Spiders eat other spiders all the time, and this behavior is so widespread that biologists have a dedicated term for it: araneophagy. Some spiders are opportunistic about it, grabbing a smaller spider when the chance arises. Others have evolved into full-time spider specialists, with hunting strategies and even custom-built venom designed specifically for taking down fellow arachnids. Beyond predation between species, spiders also consume their own kind through sexual cannibalism, sibling cannibalism, and a startling behavior in which young spiders devour their own mother. The world of spider-on-spider consumption is richer and stranger than most people realize.
How Common Is Spider-Eating Among Spiders
Spider-eating is not some rare quirk confined to a handful of exotic species. A comparative analysis of spider diets across many families found that araneophagy has turned up in lineages spread across the spider family tree, appearing in both basal and intermediate branches. Unlike some other forms of dietary specialization that clearly evolved later, the evidence suggests that eating spiders may be an ancestrally rooted behavior rather than a recently acquired one.
Even among generalist spiders that mostly eat insects, molecular gut-content studies have found spider DNA in their stomachs. Research on spiders in vegetable fields using high-throughput sequencing revealed that while the bulk of prey in spider guts consisted of flies, beetles, moths, and other insects, arachnid DNA also showed up, confirming that spiders in these communities were eating each other alongside their insect prey.
The Specialists Who Hunt Only Spiders
The most impressive spider-eaters are species that have made other spiders their primary food source. These are not accidental encounters but deliberate, sophisticated predatory strategies honed over evolutionary time.
The genus Portia, a group of jumping spiders, is perhaps the most studied example. Portia fimbriata is unusual among jumping spiders because it builds its own web and also invades the webs of other spiders. In the field, its prey consists overwhelmingly of other spiders, especially other jumping spiders. In laboratory observations, Portia used its web as a vantage point for stalking spider prey that wandered nearby, and also dropped down on silk draglines to ambush spiders walking below. When another spider entered its web, Portia waited and struck when the intruder came close.
Pirate spiders in the family Mimetidae take a different approach. These spiders invade the webs of other species and perform what researchers call aggressive mimicry: they pluck and vibrate the host spider’s web in patterns that mimic trapped prey. The resident spider approaches expecting a meal and instead gets attacked. Pirate spiders lunge at close range, seize the victim in a basket formed by their spine-covered legs, and deliver a bite with venom that appears specifically effective against spiders.
Then there is the cellar spider, Pholcus phalangioides, a species many people have encountered in basements and garages. Pholcus is both a web builder and a web invader. It enters the webs of other spiders to catch them, and when threatened by the resident, it can whirl rapidly in the web as a defensive maneuver, though research found it was less inclined to whirl when in someone else’s web than when defending its own.
Even among ant-mimicking jumping spiders, a group you might expect to specialize on ants, spider-eating has been documented. Myrmarachne melanotarsa, a jumping spider that looks remarkably like an ant, was found to actively choose spiders as preferred prey. This was the first demonstrated case of araneophagy in an ant-like salticid, and the first outside the basal subfamily Spartaeinae that had previously been the only jumping spider group known to specialize on spider prey.
Venom Built for Killing Spiders
Spider-eating specialists do not just hunt differently; their venom is chemically different too. Research on prey-specialized spiders found that their venom was more toxic to their preferred prey type than to alternative prey, and that this heightened toxicity was linked to unique chemical compounds in the venom not found in generalist spiders.
One study broke spider venom into size fractions to see which components did the work. The larger compounds, those above 10 kilodaltons, paralyzed both preferred prey and non-preferred prey equally well. But the smaller compounds, below 10 kilodaltons, were effective only against the spider’s preferred prey. This suggests the venom contains prey-specific toxins fine-tuned for the particular type of animal the spider hunts most, whether that target is another spider, an ant, or some other arthropod.
For spider-eating specialists, this means their chemical arsenal has been shaped by the same evolutionary pressures as their hunting behavior. The venom of pirate spiders, which was described in earlier research as “apparently spider-specific,” fits this pattern. These animals are not simply using brute-force toxicity; they carry molecular keys that fit the locks of spider physiology.
Sexual Cannibalism
Perhaps the most famous form of spider-on-spider consumption is sexual cannibalism, where one mate, almost always the female, eats the other during or after mating. This behavior exists in a number of spider species, but the Australian redback spider (Latrodectus hasselti) offers the most dramatic example because the male actively cooperates in his own death.
During copulation, male redback spiders position themselves directly above the female’s jaws. Research published in Science showed that this apparent suicide is actually favored by sexual selection because it yields two concrete advantages. Males that were cannibalized copulated for longer and fertilized more eggs than males that survived. On top of that, females that consumed their first mate were more likely to reject subsequent males, effectively giving the eaten male a paternity monopoly.
This pattern of male self-sacrifice has evolved independently across multiple spider lineages. A broader evolutionary analysis found that male sacrifice behavior was repeatedly correlated with genital mutilation, and that a male-biased effective sex ratio, meaning more males competing for fewer mating opportunities, appears to be a precondition for the evolution of this kind of terminal mating strategy. When a male’s chances of finding another mate are slim anyway, the reproductive math can favor dying on the spot if it means fathering more offspring.
Mothers Devoured, Siblings Consumed
Sexual cannibalism gets the headlines, but some of the most striking spider-eating behavior happens within families. In the spider Amaurobius ferox, mothers are systematically devoured by their own young. This is not an accident or a sign of neglect. The cannibalistic process happens at a fairly consistent interval after hatching, unfolds within a few hours, and involves what researchers described as an exchange of stimulation between mother and offspring, suggesting a coordinated behavior rather than a one-sided attack.
