Opiliones
The Opiliones are an order of arachnids,
colloquially known as harvestmen, harvesters, daddy long legs, or granddaddy long legs. Over 6,650 species of harvestmen have been discovered worldwide, although the total number of extant species may exceed 10,000. The order Opiliones includes five suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi, which were named in 2014.
Representatives of each extant suborder can be found on all continents except Antarctica.
Well-preserved fossils have been found in the 400-million-year-old Rhynie cherts of Scotland, and 305-million-year-old rocks in France. These fossils look surprisingly modern, indicating that their basic body shape developed very early on, 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 or a larger clade comprising horseshoe crabs, Ricinulei, and Arachnopulmonata. Although superficially similar to and often misidentified as spiders, the Opiliones are a distinct order that is not closely related to spiders. 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. Spiders have a distinct abdomen that is separated from the cephalothorax by a constriction, and they have three to four pairs of eyes, usually around the margins of the cephalothorax.
Description
The Opiliones are known for having exceptionally long legs relative to their body size; however, some species are short-legged. As in all Arachnida, the body in the Opiliones has two tagmata, the anterior cephalothorax or prosoma, and the posterior 10-segmented abdomen or opisthosoma. The most easily discernible difference between harvestmen and spiders is that in harvestmen, the connection between the cephalothorax and abdomen is broad, so that the body appears to be a single oval structure. Other differences include the fact that Opiliones have no venom glands in their chelicerae and thus pose no danger to humans.They also have no silk glands and therefore do not build webs. In some highly derived species, the first five abdominal segments are fused into a dorsal shield called the scutum, which in most such species is fused with the carapace. Some such Opiliones only have this shield in the males. In some species, the two posterior abdominal segments are reduced. Some of them are divided medially on the surface to form two plates beside each other. The second pair of legs is longer than the others and function as antennae or feelers. In short-legged species, this may not be obvious.
The feeding apparatus differs from most arachnids in that Opiliones can swallow chunks of solid food, not only liquids. The stomotheca is formed by extensions of the coxae of the pedipalps and the first pair of legs.
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. Eyes in Cyphophthalmi, when present, are located laterally, near the ozopores. A 305-million-year-old fossilized harvestman with two pairs of eyes was reported in 2014. This find suggested that the eyes in Cyphophthalmi are not homologous to the eyes of other harvestmen. Many cave-adapted species are eyeless, such as the Brazilian Caecobunus termitarum from termite nests, Giupponia chagasi from caves, most species of Cyphophthalmi, and all species of the Guasiniidae. However, recent work studying the embryonic development of the species Phalangium opilio and some Laniatores revealed that harvestman in addition to a pair of median eyes also have two sets of vestigial eyes: one median pair, and one lateral pair. This discovery suggests that the neuroanatomy of harvestmen is more primitive than derived arachnid groups, like spiders and scorpions. It also showed that the four-eyed fossil harvestman previously discovered is most likely a member of the suborder Eupnoi.
Harvestmen have a pair of prosomatic defensive scent glands that secrete a peculiar-smelling fluid when disturbed. In some species, the fluid contains noxious quinones. They do not have book lungs, and breathe through tracheae. A pair of spiracles is located between the base of the fourth pair of legs and the abdomen, with one opening on each side. In more active species, spiracles are also found upon the tibia of the legs. They have a gonopore on the ventral cephalothorax, and the copulation is direct as male Opiliones have a penis, unlike other arachnids. All species lay eggs.
Typical body length does not exceed, and some species are smaller than 1 mm, although the largest known species, Trogulus torosus, grows as long as. The leg span of many species is much greater than the body length and sometimes exceeds and to in Southeast Asia. Most species live for a year.
Behavior
Many species are 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. Such a broad range is unusual in arachnids, which are typically pure predators. Most hunting harvestmen ambush their prey, although active hunting is also found. Because their eyes cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making them vulnerable to internal parasites such as gregarines.Although parthenogenetic species do occur, most harvestmen reproduce sexually. Except from small fossorial species in the suborder Cyphophthalmi, where the males deposit a spermatophore, mating involves direct copulation. The females store the sperm, which is aflagellate and immobile, at the tip of her ovipositor. The eggs are fertilized during oviposition. The males of some species offer a secretion from their chelicerae to the female before copulation. Sometimes, the male guards the female after copulation, and in many species, the males defend territories. In some species, males also exhibit post-copulatory behavior in which the male specifically seeks out and shakes the female's sensory leg. This is believed to entice the female into mating a second time.
The female lays her eggs shortly after mating to several months later. Some species build nests for this purpose. A unique feature of harvestmen is that some species practice parental care, in which the male is solely responsible for guarding the eggs resulting from multiple partners, often against egg-eating females, and cleaning the eggs regularly. Paternal care has evolved at least three times independently: once in the clade Progonyleptoidellinae + Caelopyginae, once in the Gonyleptinae, and once in the Heteropachylinae. Maternal care in opiliones probably evolved due to natural selection, while paternal care appears to be the result of sexual selection. Depending on circumstances such as temperature, the eggs may hatch at any time after the first 20 days, up to about half a year after being laid. Harvestmen variously pass through four to eight nymphal instars to reach maturity, with most known species having six instars.
Most species are nocturnal and colored in hues of brown, although a number of diurnal species are known, some of which have vivid patterns in yellow, green, and black with varied reddish and blackish mottling and reticulation.
Many species of harvestmen easily tolerate members of their own species, with aggregations of many individuals often found at protected sites near water. These aggregations may number 200 individuals in the Laniatores, and more than 70,000 in certain Eupnoi. 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.
Harvestmen clean their legs after eating by drawing each leg in turn through their jaws.
Antipredator defences
Predators of harvestmen include a variety of animals, including some mammals, amphibians, and other arachnids like spiders and scorpions. Opiliones display a variety of primary and secondary defences against predation, ranging from morphological traits such as body armour to behavioral responses to chemical secretions. Some of these defences have been attributed and restricted to specific groups of harvestmen.Primary defences
Primary defences help the harvestmen avoid encountering a potential predator and include crypsis, aposematism, and mimicry.Crypsis
Particular patterns or colour markings on harvestmen's bodies can reduce detection by disrupting the animals' outlines or providing camouflage. Markings on legs can cause an interruption of the leg outline and loss of leg proportion recognition. Darker colourations and patterns function as camouflage when they remain motionless. Within the genus Leiobunum are multiple species with cryptic colouration that changes over ontogeny to match the microhabitat used at each life stage. Many species have also been able to camouflage their bodies by covering with secretions and debris from the leaf litter found in their environments. 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.Aposematism and mimicry
Some harvestmen have elaborate and brightly coloured patterns or appendages which contrast with the body colouration, potentially serving as an aposematic warning to potential predators. This mechanism is thought to be commonly used during daylight, when they could be easily seen by any predators.Other harvestmen may exhibit mimicry to resemble other species' appearances. Some Gonyleptidae individuals that produce translucid secretions have orange markings on their carapaces. This may have an aposematic role by mimicking the colouration of glandular emissions of two other quinone-producing species. Mimicry occurring between Brazilian harvestmen that resemble others could be explained by convergent evolution.