Pholcus phalangioides

The cosmopolitan cellar spider, scientifically known as Pholcus phalangioides, is a widespread arachnid. It belongs to the family Pholcidae. Many people call it a "daddy long-legs" spider. However, this name can be confusing. It is not the same as the harvestman or the crane fly. This spider is a beneficial predator in many environments. It often hunts other spiders, including dangerous species like the redback spider.

Physically, these spiders show sexual dimorphism, meaning males and females look different. Females are generally larger than males. An average female body is about 7 to 8 mm long. Males are often smaller, averaging around 6 mm. Their legs are remarkably long and thin. These legs are covered in fine, grey bristles. On average, the legs are 5 to 6 times the length of the body. For an adult female, the legs can reach about 50 mm.

A spider's body has two main sections. The first is the prosoma, also called the cephalothorax. This front part contains the spider's brain. The second part is the opisthosoma, or the abdomen. The opisthosoma holds most internal organs. This includes the heart, respiratory system, and silk glands. The body is translucent with a grey-pale brown color. It often has a dark spot on the prosoma. It may also have dark, blurred spots on the dorsal side of the opisthosoma.

Pholcus phalangioides has a unique eye arrangement. It is an eight-eyed spider. There is a pair of small, dark eyes at the front of the prosoma. Behind these, there are three parallel rows of larger eye pairs. To grow, these spiders must undergo molting. They shed their hard exoskeleton to allow for growth. During this process, they produce enzymes. These enzymes help release the body from the underlying tissue. The leftover outer skin is called the exuviae. It takes about one year for these spiders to reach maturity.

These spiders are synanthropic, meaning they live close to human structures. This relationship has helped them spread globally. While they may be native to subtropical Asia, they now live on almost every continent. They prefer warm, indoor habitats like cellars, garages, and attics. They also inhabit dark, quiet areas like caves or rock crevices. Because they prefer warmth, they stay inside human buildings in temperate regions. This human-driven spread has created many isolated populations. These small, scattered groups often show high genetic differentiation due to genetic drift.

The hunting methods of the cellar spider are quite sophisticated. They build loose, horizontal webs that are often irregular. Unlike many spiders, they do not always wait for prey to hit the web. They often leave their webs to hunt other spiders. They may even eat the eggs of other spiders. They also use mimicry to catch food. The spider creates specific vibrations in the web. These vibrations trick prey into thinking they have caught an insect. When the prey approaches, the spider uses its long legs to wrap it in silk.

Defense is also a major part of their behavior. When a predator like a jumping spider approaches, the cellar spider uses a technique called whirling. It spins its body in a rapid circle while keeping its legs fixed to the web. This rapid movement makes it difficult for a predator to see them clearly. This whirling can last for several hours or even days. This defense is specifically effective against Salticidae, or jumping spiders. Jumping spiders sometimes use their own mimicry to hunt them. They produce vibrations to trick the cellar spider into moving toward them.

Despite their predatory nature, these spiders are generally harmless to humans. There is a common myth that they cannot bite humans. However, their fangs are about 0.25 mm long. Since human skin is only about 0.1 mm thick, they can bite. Most reports of bites are very rare. Their venom is considered negligible to humans and other mammals. A bite usually only results in a mild stinging sensation. They are even known to hunt much more venomous spiders without harm. This makes them an interesting part of the domestic ecosystem.