Do Daddy Long Legs Kill Black Widows?

Cellar spiders, the spindly-legged arachnids most people mean when they say “daddy long legs,” can and sometimes do kill black widows. Observations of this have been reported by homeowners, pest control professionals, and arachnologists for decades, and the matchup has become one of the more persistent pieces of spider lore on the internet. But the popular version of the story, which usually comes packaged with the claim that daddy long legs carry the world’s deadliest venom, gets almost every detail wrong except the basic outcome.

Three Creatures, One Nickname

Before anything else, it helps to sort out what “daddy long legs” actually refers to, because the common name is used for three entirely different animals depending on where you live. In the United States, the name most often lands on harvestmen (order Opiliones), the round-bodied, eight-legged creatures you see clumped together on tree trunks or basement walls. Harvestmen are arachnids but not spiders at all. They have no venom glands, no silk glands, and no fangs capable of piercing human skin or spider exoskeletons. A harvestman is not killing a black widow under any circumstances.

The second creature called “daddy long legs” is the crane fly, a gangly, mosquito-like insect that bumbles around porch lights in summer. Crane flies are completely harmless to other insects and to people. They eat almost nothing as adults.

The third and most relevant candidate is the cellar spider, family Pholcidae. These are genuine spiders with venom, fangs, and silk. They build messy, irregular webs in dark corners and are common in garages, basements, and crawl spaces across much of the world. When people claim that daddy long legs kill black widows, cellar spiders are the ones actually doing it. For the rest of this article, “daddy long legs” means pholcid cellar spiders unless stated otherwise.

How Cellar Spiders Take Down Black Widows

The predation is real, but the mechanism is less dramatic than the internet version suggests. Cellar spiders are araneophagous, meaning they eat other spiders as a regular part of their diet. When a cellar spider encounters a black widow, it typically does not charge in for a direct fight. Instead, it relies on a combination of physical advantages and behavioral tactics that favor it against web-dwelling prey.

The cellar spider’s most obvious advantage is leg length. Its legs are disproportionately long relative to its body, and it uses them to stay at a distance while flicking silk at its target. By throwing strands of its own silk from beyond the black widow’s striking range, the cellar spider can gradually immobilize the widow without ever getting close enough to be bitten. Once the widow is sufficiently wrapped, the cellar spider moves in and delivers a bite. The widow’s own cobweb works against it here: it is a tangled, three-dimensional structure that the cellar spider navigates well, while the widow, already restrained by thrown silk, cannot maneuver effectively.

This hunting strategy is not unique to encounters with black widows. Cellar spiders use the same silk-throwing, stay-at-range approach against a variety of other spiders, including other pholcids. They are generalist spider predators, and black widows happen to be one of the prey items they can subdue. But “can” and “routinely do” are different claims, and this matters for anyone hoping cellar spiders will serve as a natural pest control solution. A cellar spider living in your garage might kill a black widow if the two cross paths. It also might not. Cellar spiders eat whatever prey wanders into their web, and they are not specifically seeking out widows.

The Venom Myth

The most persistent piece of misinformation attached to this story is the claim that daddy long legs possess the most toxic venom of any spider but cannot deliver it because their fangs are too small to penetrate human skin. Both halves of this claim are wrong.

On the fang side: cellar spiders can bite humans. Their chelicerae (mouthparts) are small, but they are not too small to break skin. Documented human bites from pholcid spiders exist. They produce a mild sting comparable to a tiny pinprick, and the sensation fades quickly.

On the venom side: cellar spider venom is not especially potent. A study testing the crude venom of the pholcid species Physocyclus mexicanus on crickets found an effective paralytic dose of about 3.9 micrograms per gram of body weight, a potency roughly comparable to other small spider taxa. The venom did produce some interesting effects in crickets, including flaccid paralysis and areas of tissue darkening, but these were insect bioassay results, not evidence of extreme toxicity to vertebrates. The researchers noted that documented human bites from pholcids result in a mild sting with no long-lasting effects, and they concluded that the effects of pholcid bites on humans and other mammals are “inconsequential.”¹

So the venom is neither extraordinary in potency nor undeliverable. The myth likely originated as a schoolyard factoid and gained traction because it has an appealing paradox structure: the most dangerous spider is also the most harmless. It turns out that the actual story, a modest spider with modest venom that can still kill much larger, genuinely dangerous spiders through clever hunting tactics, is more interesting than the myth anyway.

