Spiders do not chew. They cannot swallow solid food. Instead, every spider liquefies its meal before consuming it, flooding the prey’s body with digestive enzymes and then sucking up the resulting slurry. This process, called extra-oral digestion, is the defining feature of how spiders eat, and it shapes everything from their venom to the architecture of their guts. The journey from a struggling insect caught in silk to a nutrient absorbed into a spider’s body is stranger and more sophisticated than most people realize.
Catching the Meal
Spiders have evolved dozens of strategies for getting food to the table, but they broadly split into two camps: web builders and active hunters. Web-building species like orb weavers construct silk traps designed to intercept flying or crawling insects. The sticky capture spirals on an orb web use glue droplets that recruit adhesion across multiple points when prey touches them. Research across 17 orb-weaving species found that the stickiness of capture silk is almost entirely governed by the breaking strength of the thread beneath the glue, not by the glue’s chemistry. The silk has to be strong enough that prey can stick, pull free partway, and re-stick during the struggle without snapping the thread.
1Journal of Zoology. Can a spider web be too sticky? Tensile mechanics constrains the evolution of capture spiral stickiness in orb‐weaving spidersActive hunters skip the web entirely. Jumping spiders stalk their prey with sharp eyesight. Wolf spiders run down insects on the ground. Trapdoor spiders ambush from hidden burrows. Temperature plays a measurable role in how these encounters unfold. A study of the funnel-web spider Mecicobothrium thorelli found that spiders accepted prey across a wide thermal range, from about 5°C to 30°C, but attack speed increased at warmer temperatures, while prey detection was sharpest at moderate temperatures.
2Arachnology. Faster but not optimal: how temperature affects prey capture behaviour and physiology in the winter midget funnel-web spider Mecicobothrium thorelliWhatever the hunting method, the next step is nearly always the same: the spider needs to subdue its prey fast, because a thrashing insect can damage the spider or escape.
What the Venom Actually Does
Almost all spiders are venomous (the rare exceptions include the family Uloboridae, which relies entirely on silk wrapping). A spider typically delivers venom by driving its fangs, called chelicerae, into the prey. Spider venoms are complex cocktails. Among the most studied components are cysteine-rich peptides that target ion channels and membrane receptors in the prey’s nervous system.
3PubMed Central. Spider Venom: Components, Modes of Action, and Novel Strategies in Transcriptomic and Proteomic AnalysesThe wandering spider Cupiennius salei offers a good case study. Its main neurotoxin, CsTx-1, does double duty. As a neurotoxin, it blocks calcium channels, disrupting nerve signaling. But its cationic tail also punches holes in cell membranes directly, causing irreversible depolarization of muscle cells. In insect larvae, this means rapid paralysis. The effect appears to be receptor-independent, meaning the toxin does not need a specific docking site on the cell surface. It simply destabilizes membranes.
4PubMed Central. A venom-derived neurotoxin, CsTx-1, from the spider Cupiennius salei exhibits cytolytic activitiesThe speed of paralysis varies with the prey. Ant-eating spiders in the family Zodariidae, for instance, face especially dangerous targets. The Asian species Asceua japonica adjusts its tactics depending on the ant species it encounters. When biting formicine ants, it holds on continuously and the ant goes limp within about three to four seconds. But when attacking myrmicine ants, which often have stinging defenses, the spider bites for just one to two seconds and then releases the ant, retreating about a centimeter away. The envenomated ant walks erratically for ten to fifteen seconds before collapsing.
5PubMed Central. Diet and predatory behavior of the Asian ant-eating spider, Asceua japonica (Araneae: Zodariidae)That bite-and-retreat approach is essentially risk management. Some ant species can muster nest-mates or deliver retaliatory stings, so the spider injects just enough venom and backs off until the toxin finishes working. Individual personality even factors in: among ant-eating spiders in the genus Zodarion, more aggressive individuals attack sooner, attack more often, and paralyze prey faster than cautious ones.
6Current Zoology. Personality is associated with the hunting strategy in a specialized ant-eating spiderWrapping the Prey in Silk
Many spiders wrap their catch in silk before or during feeding. Orb weavers are famous for spinning prey into tight bundles, but the practice is widespread even among non-web-building species. The silk used for wrapping comes primarily from the aciniform glands and is structurally different from the dragline silk that forms the web’s spokes. Aciniform silk has a fiber diameter of roughly half a micrometer, wrapped around the much thicker structural silk fibers that are about ten times wider.
7Communications Materials. Prey-wrapping spider silk as a model for tough and adaptable protein materialsWrapping serves several purposes at once. It physically restrains the prey, reducing the chance of escape or injury to the spider. It can serve as a storage method: some spiders wrap multiple prey items and return to feed later. And for species like the Uloboridae that lack venom entirely, wrapping is the sole means of subduing prey. The silk is remarkably tough for its size, with the protein structure converting from one form to another as it is spun, giving the finished fiber both flexibility and strength.
