A spider has no teeth and cannot chew, so it eats by turning its prey into a liquid it can drink. Most of the work of eating happens outside its body, before any food reaches its mouth.1Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda That lets a small hunter take on prey nearly as big as itself, even prey protected by a hard outer shell.2Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda The strangest part may come after the meal: the spider’s gut is built to store food for weeks of fasting.
What Fangs and Venom Do First
A spider’s fangs sit at the tips of its jaws, the paired mouthparts in front of its face. In most spiders, venom flows from glands through a tiny opening near each fang tip. The venom stops the prey from struggling, which protects the spider from kicks and stings. In tarantulas and their relatives, the fangs point straight down, parallel to each other, and strike downward.3Journal of Entomology and Zoology Studies. Determining with SEM, structure of the venom apparatus in the tube web spider, Segestria florentina (Araneae: Segestriidae) In most familiar house and garden spiders, the fangs swing toward each other like pincers.4Journal of Entomology and Zoology Studies. Determining with SEM, structure of the venom apparatus in the tube web spider, Segestria florentina (Araneae: Segestriidae)
In practice, you see this when a fly lands in a web. The spider rushes in, bites, and often backs off for a moment while the venom acts. Many web spiders also wrap the prey in silk, either before or after the bite. The fly usually stops moving within seconds to minutes, and the spider may then carry the bundle to the hub of the web to feed.
The fang design changes the motion of the bite. Pincer-style fangs can pinch prey between them, which suits spiders that grab insects from a web or off a surface. Straight-down fangs work more like a pair of stabbing spikes, so the spider has to rear up and strike onto prey beneath it. Either way, venom is delivered through a hollow tip. Some venoms also begin breaking down tissue.
Not every spider uses venom. One family, the cribellate orb weavers, has lost its venom glands entirely.5PubMed Central. Beyond venomous fangs: Uloboridae spiders have lost their venom but not their toxicity These spiders subdue prey by wrapping it tightly in silk instead. In lab tests, their gut fluids did not attack insect nerve signals, so the wrapping, not a hidden poison, does the subduing.6PubMed Central. Beyond venomous fangs: Uloboridae spiders have lost their venom but not their toxicity
Digesting the Prey Before Swallowing
Spiders digest food outside their bodies before swallowing it.7Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda They are in large company: at least 79% of predatory land arthropods eat this way.8Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda The method gets more nourishment out of prey wrapped in a hard shell than biting off pieces could.9Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda It also shortens handling time and makes each mouthful richer in nutrients.10Annual Review of Entomology. Extra-Oral Digestion in Predaceous Terrestrial Arthropoda
The spider brings up digestive fluid from its gut and puts it onto the prey. The fluid is packed with enzymes that break proteins into smaller and smaller pieces.11PubMed Central. Digestive enzymes and sphingomyelinase D in spiders without venom (Uloboridae) Different spiders use somewhat different enzyme mixes to do the same job on insect tissue.12PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestion The hard outer shell mostly survives, which is why a drained insect looks intact.
Everything inside, such as muscle and soft organs, turns to liquid and gets drunk. A husk can therefore keep the exact shape of the insect, legs and wings included, while weighing almost nothing. Some spiders crush their prey into a shapeless ball while feeding, but whole or crumpled, the leftover is hollow and light.
A spider often repeats the cycle: fluid onto the prey, some drawn back, then more fluid. That back-and-forth is how a small spider works steadily through a body much tougher than any single dose could dissolve.
Feeding can last from minutes to many hours depending on the size of the prey. Big prey needs several rounds of juice and sucking, and the spider may move the prey around to reach new spots.
