Harvestmen are arachnids placed in the order Opiliones (sometimes called phalangids). They belong to the class Arachnida, but they are not true spiders and differ from spiders in several important ways.
Names and common confusion
The informal name "daddy longlegs" is often applied to harvestmen, but that same nickname is used for two unrelated groups of long-legged arthropods: the crane flies (family Tipulidae) and the so-called cellar spiders (family Pholcidae). This overlap in common names causes frequent misunderstandings.
Diversity and fossil history
Scientists have formally described more than 6,400 living species of harvestmen worldwide; estimates of the total number of extant species vary and may exceed 10,000. Well-preserved fossils of opilionids are known from the Devonian period, including specimens found in the Rhynie cherts of Scotland, which are about 400 million years old. These fossil examples show body plans that are recognizably similar to modern harvestmen.
Diet and ecological role
Many harvestmen are opportunistic feeders and are considered omnivores. Their diet typically includes small insects, bits of plant material and fungi (plants and fungi), and carrion; some species specialize as scavengers or predators of tiny invertebrates. Their feeding habits help recycle organic matter in leaf litter and other habitats.
Body structure and interaction with people
Harvestmen have a compact, oval body with the segments of the abdomen broadly joined to the cephalothorax, producing the single-bodied appearance that distinguishes them from many spiders. As members of the class Arachnida, they possess eight legs, but they lack the silk-producing glands and venom glands that characterize many spiders. No described species of harvestmen is known to be dangerous to humans.