Dissolving Dinner Outside the Body
Here is where spider feeding gets genuinely alien. Once the prey is subdued and wrapped, the spider regurgitates digestive fluid from its midgut onto or into the prey’s body. These fluids contain a concentrated mix of enzymes that break down the prey’s tissues from the inside out. The spider then alternates between flooding the prey with fluid and sucking the liquefied contents back up, sometimes over a period of hours. What remains at the end is a dry, hollow husk.
The enzyme cocktail itself is complex. Analyses of spider digestive fluids have found large numbers of trypsin-like proteases and astacin-like metalloproteases, the latter being zinc-dependent enzymes found across the animal kingdom but present in unusually high numbers in spider digestive fluid.
8PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestionThe digestion process is not one-step. Researchers studying the golden orb-weaver Nephilengys cruentata established that extra-oral digestion occurs under alkaline conditions, with trypsins and astacins doing the heavy lifting outside the body. But once the liquefied food is swallowed, a second round of digestion occurs inside the spider under acidic conditions, driven by a different class of enzymes called cysteine cathepsins. These work within digestive vacuoles or lysosome-like compartments in the gut cells.
9Insect Biochemistry and Molecular Biology. Cysteine cathepsins as digestive enzymes in the spider Nephilengys cruentataAn intriguing wrinkle in this system is that venom and digestive fluid share a lot of the same proteins. In the social spider Stegodyphus mimosarum, researchers found that out of 153 proteins identified in the venom, 101 also appeared in the digestive fluid. Going the other direction, out of the 71 most abundant digestive fluid proteins, 27 were also present in the venom. The shared proteins included carbohydrases, proteases, and metalloproteases. This overlap suggests that envenomation and digestion are not entirely separate processes. When a spider bites its prey and injects venom, it may already be beginning the digestive process from the inside.
8PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestionInside the Spider’s Gut
The internal anatomy of a spider’s digestive system looks nothing like a vertebrate gut. There is no long tube with distinct stomach and intestinal sections. Instead, the midgut branches into a series of blind-ended pouches called diverticula that extend throughout the abdomen and even into the legs in some species. These diverticula massively increase the surface area available for nutrient absorption.
The lining of these diverticula contains several specialized cell types. Secretory cells produce the digestive enzymes that are regurgitated onto prey. Digestive cells absorb the liquefied nutrients once they are sucked back in, processing them in internal vacuoles. A third type, adipocytes, stores lipids and other energy reserves. The apical surfaces of these cells are covered in tiny projections called microvilli that increase absorption efficiency, and the digestive cells contain vacuoles filled with material at various stages of breakdown.
10PubMed Central. Changes in the midgut diverticula epithelial cells of the European cave spider, Meta menardi, under controlled winter starvationThe midgut also contains spherites, layered mineral deposits that store ions and possibly serve as detoxification sites. During starvation, these structures are gradually consumed. Spiders essentially mine their own gut lining for resources when food runs out.
Surviving Between Meals
Spiders are sit-and-wait predators, and meals are unpredictable. Many species routinely go days or weeks without eating, and some can survive months. Their metabolism is built around this feast-or-famine pattern. During fasting, spiders reduce their energy consumption and rely primarily on stored lipids to stay alive.
11Comparative Physiology of Fasting, Starvation, and Food Limitation. Metabolic Transitions During Feast and Famine in SpidersWhat a spider ate before the fast matters. The wolf spider Pardosa prativaga, when fed lipid-rich prey, built up larger fat reserves and survived longer during subsequent starvation than spiders that had been eating protein-rich prey. Oxygen consumption and carbon dioxide output dropped gradually as the fast continued. Interestingly, these spiders did not try to adjust their metabolism to maintain a constant body composition. They simply stored lipids efficiently when they could, treating fat as a buffer against the inevitable lean times.
12Journal of Insect Physiology. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativagaAt the cellular level, prolonged starvation triggers autophagy in the midgut diverticula. The digestive cells, secretory cells, and adipocytes all begin recycling their own internal components. Autophagosomes engulf worn-out organelles, and the spherites that store minerals are consumed until, in some cases, only the empty membrane remains.
10PubMed Central. Changes in the midgut diverticula epithelial cells of the European cave spider, Meta menardi, under controlled winter starvationWater Is a Separate Problem
You might assume that sucking up liquefied insect provides all the water a spider needs. It does not. Orb web spiders deprived of drinking water but given plenty of prey still became water-stressed. When researchers offered water afterward, these spiders drank significantly more than spiders that had been receiving water all along, regardless of how much food they had eaten. Drinking appears to let spiders regulate their water balance independently from their nutritional intake, and researchers have suggested the spiders may need fresh water specifically to process ingested nutrients.