Sucking Up the Liquid Meal
A spider drinks with a sucking stomach, a muscular chamber that expands to pull liquid in.13British Arachnological Society. The mystery of how spiders extract food without masticating prey The liquid vanishes from the prey in small rhythmic pulses.14British Arachnological Society. The mystery of how spiders extract food without masticating prey Those pumps run roughly 2 to 7 times a second, and the side mouthparts move back and forth at about the same pace while sucking.15British Arachnological Society. The mystery of how spiders extract food without masticating prey
Filters stand between the prey and the spider’s gut. Thick brushes of hairs around the mouth catch larger bits.16British Arachnological Society. The mystery of how spiders extract food without masticating prey A second filter in the throat then strains out particles as small as about a thousandth of a millimetre.17British Arachnological Society. The mystery of how spiders extract food without masticating prey What the spider swallows is almost entirely liquid.18British Arachnological Society. The mystery of how spiders extract food without masticating prey
Where the Food Goes Inside a Spider
From the stomach, food flows into the midgut, which branches into many side pouches through the body. The pouch walls hold cells that digest, cells that release enzymes, and fat-storing cells.19PubMed Central. Changes in the midgut diverticula epithelial cells of the European cave spider, Meta menardi, under controlled winter starvation So the gut is both where digestion finishes and the spider’s pantry. The branched midgut spreads the liquid food to many parts of the body rather than holding it in one central tube, and the last stage of digestion happens inside the gut cells themselves as they take in the nutrient-rich liquid.
You can often see the effect on the outside. After a good meal, the pouches fill and the abdomen visibly swells. A thin, shrunken abdomen suggests a spider that has not eaten recently and is drawing on its stores.
Waste mostly stays outside as the empty husk. What remains inside leaves as small whitish droppings, so whitish spots beneath a web are often spider waste.
Eating also costs energy. In wolf spiders, feeding sharply and quickly raises the body’s energy use.20PubMed Central. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativaga About 21% of the energy in a meal goes into catching and digesting it.21PubMed Central. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativaga
How Long Spiders Can Go Without Eating
During starvation, a wolf spider’s breathing and energy use drop gradually.22PubMed. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativaga The lower burn rate stretches its stored fat over a much longer time. A spider that looks like it is doing nothing can stay alive for weeks or longer.
The last meal matters. Spiders that ate fatty prey survived starvation longer than those fed protein-rich prey.23PubMed. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativaga That is because spiders store fat efficiently instead of keeping a fixed body makeup.24PubMed. Metabolic consequences of feeding and fasting on nutritionally different diets in the wolf spider Pardosa prativaga In overwintering cave spiders, the gut lining changes as stores are used up.25PubMed Central. Changes in the midgut diverticula epithelial cells of the European cave spider, Meta menardi, under controlled winter starvation
This suits sit-and-wait hunters, which may go long stretches without prey and bridge the gaps with fat stored after a big catch. A spider that hasn’t eaten in weeks isn’t necessarily in trouble, and a hungry spider in winter is usually waiting, not dying.
For pet spider keepers, that means adults usually need feeding only every week or two. Refusing food before a molt is normal, and overfeeding swells the abdomen without helping.
Spiders That Eat Differently
Orb weavers, crab spiders and cobweb spiders spit fluid onto the prey’s surface and suck it back, rather than injecting it.26British Arachnological Society. The mystery of how spiders extract food without masticating prey Venomless orb weavers soak the whole silk-wrapped bundle at once, often without their mouths touching the prey.27British Arachnological Society. The mystery of how spiders extract food without masticating prey This works because liquid creeps through tiny gaps, carrying juices in and soup out.28British Arachnological Society. The mystery of how spiders extract food without masticating prey In these spiders the dense silk wrapping also acts as a filter for the digested food.29Canadian Journal of Zoology. Feeding by Philoponella vicina (Araneae, Uloboridae) and how uloborid spiders lost their venom glands Silk holds the prey still, which keeps digestive fluid in contact with its surface.
Watching a feeding spider gives loose clues about its technique. Wolf spiders and other active hunters hold prey directly in their jaws. A neat silk bundle held at the mouth suggests a wrapper that feeds on the surface. A crumpled, shapeless ball points to a spider that mashes its meal with its jaws. These are not exact tests, since many spiders mix methods depending on the prey.
Do Spiders Drink Water
Spiders get much of their water from the liquid meals they drink. They also take in moisture directly: wolf spiders can drink water held in damp soil.30Journal of Experimental Biology. On The Drinking of Soil Capillary Water by Spiders As the soil dries and holds water more tightly, they eventually can’t pull it out, even when it still feels wet.31Journal of Experimental Biology. On The Drinking of Soil Capillary Water by Spiders A damp spot or water droplets therefore help a spider in a dry room or tank. Morning dew on webs gives many garden spiders an easy source, and spiders indoors may turn up near sinks and bathtubs, where moisture is more likely.
For pet spiders, a shallow dish or a lightly misted surface works. Deep water could drown small spiders, and an overly damp enclosure can grow mould.