Harvestmen, also known as ‘harvest spiders’ or ‘daddy longlegs,’ are arthropods belonging to the arachnid order Opiliones. Despite their superficial resemblance to spiders, they are distinguished by their elongated bodies and characteristic fused cephalothorax and abdomen, which give them a single oval-shaped structure. Unlike spiders, harvestmen lack venom glands in their chelicerae and do not possess silk glands; thus, they do not produce silk or build webs.
These arachnids can be found in diverse habitats worldwide, ranging from forests and grasslands to caves and deserts. They play important roles in ecosystems as both predators and scavengers, feeding on small invertebrates, plant matter, and decaying organic material.
Although most harvestmen measure around 7mm (0.28 inches) on average, the smaller ones are often less than 1mm (0.03 inches) long, while the largest species, Trogulus torosus, grows up to 22 mm (0.87 inches).
Their leg span exceeds the body length in many species, ranging between 160 mm (6.3 inches) to 340 mm (13 inches).
Their bodies are divided into two segments or tagmata: the anterior cephalothorax or prosoma and the posterior abdomen or opisthosoma (10-segmented). Unlike spiders, harvestmen have a visibly broad connection between their prosoma and opisthosoma and thus appear oval in shape. In some advanced species, the first five abdominal segments are fused into a scutum (dorsal shield), which, in turn, is sometimes integrated with the carapace.
The feeding apparatus, or stomotheca, lies in the cephalothorax and is formed by the fusion of pedipalpal coxae and the first pair of legs. The second pair of legs are notably longer than the others and are sensory in function, acting as feelers or antennae for sensing the environment.
The phylogenetic position of harvestmen within Arachnida has been widely debated. They are assumed to be closely related to camel spiders (Solifugae) or form part of a larger clade, including horseshoe crabs, Ricinulei, and Arachnopulmonata (scorpions, pseudoscorpions, and Tetrapulmonata). Despite their resemblance to spiders (order Araneae), harvestmen are now a separate and distinct order within Arachnida.
The interfamilial relationships within the order Opiliones remain a subject of ongoing research and debate, with recent phylogenetic studies providing valuable insights. The following list represents a compilation of interfamilial relationships proposed by several recent studies, acknowledging that the placement and monophyly of certain taxa remain uncertain.
Currently, around 6,650 known species of harvestmen have been broadly distributed under five suborders.
Harvestmen are found in almost every continent except Antarctica.
They prefer living in leaf litter, moss, rotten wood, tree bark, and moist soil. A few troglobitic (cave-dwelling) species, like Giupponia chagasi, are adapted to living in dark and damp cave environments.
These arachnids are omnivores that feed on an unusually wide range of food, including small insects like aphids, plant matter, fungi, dead organisms, bird droppings, and feces.
Harvestmen employ a variety of primary and secondary defense mechanisms to protect themselves from potential predators. While the primary strategies avoid the attacker, the secondary defenses help these arachnids survive face-to-face encounters.
Primary Defense: Some harvestmen, like those in genus Leiobunum, often show cryptic coloration that helps them camouflage against their surroundings (crypsis), making them less conspicuous and thus less visible to predators. Additionally, some of these arthropods may bear prominent markings on their bodies and appear unpalatable (aposematism), while others, like members of the family Gonyleptidae, may produce translucent secretions that mimic the coloration of glandular emissions of other toxic species (mimicry) to repel their enemies.
Secondary Defense: When threatened, harvestmen may simulate death by becoming rigid (thanatosis) or constantly distract their enemies by bobbing their bodies. They may also produce defensive sound vibrations using specialized stridulating organs (stridulation) or exude strongly odored chemical secretions from specialized scent glands known as ozopores. If captured by a predator, some harvestmen may voluntarily amputate their appendages and free themselves from their grasp (autotomy).
Generally, most harvestmen live for about one to two years in the wild.
While asexual species do exist, most harvestmen reproduce sexually. Mating typically involves direct copulation, where the males transfer sperm directly to females, which the females store at the tip of their ovipositors and use later for fertilizing their eggs during oviposition. However, a few species in the suborder Cyphophthalmi transfer their sperm indirectly by depositing spermatophores (packets of sperm) on the ground, which are then collected by the females.
The females lay eggs shortly after mating, with some species building nests for storing the laid eggs. A few harvestmen species exhibit parental care, where males guard eggs from multiple partners, protect them from egg-eating females, and clean them regularly.
Depending on temperature and other environmental factors, the hatching period varies from 20 days to six months after being laid. They usually undergo four to eight instars before reaching maturity, with the majority known to have six instars in their life cycle.
They have a variety of natural predators, including birds, mammals, and larger arachnids, including spiders and scorpions.
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