The direct answer
If the spider hanging in a dusty ceiling corner is a long-bodied cellar spider (often Pholcus phalangioides), treat it as a nuisance predator of other arthropods, not as a medical emergency. University of California, Riverside entomologist Rick Vetter has said he is unaware of any publication showing that pholcid spiders cause a toxic effect in humans beyond a short-lived mark or sting.
That is a different claim from "they have no venom." Cellar spiders are true spiders: two body regions, venom glands, and fangs. A 2020 venomics study on another pholcid, Physocyclus mexicanus, found plenty of venom peptides and enzymes that paralyze insect prey. Insect potency is not the same as human medical risk. Documented human bites, when they happen, are described as mild.
Australian Museum guidance on daddy-long-legs spiders reaches the same practical endpoint: the species is not considered harmful to humans; seek care only if an unusual reaction persists. The Australian Venom Research Unit likewise calls the bite generally harmless, with only minor local reactions described.
Three animals share one nickname
The myth survives partly because "daddy longlegs" is not one animal. Vetter's UCR write-up (originally for Live Science) separates three look-alikes that English speakers mash together:
Where the "most venomous" story breaks
The story usually has two halves: (1) their venom is extremely potent, and (2) their fangs are too short to pierce human skin. Both halves fail under scrutiny.
On potency, there is no peer-reviewed ranking that puts pholcid venom at the top for mammals. Mythbusters-era work later summarized in the scientific literature compared cellar-spider venom with black-widow venom in mice; the widow venom was far more potent. The 2020 Physocyclus paper synthesizes available human-bite evidence as inconsequential for mammals while still interesting for insect prey chemistry.
On fang length, pholcids do have short "uncate" fangs, but that morphology alone does not prove they cannot bite. Brown recluse spiders also have uncate fangs and clearly bite people. Mythbusters demonstrated that tiny pholcid fangs can pierce skin; the sensation was a short sting. UCR's spider research pages stress that the "fangs too short" half of the myth has no scientific basis as a universal rule.
A related folk claim says cellar spiders must be hyper-venomous because they kill redback or other dangerous spiders. Predatory success against another spider is about wrapping, timing, and web behavior as much as venom titer. Australian Museum notes that the myth probably grew from watching daddy-long-legs spiders overpower redbacks, not from measured LD50 values.
What a cellar spider bite actually looks like
Confirmed bites are uncommon because the spiders are shy web-dwellers. When bites are documented, sources converge on a mild local sting that fades quickly, sometimes with a small mark for a day or two. That profile is closer to many common house spiders than to widow or recluse envenomation syndromes.
If you did not see the spider, do not diagnose from the wound alone. Many "spider bites" reported to clinics are infections, bed-bug reactions, or other arthropods. For comparison, FactCrumbs' wolf spider page walks through prospective bite series that also undercut necrotic-wound folklore for a different family.
Practical home guidance: leave the web if you want free pest control, vacuum it if you want less silk, and avoid crushing spiders against skin. Cellar spiders eat flies, mosquitoes, and sometimes other spiders; they are not a reason to fog an entire basement with insecticide.
Species labels vary by region. In much of North America and Europe the long-bodied cellar spider Pholcus phalangioides is the indoor default; along parts of the Pacific coast, Holocnemus pluchei is also common and shows a dark ventral stripe. Both are pholcids. Neither matches the medical profile of widow or recluse spiders that clinics actually train for.
How to tell a cellar spider from a harvestman
Cellar spiders build messy sheet or tangle webs in undisturbed corners, hang upside down, and often vibrate the web when disturbed. The abdomen is usually oblong, and you can see a clear waist between cephalothorax and abdomen.
Harvestmen roam on walls and leaf litter without a capture web, have a fused-looking body without a spider waist, and show only two eyes on a little turret. They can shed a leg as a distraction; the leg may twitch afterward. They do not inject venom.
If identification matters for peace of mind, a clear photo of the body (not only the legs) is enough for most extension or natural-history keys. Legs alone are a poor ID character for all three "daddy longlegs" groups.