the total.2Comparative Biochemistry and Physiology. The excreta of spiders This is a sharp contrast to insects, which primarily excrete uric acid, and to mammals, which excrete urea. The difference traces back to enzyme availability: spiders appear to lack certain enzymes in the metabolic pathway that would convert guanine into uric acid, so guanine is the endpoint.3Nature. Nitrogenous Excretion in Arachnids
The practical result is that guanine, being relatively insoluble, forms that pasty white deposit. It is an efficient way to get rid of nitrogen waste while losing minimal water, which matters enormously for small animals living in environments where water can be scarce.
The Stercoral Pocket
Before waste is expelled, it collects in an internal structure called the stercoral pocket, a sac-like chamber at the end of the spider’s intestine. This is essentially the spider’s rectum, though it serves double duty. Besides holding waste, the stercoral pocket has been identified as a reservoir for certain ions like potassium and phosphate, nutrients the spider may reclaim for other biological needs such as silk production.4Comparative Biochemistry and Physiology. The gut, stercoral pocket, and hemolymph as potential storage sites of K+ and phosphate for the spider’s orb web
The stercoral pocket empties through the anus, which is located at the underside of the spider’s abdomen near the spinnerets. When a spider defecates, the contents of the stercoral pocket are released as that combined solid-and-liquid mixture described above. Some species expel waste fairly regularly after meals, while others, particularly those that eat infrequently, may go weeks between defecations. The timing depends on feeding frequency, prey size, and how much water the spider has access to.
Do Spiders Actually Pee?
The short answer is that spiders produce a dilute fluid that functions like urine, but they do not have a separate urinary tract. Their “pee” exits the body through the same anal opening as their solid waste. In laboratory studies, starved but hydrated spiders of one mygalomorph species produced this anal urine at a rate of about five microlitres per day, a tiny droplet that is roughly equal in salt concentration to the spider’s own blood.5PubMed. Regulatory responses of the coxal organs and the anal excretory system to dehydration and feeding in the spider Porrhothele antipodiana (Mygalomorpha: Dipluridae)
Some spiders also have a second fluid-handling system: coxal organs, small glands located near the bases of the legs. These glands can release droplets of fluid and appear to play a role during feeding, possibly helping manage the hydraulic pressure spiders use to move their legs and process prey. The coxal organs also seem to help regulate ion balance, particularly after a large or salty meal.6Semantic Scholar. The Function of Spider Coxal Organs: Effects of Feeding and Salt-Loading on Porrhothele Antipodiana (Mygalomorpha: Dipluridae) Not all spider families have functional coxal organs. Many of the more familiar household spiders, which belong to the araneomorph group, rely entirely on their Malpighian tubules, thin tubes inside the body that filter waste from the hemolymph (the spider equivalent of blood) and channel it toward the stercoral pocket.
So when people ask whether spiders pee, the honest answer is that they produce a liquid excretion, but it is not urine in the mammalian sense. It exits the body mixed with the solid guanine waste or, in species with coxal organs, as small droplets near the legs. There is no dedicated bladder and no separate urethra.
How Spiders Manage Water Loss
Water conservation is a life-or-death concern for an animal that weighs a fraction of a gram. Spiders have evolved several strategies to hold onto moisture, and their excretory system plays a central role. The choice of guanine as a waste product is itself a water-saving adaptation. Guanine is poorly soluble in water, so it can be excreted as a semi-solid paste rather than dissolved in a large volume of liquid, the way mammals must dissolve urea in water to produce urine.
When water becomes scarce, spiders actively reduce how much fluid they excrete. In dehydration experiments, anal urine production dropped to zero within four days of water deprivation, and coxal organ output declined as well.5PubMed. Regulatory responses of the coxal organs and the anal excretory system to dehydration and feeding in the spider Porrhothele antipodiana (Mygalomorpha: Dipluridae) This kind of rapid shutdown suggests that fluid excretion is tightly regulated rather than passive. The spider’s body monitors its hydration state and dials back waste output accordingly.
Starvation adds another layer. When spiders go without food, they burn through their energy stores in a specific sequence: carbohydrates first, then fats, and proteins only as a last resort. During extended fasts lasting weeks, a spider may metabolize the majority of its stored carbohydrates and nearly half its fat reserves, while protein levels remain relatively stable and body water content stays roughly constant. This careful rationing helps explain how some spiders survive months without a meal.
Why Spider Waste Smells (or Doesn’t)
Fresh spider droppings typically have no strong odor detectable to humans. Guanine itself is not particularly volatile, and the amounts involved are minuscule. Over time, bacterial decomposition of accumulated droppings can produce a faint musty smell, but this is the bacteria at work, not the droppings themselves. If you have noticed an unpleasant odor near a spider infestation, it is more likely coming from the decaying remains of prey items caught in the web than from the spider’s own waste.
For the spiders themselves, however, chemical traces in waste and silk carry biological information. Excretions and silk can indicate the presence, location, sex, or health status of an individual spider, functioning as unintentional chemical cues that other spiders or predators may detect.7PubMed Central. The origin and dynamic evolution of chemical information transfer This is why some species show deliberate sanitary behavior, positioning their droppings at the edges of the web or in designated spots rather than fouling the area where they wait for prey. A spider sitting on a waste-stained web would essentially be broadcasting its location to anything nearby with the right chemical receptors.