Why a Cellar Spider Can Beat a Black Widow Without Potent Venom

You might wonder how a spider with weak venom manages to kill one with notoriously strong venom. The answer is that venom potency is largely irrelevant to this particular matchup. The cellar spider’s strategy does not depend on overpowering the widow chemically. It depends on never giving the widow a chance to fight back.

The silk-wrapping technique keeps the cellar spider out of the widow’s bite range for most of the encounter. By the time the cellar spider closes in, the widow is already restrained. The final bite delivers enough venom to subdue a spider that has already been partially immobilized. This is the same basic logic that allows much smaller predators across the animal kingdom to take down larger, more dangerous prey: avoid the weapon, control the body, finish the job.

Black widows, for their part, are not built for spider-on-spider combat. Their web is a prey trap, not a defensive fortification. Research on black widow web defense shows that web structure does play a role in contests between spiders. When western black widows competed against conspecific intruders, residents with webs containing more structural lines were more likely to successfully defend their territory. But even this structural advantage had limits: against brown widow intruders, the number of structural lines did not predict whether the resident could fend off an invader.² A cellar spider invading from outside the web, using a fundamentally different attack approach than another widow would, sidesteps whatever defensive advantage the web structure provides against widows’ typical rivals.

How Black Widows Detect Intruders

Black widows sense the world primarily through vibrations transmitted along their web silk. Every time something touches the web, it sends a signal. The widow has to distinguish between a struggling insect (dinner), a courting male (potential mate), and a threat (time to flee or fight). Research on western black widows found that females respond based on the amplitude of the vibration, not its waveform. In playback experiments, far fewer females responded aggressively to low-amplitude vibrations, regardless of whether the waveform mimicked prey or a male spider. When the amplitude was cranked up to match the vigorous struggles of prey, about 87.5 percent of females responded, compared with roughly 30 to 44 percent for the low-amplitude signals.³

This has an interesting implication for the cellar spider matchup. A cellar spider approaching on its own long, delicate legs and throwing individual silk strands may produce only low-amplitude disturbances on the widow’s web, the kind of signal the widow is less likely to respond to aggressively. Male black widows are known to exploit this same perceptual bias: they approach females using very quiet, low-amplitude vibrations to avoid being mistaken for prey and attacked. A cellar spider may benefit from a similar acoustic camouflage, not by intent but simply because its lightweight body and careful approach happen to fall below the widow’s alarm threshold. By the time the widow realizes something is wrong, the silk wrapping may already be underway.

What Black Widow Venom Actually Does

Understanding why people fear black widows in the first place helps put the cellar spider predation story in perspective. Black widow venom is genuinely dangerous to vertebrates, including humans, but for a specific biochemical reason. The key toxin, called alpha-latrotoxin, targets vertebrate nerve endings. It causes massive neurotransmitter release, which leads to the intense muscle pain and cramping characteristic of a black widow bite. Research into the molecular evolution of this toxin found substantial variation even within the widow family. Between different genera of widow-related spiders, alpha-latrotoxin sequences differed by as much as about 36 percent at the amino acid level, with most of those differences involving changes between fundamentally different chemical classes of amino acids.⁴

This matters because it shows that alpha-latrotoxin is highly specialized. It evolved to target vertebrate nervous systems, not arthropod ones. A cellar spider is not a vertebrate. The venom that makes a black widow so feared by humans is not the same weapon the widow would need to defend itself against another spider. Widow spiders do produce other venom components, including latroinsectotoxins that target insect and arthropod nervous systems, but these are separate molecules with different targets. The fearsome reputation of black widow venom comes from the vertebrate-targeting toxin, which is essentially irrelevant in a spider-versus-spider encounter. The widow is not unarmed against a cellar spider, but it is not carrying the weapon it is famous for.

