Opiliones

Lore & Background

Opiliones, commonly known as harvestmen, have a body that appears as a single oval structure due to a broad connection between the cephalothorax and abdomen, unlike spiders which have a distinct constriction. They possess a single pair of eyes in the middle of the cephalothorax, oriented sideways in most groups, though some cave-dwelling species are eyeless. However, recent studies have revealed that harvestmen also have two sets of vestigial eyes, suggesting a more primitive neuroanatomy than spiders or scorpions. Their second pair of legs is notably longer than the others and serves as feelers. They lack venom glands in their chelicerae and silk glands, so they pose no danger to humans and do not build webs. Defensive scent glands on the prosoma secrete a fluid, sometimes containing quinones, when disturbed. Respiration occurs through tracheae, not book lungs, with spiracles located near the fourth leg pair. Unlike other arachnids, males possess a penis for direct copulation. Most species are omnivorous, eating small insects, plant material, and fungi, and some are scavengers. Well-preserved fossils from 400-million-year-old Scottish rocks and 305-million-year-old French rocks appear surprisingly modern, indicating their basic body shape developed early and has changed little in some lineages.

Reader's Guide

Opiliones are significant because they represent a distinct evolutionary lineage within Arachnida, with a body plan that has remained remarkably stable for hundreds of millions of years. Their ability to swallow solid food, unlike most arachnids, and their direct copulation (males have a penis) set them apart. They are harmless to humans and play roles as omnivores, scavengers, and predators of small insects. Their defensive scent glands and paternal care in some species are notable behaviors. The discovery of vestigial eyes in embryos suggests their neuroanatomy is more primitive than that of spiders and scorpions, offering insights into arachnid evolution.

Did You Know?

• Harvestmen can swallow chunks of solid food, not just liquids.
• Some species practice paternal care, with males guarding eggs from multiple partners.
• The second pair of legs is longer than the others and functions as antennae.

Taxonomy and Global Reach

The Opiliones, a group of arachnids long known by colorful common names such as harvestmen, daddy long legs, and granddaddy long legs, constitute a single order that was formerly called Phalangida. In 2014, the order was formally divided into five suborders—Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi—each representing a distinct evolutionary lineage. What makes their distribution remarkable is that representatives of every one of these suborders can be encountered on every continent except Antarctica, underscoring both the group's ancient origins and its extraordinary ecological versatility. Despite their small size and often overlooked presence, harvestmen occupy a wide range of habitats worldwide, from tropical forests to temperate woodlands, and their sheer taxonomic breadth makes them one of the most geographically widespread arachnid orders on Earth.

Deep Time and Evolutionary Stasis

What strikes paleontologists most about these specimens is how closely they resemble living harvestmen today; the fundamental body plan appears to have been established very early in the order's history and, in at least some lineages, has remained virtually unchanged for hundreds of millions of years. The exact evolutionary placement of Opiliones within the broader arachnid tree remains debated. Some researchers propose camel spiders (Solifugae) as their nearest relatives, while others favor a larger assemblage that includes horseshoe crabs, Ricinulei, and the Arachnopulmonata clade. Crucially, despite frequent misidentification, harvestmen are not closely related to true spiders at all.

Anatomy That Defies Expectation

At first glance, a harvestman's body appears as a single, smooth oval, but this is because the broad junction between the cephalothorax and the ten-segmented abdomen eliminates the narrow waist that characterizes spiders. This fused architecture is the most reliable visual cue separating Opiliones from Araneae. Equally important is the absence of both venom glands and silk-producing organs, meaning harvestmen neither inject toxins nor construct webs. Their feeding apparatus, the stomotheca, is built from extensions of the pedipalp coxae and the first pair of legs, and unlike the liquid-only diets of most arachnids, it allows them to swallow solid chunks of food. A single pair of camera-type eyes sits in the center of the head, oriented sideways, though cave-dwelling species such as Caecobunus termitarum and all members of Guasiniidae have lost their eyes entirely. For defense, a pair of prosomatic ozopores releases a foul-smelling fluid, sometimes containing noxious quinones. Respiration occurs through tracheae rather than the book lungs found in spiders, with spiracles positioned between the fourth leg pair and the abdomen.

Feeding, Mating, and the Rhythm of a Short Life

Ecologically, harvestmen are far more flexible than most arachnids. Many species are omnivorous, consuming small insects, decaying plant matter, and fungi, while others function as scavengers, feeding on carrion, bird droppings, and other fecal material. Because their eyes cannot produce sharp images, they rely heavily on the second pair of legs, which are typically the longest, as tactile feelers to probe their surroundings. During hunting, most species ambush prey, though some actively pursue it. Reproduction is predominantly sexual, with males in certain species presenting a cheliceral secretion as a nuptial gift before mating, and in some cases guarding the female afterward. Sperm is stored at the tip of the female's ovipositor and fertilization occurs during egg-laying. A few small, burrowing Cyphophthalmi species use spermatophores instead.