Spider Droppings as Fertilizer
One underappreciated consequence of spider waste is its effect on the soil beneath. A study examining both web-building and ground-roaming wolf spiders found that soil collected from under spider retreats had higher nutrient content and greater microbial activity compared with soil from nearby areas without spiders. Plants grown in spider-enriched soil showed measurably greater biomass and height.8Functional Ecology. Spider waste enhances soil nutrient content, soil respiration, and plant growth
This makes ecological sense. Spider droppings are rich in nitrogen (from guanine) and phosphorus, two nutrients that frequently limit plant growth. Since spiders occupy almost every terrestrial ecosystem and can reach impressive densities in favorable habitats, the cumulative nutrient input from millions of tiny guanine deposits adds up. The effect also extends beyond direct nutrient addition: the waste appears to shift the composition and activity of soil microbial communities, which in turn improves nutrient cycling. Gardeners and farmers rarely think of spiders as contributors to soil fertility, but the evidence suggests they function as small-scale nutrient redistributors, concentrating nutrients from their insect prey into localized patches of enriched soil.
What Lives in a Spider’s Gut
Like most animals, spiders host communities of bacteria inside their digestive systems, and the composition of these communities influences how waste is processed. An analysis of three spider species identified 23 bacterial phyla and 150 bacterial families in their guts, with Proteobacteria dominating across all species examined.9Ecology and Evolution. Taking insight into the gut microbiota of three spider species: No characteristic symbiont was found corresponding to the special feeding style of spiders Interestingly, researchers did not find any single bacterial symbiont that seemed specifically adapted to spiders’ unusual liquid diet. The microbiome looked broadly similar to what you find in other arthropods, suggesting that spiders have not evolved a unique digestive partnership with particular bacterial species the way termites have with their wood-digesting microbes.
Some of the bacteria detected, like Wolbachia and Rickettsiella, are known to be inherited from parent to offspring and can influence their host’s reproduction or immune function. Their presence in spider guts raises questions about whether these microbes affect waste composition or nutrient absorption, but the research is still in early stages. What is clear is that spider digestion is not a sterile chemical process. Bacteria are involved, and the mix of species in the gut varies between spider families and even between individuals living in different environments.
Cleaning Up Spider Droppings
If you share your home with spiders, you have probably dealt with their droppings on walls, ceilings, or windowsills. The white-and-dark splatter marks are more noticeable on light-colored surfaces and can be surprisingly stubborn to remove once dried. Fresh droppings wipe away easily with a damp cloth, but older stains may require a mild cleaning solution. The guanine in the solid portion bonds to porous surfaces like painted drywall or untreated wood, and the dark liquid component can leave a stain similar to a small ink spot.
Enzymatic cleaners designed for pet stains work well because they break down the organic compounds. For painted walls, a paste of baking soda and water applied gently with a soft cloth can lift most stains without damaging the finish. Avoid abrasive scrubbing on delicate surfaces, since the droppings often land on ceilings and upper walls where scuff marks are visible. On exterior surfaces like vinyl siding or stone, a pressure washer or scrub brush with soapy water handles the job. The key is frequency: cleaning droppings while they are fresh prevents the buildup that becomes harder to remove later.
Homes with large populations of spiders, particularly in attics, basements, or garages, can accumulate enough droppings to create a visible mess over time. In these cases, addressing the spider population through sealing entry points, reducing insect prey (by managing outdoor lighting, for example), and clearing existing webs will reduce new droppings. The waste itself is not considered hazardous to humans, but heavy accumulation in enclosed spaces can contribute to dust and allergen load, making periodic cleanup worthwhile for air quality.
How Spider Waste Differs from Insect Waste
People often lump spiders in with insects, but their waste products highlight a genuine biochemical divide. Insects excrete uric acid as their primary nitrogenous waste. Spiders excrete guanine. Both compounds serve the same evolutionary purpose, getting rid of nitrogen from protein metabolism while conserving water, but they represent different endpoints in the purine degradation pathway. Spiders appear to stop the process one step earlier than insects, at guanine, because they lack the enzymes that would convert guanine further down the pathway toward uric acid.2Comparative Biochemistry and Physiology. The excreta of spiders
This difference is consistent across a wide range of spider families, suggesting it is an ancient trait that originated deep in spider evolutionary history rather than a specialization of a few species. Scorpions and some other arachnids also excrete guanine, reinforcing the idea that this metabolic quirk belongs to the arachnid lineage broadly rather than to spiders alone.3Nature. Nitrogenous Excretion in Arachnids For a casual observer, the practical difference is that spider droppings tend to look whiter and chalkier than insect frass, which is typically darker and more granular. If you find pale splotchy stains rather than dark pellets, you are almost certainly looking at spider waste.
The distinction matters for pest identification too. Homeowners sometimes confuse spider droppings with those of other household pests like cockroaches or carpet beetles. Cockroach droppings are small dark cylinders or smears, while carpet beetle frass consists of tiny dry pellets mixed with shed larval skins. Spider droppings, with their characteristic white center and liquid splash mark, have a look that is fairly distinctive once you know what to search for.