This phenomenon, called matriphagy, seems to function as a form of extreme parental investment. The mother’s body becomes a high-quality meal that gives her spiderlings a critical nutritional boost during the most vulnerable stage of their lives. From the mother’s perspective, if her reproductive opportunities are already spent, converting her own body into food for her offspring may be the most effective way to maximize the survival of her genes.
Sibling cannibalism is also widespread. In the false widow spider Steatoda grossa, newly hatched spiderlings eat each other, but the rate depends heavily on conditions. Research found that cannibalism dropped when fresh prey was replenished more frequently, indicating that starvation is a primary driver. Interestingly, higher spiderling density was also associated with lower cannibalism rates, which is counterintuitive until you consider that denser groups may simply encounter fresh food sooner. The researchers concluded that juvenile cannibalism in this species occurs mainly under extreme food limitation, though in human-associated habitats where the spider is common, it could be an adaptive fallback when prey is scarce.
A recent study on group-reared spiderlings added a strange twist to the sibling story. Spiderlings raised together tolerated their living siblings but immediately cannibalized siblings that had just died. This happened regardless of whether the corpse was fresh or a day old, suggesting that living spiders produce some kind of signal, chemical or otherwise, that actively protects them from being treated as food. The moment that signal disappears at death, a sibling becomes fair game.
How Spiders Defend Themselves Against Spider-Eaters
Given how common spider-on-spider predation is, it makes sense that spiders have evolved defenses against it. These range from simple behavioral avoidance to specialized physical responses.
Web-building spiders face a particular challenge because their webs, while excellent for catching insect prey, are also highways for spider-eating invaders. The cellar spider Pholcus phalangioides uses a rapid whirling behavior when threatened in its own web, spinning so fast that it becomes a blur and is difficult for an attacker to target. This defense works well on home turf, though as noted earlier, the spider is less likely to employ it when it has entered someone else’s web, where the silk architecture may not support the same spinning motion.
Other spiders rely on speed, cryptic coloring, or simply abandoning their web when they detect the vibrations of an invader. Some orb-weaving spiders cut their webs and drop to the ground when a pirate spider begins its aggressive-mimicry plucking. The evolutionary arms race between spider predators and spider prey has produced a catalog of countermeasures, though researchers have studied the attack strategies of specialists far more thoroughly than the full range of defenses their prey employ.
What Spider-Eating Means for Ecosystems
When spiders eat other spiders, ecologists call it intraguild predation: predators consuming other predators that would otherwise be competitors. This has real consequences for how spider communities function, especially in agricultural and urban ecosystems where people care about pest control.
In vegetable fields, molecular diet analysis showed that spiders from different families were eating each other alongside their insect prey, revealing complex webs of interfamily predation. This matters for pest management because if the spiders you are counting on to eat crop pests are busy eating each other instead, the net benefit to the farmer is reduced. Reviews of spiders as biological control agents have highlighted these “complex intraguild interactions” as a complication that makes it hard to predict how much pest suppression a spider community will actually deliver.
Invasive spiders can amplify these dynamics. A study of Cheiracanthium mildei, an introduced spider in North American habitats, found that despite suppressing leafhopper populations (a beneficial effect), it negatively affected a range of native spider species, likely through intraguild predation. The introduced spider was eating both pests and native predators at the same time.
The Joro spider (Trichonephila clavata), which has been rapidly expanding its range in the southeastern United States, has raised similar concerns. DNA metabarcoding of gut contents from Joro spiders and three native orb-weaving species revealed that while there was some overlap in what they ate, the Joro spider’s overall diet was distinct, including at least 26 prey taxa not detected in native spiders. Native spider diets did not appear to change at sites where Joro spiders were present. The initial evidence suggests that competition for food may be less severe than feared, though intraguild predation and web-placement competition could still cause problems as the invasion progresses.
When Bigger Females Mean Fewer Babies
Climate and environmental conditions can shift the balance of spider-on-spider predation in unexpected ways. Research on a dominant arctic wolf spider, Pardosa lapponica, turned up a puzzling finding. In most spider species, larger females produce more eggs. But in this population, areas with larger females actually had fewer juveniles. The explanation appears to be density-dependent cannibalism: when conditions allow females to grow bigger, they also produce more offspring, which increases competition and drives up the rate at which spiders eat each other.
Mesocosm experiments supported this interpretation. At higher wolf spider densities, individuals occupied higher trophic positions, meaning they were eating more of their own kind rather than just insects. In the context of a warming Arctic, where growing seasons are lengthening and spider body sizes may increase, this suggests that more cannibalism could partially offset the expected population boom. The spiders’ own predatory behavior on each other acts as a self-regulating mechanism.
Studying What Spiders Actually Eat
For a long time, understanding spider diets was difficult. Spiders liquefy their prey before ingesting it, so you cannot simply open a spider’s stomach and identify half-digested body parts the way you can with a bird or a fish. Visual observation in the field catches only a fraction of predation events, and many happen at night or inside webs where they are hard to see.
Molecular techniques have changed this. High-throughput DNA sequencing of spider gut contents now allows researchers to identify prey species from genetic traces, even when the physical evidence has been completely destroyed by the spider’s external digestion. This approach has revealed far more spider-on-spider predation than earlier methods detected. In one study of spiders in Brassica vegetable fields, DNA metabarcoding uncovered not just the expected insect prey but a full network of interfamily spider predation that would have been invisible using older methods.
These molecular tools have also been essential for studying invasive species. The Joro spider research, for instance, relied on DNA metabarcoding to compare diets across four spider species at 52 sites, a scale of dietary analysis that would have been impossible with direct observation alone. As these techniques become more common, the picture of how frequently and how broadly spiders eat each other is likely to keep expanding.