13Ethology. Water as an Essential Resource: Orb Web Spiders Cannot Balance Their Water Budget by Prey AloneIn the wild, many spiders drink dew droplets from their webs in the morning or drink from rain collected on leaves. Some species actively seek out water sources. This is often overlooked in popular descriptions of spider biology, which tend to focus exclusively on prey capture and venom. But dehydration can kill a spider faster than starvation can.
When Spiders Steal Instead of Hunt
Not every spider catches its own food. Kleptoparasitism, the theft of food gathered by another animal, is widespread among spiders. The most-studied cases involve small spiders that live on the webs of larger species and steal wrapped prey or feed on insects too small for the host spider to bother with. But the phenomenon extends well beyond a few freeloading specialists.
A recent survey found that spider hosts of kleptoparasites span roughly 200 species across 86 genera and 23 families, a distribution that is not random across the spider family tree but concentrated in certain lineages.
14Frontiers in Arachnid Science. Spiders as superhosts and secondary kleptoparasitesThe cost to the host varies. In some cases the kleptoparasite is tiny relative to the host and takes so little food that the impact is negligible. In others, kleptoparasites can significantly reduce the host’s feeding success, essentially acting as parasites that siphon off the caloric benefit of each prey capture. Some kleptoparasitic spiders even vibrate the host’s web in patterns that mimic trapped prey, luring the host spider away from its meal so the thief can feed undisturbed. From the kleptoparasite’s perspective, the strategy eliminates the energy cost and risk of building its own web or hunting, at the price of total dependence on the host species.
Spiders That Eat Something Other Than Insects
The classic image of spider feeding is insect capture, but reality is more varied. Fishing spiders in the genus Dolomedes hunt aquatic insects, tadpoles, and even small fish, detecting prey by vibrations on the water surface. Large theraphosid spiders (tarantulas) occasionally take small vertebrates like lizards, frogs, and mice, though insects still make up the bulk of their diet. Some crab spiders sit inside flowers and ambush pollinators, using pigment-matching camouflage to avoid detection.
At the extreme end of dietary flexibility, at least one spider species, Bagheera kiplingi, feeds primarily on plant material. This Central American jumping spider harvests protein- and lipid-rich structures called Beltian bodies from the tips of acacia leaves, making it the only known predominantly herbivorous spider. Even B. kiplingi occasionally eats ant larvae, so it has not entirely abandoned animal food, but its diet overturns the assumption that spiders are obligate carnivores.
Regardless of the food source, the basic feeding mechanism stays the same. The spider regurgitates enzymes, liquefies whatever tissue it can access, and sucks up the result. A spider feeding on a fish uses the same extra-oral digestion system as one feeding on a fruit fly. The digestive chemistry is versatile enough to handle a wide range of protein and lipid sources, which helps explain why spiders have colonized nearly every terrestrial habitat on the planet.
How the Whole System Fits Together
What makes spider feeding genuinely unusual among predators is how tightly integrated venom, silk, and digestion are. These are not three separate systems that happen to coexist. The overlap in protein composition between venom and digestive fluid suggests that the envenomation step and the digestion step are biochemically continuous.
8PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestionWhen a spider bites prey, venom components begin breaking down tissue at the wound site. When the spider later floods the prey with digestive fluid, many of the same enzyme families continue the work at higher concentrations. Silk wrapping keeps the dissolving contents contained, preventing nutrient loss and protecting the feeding spider from scavengers. The branching midgut diverticula then maximize absorption of the liquid meal, and any surplus is stored as lipid reserves in adipocytes that the spider can draw on during the inevitable famine to come. Each piece of the system depends on the others, and together they make spiders one of the most metabolically efficient predators for their body size.
Waste and What Gets Left Behind
After a spider has extracted everything it can from a prey item, what remains is largely cuticle, the tough exoskeleton that resists enzymatic breakdown. The discarded husk is sometimes called a bolus, and web-building spiders typically drop it from the web or push it to the edge. Inside the spider, indigestible waste and metabolic byproducts accumulate in the stercoral pocket, a pouch at the posterior end of the gut. Spiders excrete nitrogenous waste primarily as guanine, a purine compound, rather than urea or uric acid. The white droppings you occasionally see beneath spider webs are guanine-rich fecal material expelled from the stercoral pocket.
The spherites stored in the midgut lining also function as waste repositories. Heavy metals and other potentially toxic ions picked up from prey can be sequestered in these layered mineral deposits, keeping them out of the spider’s active metabolism. When a spider molts, some accumulated waste in the gut lining is shed along with the old cuticle, giving the animal a partial internal reset. This dual system of excretion and sequestration helps spiders tolerate prey that might carry trace contaminants, a useful adaptation for a generalist predator that cannot be choosy about which insect lands in its web.