The common name “daddy long-legs” often causes confusion, applying to three different long-legged creatures. The question of whether a cellar spider is a true spider stems from this shared, ambiguous moniker. Scientifically, the family Pholcidae, commonly known as cellar spiders, are unequivocally members of the Order Araneae and are therefore true spiders. Their delicate appearance and extremely long legs belie their classification as predatory arachnids found worldwide.
The Anatomy of a True Cellar Spider
Cellar spiders, like all members of the Order Araneae, possess specific anatomical features that confirm their identity as true spiders. Their body is divided into two segments: the cephalothorax and the abdomen. This two-part structure separates them from their arachnid cousins, the harvestmen, whose body segments are fused.
They have eight slender legs, all of which attach to the cephalothorax. Cellar spiders also possess chelicerae, which are the mouthparts that contain venom glands and small fangs used for subduing prey. While some species have eight eyes, others have six. The presence of these characteristics, along with silk-producing spinnerets on the abdomen, firmly places the cellar spider within the classification of true spiders.
Distinguishing Cellar Spiders from Their Namesakes
The primary source of confusion lies in the overlapping common name “daddy long-legs,” which is used for cellar spiders, harvestmen, and crane flies. Harvestmen, belonging to the Order Opiliones, are arachnids but are not spiders. The most noticeable difference is their body structure, which appears as a single, fused oval segment, lacking the distinct waist found in true spiders.
Harvestmen also lack venom glands and silk-producing spinnerets, relying on chewing mouthparts and scavenging rather than spinning webs. Crane flies, in contrast, are not arachnids; they are true insects belonging to the Order Diptera. Crane flies possess only six legs and three body segments, the defining characteristics of insects. Adult crane flies are non-predatory and harmless, having no venom or fangs.
Where Cellar Spiders Live and How They Hunt
Cellar spiders are most often found in dark, damp, and undisturbed environments, which is the origin of their common name. They thrive in basements, cellars, crawl spaces, and even natural locations like caves and rock crevices. These spiders construct large, irregular, and messy webs in corners, lacking the organized spiral structure of an orb-weaver’s web.
Their hunting technique relies on the chaotic web to entangle prey like flies, mosquitoes, and even other spiders. When an insect is caught, the cellar spider rapidly wraps it in silk before delivering a bite with its short fangs. A distinctive defense mechanism is their characteristic “vibrating” or “shaking” motion when their web is disturbed, which is thought to make them harder for predators to target.
Dispelling the Venom Myth
A persistent urban legend claims that the cellar spider has the most potent venom of any spider but cannot bite humans because its fangs are too small. This myth has been thoroughly debunked by scientific research. Cellar spiders do possess venom, but its composition is mild and not considered medically significant to humans.
Studies have indicated that the venom of the common cellar spider species, Pholcus phalangioides, is relatively weak, posing no health risk to people. While their fangs are indeed very short, measuring approximately a quarter of a millimeter, they are capable of penetrating human skin in rare instances. When a bite does occur, the effect is typically described as a mild, inconsequential sting with no lasting symptoms. The venom myth may have originated from the cellar spider’s ability to successfully prey on more dangerous spiders, such as the black widow, which is accomplished more by wrapping the victim in silk.