Which arachnid order is commonly called harvestmen?

The arachnid order commonly called harvestmen is Opiliones.

Harvestmen are a distinct order of arachnids, not spiders. Their familiar “daddy longlegs” appearance can cause confusion, but harvestmen have a body in which the front and rear regions are broadly fused. Spiders, by contrast, have a distinct constriction between the cephalothorax and abdomen.

More than 6,650 harvestman species have been described, and the true number of living species may exceed 10,000. They occur on every continent except Antarctica and include five living suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi.

The group has a deep fossil history. Harvestman fossils from Scotland’s Devonian Rhynie chert are about 400 million years old and already show features associated with modern species. Another common misconception is that harvestmen are venomous spiders; they are neither spiders nor animals known for medically important venom. Their single central eye pair also helps distinguish them from most spiders.