Other Predators That Kill Black Widows

Cellar spiders are far from the only creatures that prey on black widows. Mud-dauber wasps are among the most effective and well-documented widow predators. These solitary wasps build tube-shaped nests out of mud and provision them with paralyzed spiders as food for their larvae. Studies of the spider prey collected by common mud-dauber wasps have found that certain species of spiders show up repeatedly in nest inventories, suggesting that the wasps have preferences for specific prey, including widow spiders.⁵ The blue mud-dauber wasp in particular has a strong association with black widows and is considered one of the most important natural population controls on widow spiders in many regions.

Other araneophagous spiders also eat widows when the opportunity arises. Spider-eating is a surprisingly common lifestyle in the arachnid world. Some species specialize in invading the webs or nests of other spiders, a strategy called nest usurpation. Research on one such spider-hunting species found that it successfully captured its spider prey in 38 percent of usurpation attempts, with some captures happening immediately upon entering the nest and others occurring after the resident spider fled and returned later. The target spider successfully defended its nest in 41 percent of attempts, and in one case killed the invader.⁶ These success rates illustrate the general dynamic of spider-on-spider predation: it works often enough to be a viable strategy, but the prey spider wins a substantial fraction of the time. There is no reason to think cellar spider versus black widow encounters are any more one-sided.

Should You Rely on Cellar Spiders for Black Widow Control?

Knowing that cellar spiders can kill black widows, you might be tempted to welcome them into your garage or shed as a biological pest control agent. This is a reasonable thought, but the practical reality is less tidy. Cellar spiders are not specialists. They eat whatever fits in their web, which includes gnats, mosquitoes, other cellar spiders, and occasionally a black widow. They are not going to patrol your property looking for widows to eliminate. If a cellar spider’s web happens to be near a black widow’s territory, there is a chance the two will interact. If not, the cellar spider will go about its business catching tiny flies and ignoring the widow entirely.

Cellar spiders also tend to prefer different microhabitats than black widows. Widows favor undisturbed, sheltered spots close to the ground: under woodpiles, behind stored boxes, inside meter boxes, in the gap between a wall and a piece of furniture. Cellar spiders prefer upper corners, ceiling junctions, and spots where they can build a hanging web. There is some overlap, especially in garages and basements, but the two species are not constantly running into each other even when they share a building.

If you have a black widow problem, the honest answer is that standard pest management, including removing clutter, wearing gloves when reaching into dark spaces, sealing entry points, and targeted insecticide treatment, will do far more than any number of cellar spiders. Thinking of cellar spiders as a bonus rather than a solution is the right framing.

Why Harvestmen Get Dragged Into the Story

The persistent confusion between cellar spiders and harvestmen (the other “daddy long legs”) introduces a second layer of misinformation. Harvestmen are the creatures most Americans picture when they hear “daddy long legs,” yet they are completely incapable of killing a black widow or any other spider. They lack venom entirely. Their mouthparts are designed for scavenging dead organic material, not subduing live prey. They do not build webs. They cannot produce silk. In a confrontation with a black widow, a harvestman would simply be eaten.

The venom myth, already debunked for cellar spiders, makes even less sense when applied to harvestmen. An animal with no venom glands cannot have the “most potent venom in the world.” Yet the myth persists because the common name lumps two totally unrelated arachnids together, and once a catchy factoid attaches to a name, it becomes nearly impossible to correct at scale. If you encounter someone repeating the claim, the quickest clarification is to ask which daddy long legs they mean. If they mean the round-bodied ones clustering on trees and walls, those are harvestmen, and there is no venom, no fangs, and no spider predation. If they mean the delicate, web-building cellar spiders in the basement, the predation part has a kernel of truth, but the venom part is still wrong.