These fossils look surprisingly modern, indicating that their basic body shape developed very early on,[6] and, at least in some taxa, has changed little since that time.

Their phylogenetic position within the Arachnida is disputed; their closest relatives may be camel spiders (Solifugae) or a larger clade comprising horseshoe crabs, Ricinulei, and Arachnopulmonata (scorpions, pseudoscorpions, and Tetrapulmonata).

They can be easily distinguished from long-legged spiders by their fused body regions and single pair of eyes in the middle of the cephalothorax.

Most Opiliones, except for Cyphophthalmi, have long been thought to have a single pair of camera-type eyes in the middle of the head, oriented sideways.

It also showed that the four-eyed fossil harvestman previously discovered is most likely a member of the suborder Eupnoi (true daddy-longlegs).

Except from small fossorial species in the suborder Cyphophthalmi, where the males deposit a spermatophore, mating involves direct copulation.

Gregarious behavior is likely a strategy against climatic odds, but also against predators, combining the effect of scent secretions, and reducing the probability of any particular individual being eaten.

[23] Opiliones display a variety of primary and secondary defences against predation,[24] ranging from morphological traits such as body armour to behavioral responses to chemical secretions.

Particular patterns or colour markings on harvestmen's bodies can reduce detection by disrupting the animals' outlines or providing camouflage.

[29] Within the genus Leiobunum are multiple species with cryptic colouration that changes over ontogeny to match the microhabitat used at each life stage.

[27][31] Some hard-bodied harvestmen have epizoic cyanobacteria and liverworts growing on their bodies that suggest potential benefits for camouflage against large backgrounds to avoid detection by diurnal predators.

[27] Secondary defences allow for harvestmen to escape and survive from a predator after direct or indirect contact, including thanatosis, freezing, bobbing, autotomy, fleeing, stridulation, retaliation and chemical secretions.

[37][38] Thanatosis is used as a second line of defence when detected by a potential predator and is commonly observed within the Dyspnoi and Laniatores suborders,[35] with individuals becoming rigid with legs either retracted or stretched.

[27][47] Cellar spiders (Pholcidae) that are commonly mistaken for daddy long-legs (Opiliones) also exhibit this behavior when their webs are disturbed or even during courtship.

[27][47] Multiple species within the Laniatores and Dyspnoi possess stridulating organs, which are used as intraspecific communication and have also been shown to be used as a second line of defense when restrained by a predator.

These include Pachyloidellus fulvigranulatus, which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) and Pachyloides borellii is in rainforest patches in northwest Argentina which are in an area being dramatically destroyed by humans.

It is included in the Catálogo Nacional de especies amenazadas (National catalog of threatened species) from the Spanish government.

An urban legend claims that the harvestman is the most venomous animal in the world;[65] however, it possesses fangs too short or a mouth too round and small to bite a human, rendering it harmless (the same myth applies to Pholcus phalangioides and the crane fly, which are both also called a "daddy longlegs").

[66] None of the known species of harvestmen have venom glands; their chelicerae are not hollowed fangs but grasping claws that are typically very small and not strong enough to break human skin.

Other important taxonomists in this field include: Since the 1990s, study of the biology and ecology of harvestmen has intensified, especially in South America.

Fossils from the Devonian Rhynie chert, 410 million years ago, already show characteristics like tracheae and sexual organs, indicating that the group has lived on land since that time.

Despite being similar in appearance to, and often confused with, spiders, they are probably closely related to the scorpions, pseudoscorpions, and solifuges; these four orders form the clade Dromopoda.

[4][6] Indeed, one species discovered in China, Mesobunus martensi, fossilized by fine-grained volcanic ash around 165 million years ago, is hardly discernible from modern-day harvestmen and has been placed in the extant family Sclerosomatidae.

The following list is a compilation of interfamilial relationships recovered from several recent phylogenetic studies, although the placement and even monophyly of several taxa are still in question.

The oldest known harvestman, from the 410-million-year-old Devonian Rhynie chert, displayed almost all the characteristics of modern species, placing the origin of harvestmen in the Silurian, or even earlier.

A recent molecular study of Opiliones, however, dated the origin of the order at about 473 million years ago (Mya), during the Ordovician.

[74] No fossils of the Cyphophthalmi or Laniatores much older than 50 million years are known, despite the former presenting a basal clade, and the latter having probably diverged from the Dyspnoi more than 300 Mya.

[5][76] However, recent reanalysis of harvestman phylogeny has shown that E. sheari and H. argus are in fact members of the suborder Eupnoi, after it was discovered that living daddy-longlegs have the same arrangement of eyes as the fossils.

Bartel et al, 2023 report "These new records bring the total number of Burmese amber laniatorean species to ten" Currently, no fossil harvestmen are known from the Triassic.

More likely, as in other European languages which call them a word meaning “cutter” or “scyther”, the original explanation is that their oddly shaped legs look like tiny sickles or scythes.