Opiliones (Harvestmen)

Opiliones are an order of arachnids commonly known as harvestmen. About 6,500 species of harvestmen have been discovered worldwide, although the total number of extant species are thought to exceed 10,000. Many species of harvestmen tolerate members of their own species and are often found in groups.

Opiliones usually have exceptionally long legs relative to their body size, exceeding 340mm, with the second pair of legs being longer and functioning as antennae. This is not obvious is short legged species. Most Opiliones have a single pair of eyes in the middle of the head, oriented sideways, which are close to useless and cannot form images, and thus rely heavily on their longer second pair of legs to explore their environment.

As in all Arachnida, the body in the Opiliones consists of two “segments”, the anterior cephalothorax/ prosoma, and the posterior abdomen/ opisthosoma. But unlike spiders Opiliones body appears to be a single oval structure because the connection between the cephalothorax and abdomen is broad. Other differences include the fact that Opiliones have no venom glands and as such pose no danger to humans.

The abdominal segments can also be modified into different structures and thus give a different morphological look. In the males of some species the first five abdominal segments are fused into a dorsal shield and then fused with the carapace. In other species, the two posterior abdominal segments are reduced.

Harvestmen have a pair of defensive scent glands that secrete a peculiar-smelling fluid containing quinones when disturbed. Opiliones breathe through tracheae, with a pair of spiracles located between the base of the fourth pair of legs and the abdomen. In more active species, spiracles are found on the legs.

Unlike most arachnids which are typically pure predators and carnivorous, Opiliones have an unusually broad, many species being omnivorous, eating primarily small insects and all kinds of plant material and fungi. Some are scavengers, feeding upon dead organisms, bird dung, and other fecal material. Also, unlike other arachnids, Opiliones can swallow solid food, not needing to liquefy their food before ingesting, hence do not have a “filtering” mechanism, making them susceptible to internal parasites which can be ingested. Unlike spiders, harvestmen have no silk glands and thus do not build webs, but rather ambush their prey, while some actively hunt.

Most harvestmen reproduce sexually, although parthenogenetic species do occur. Unlike other arachnids male Opiliones have a penis and as such can directly copulate, instead of deposition of a spermatophore. Males of some species sometimes offer a nuptial gift secretion to the female before copulation. In many species, males defend territories and perform mate guarding after copulation. Some species, males also exhibit post-copulatory behavior where the male seeks out and shakes the female’s sensory leg, enticing the female into mating a second time.

Females lay eggs from an ovipositor several months after mating. Some species build nest, while the males in some species exhibit parental care, guarding eggs laid in the same nest by few females, usually from the females themselves.

Most species are nocturnal and drab in color, in contrast to diurnal species which are more often more colorful with vivid patterns and reticulations.