Opiliones

How to read a taxoboxOpiliones
Fossil range: Devonian - Recent
Hadrobunus grandis
Hadrobunus grandis
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Subclass: Dromopoda
Order: Opiliones
Sundevall, 1833
Diversity
4 suborders, > 6,400 species
Suborders

Cyphophthalmi
Eupnoi
Dyspnoi
Laniatores

Harvestmen is the common name for any of the eight-legged invertebrate animals comprising the order Opiliones (formerly Phalangida) in the arthropod class Arachnida, characterized by a body in which the two main sections, the cephalothorax and abdomen, are broadly joined so that they appear as if one oval structure. Commonly they also have long walking legs, which has led to them being known in some places as daddy longlegs or grandaddy longlegs. Although they belong to the class of arachnids, harvestmen are not spiders, which are of the order Araneae rather than the order Opiliones. There are over 6,000 species of opiliones.

Found in terrestrial habitats worldwide, on all continents except for Antarctica, harvestmen play important ecological roles as part of food chains. Mostly omnivorous, consuming invertebrates (insects, snails, and so on), plant matter, fungi, and carrion, they serve as food for birds, spiders, frogs, toads, and other organisms. For human beings, they add to the diversity of nature. However, despite their importance and diversity—they are the third largest order of arachnids, after Acari (mites and ticks) and Araneae (spiders)—they remain poorly studied.

Contents

Harvestmen also reflect the tendency of species to demonstrate stasis once they come into existence: Fossils 400 million years old have been found that look like modern harvestmen, reflecting the slight changes in basic structure over that time.

Overview and description

A male Phalangium opilio, showing the distinguishable long legs.

Harvestment or opiliones comprise the order Opiliones in the class Arachnida in the subphylum Chelicerata of the phylum Arthropoda. Arachnida is a largely terrestrial group that also includes spiders, mites, ticks, and scorpions. Arachnids are characterized by four pairs of segmented walking legs and a body divided into two regions, the cephalothorax and the abdomen, the cephalothorax being derived from the fusion of the cephalon (head) and the thorax.

Harvestmen are known for their exceptionally long walking legs, compared to body size, although there are also short-legged species. The difference between harvestmen and spiders is that in harvestmen the two main body sections (the abdomen with ten segments and the cephalothorax—or the prosoma and opisthosoma) are broadly joined, so that they appear to be one oval structure; they also have no venom or silk glands. In more advanced species of harvestment, the first five abdominal segments are often fused into a dorsal shield called the scutum, which is normally fused with the carapace. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates lying next to each other. The second pair of legs are longer than the others and work as antennae. This can be hard to see in short-legged species.

Typical body length does not exceed 7 millimeters (about 5/16 inch), with some species smaller than one millimeter, although the largest species Trogulus torosus (Trogulidae) can reach a length of 22 millimeters (Pinto-da-Rocha et al. 2007). However, leg span is much larger and can exceed 160 millimeters (over 6 inches).

The feeding apparatus (stomotheca) differs from other arachnids in that ingestion is not restricted to liquid, but chunks of food can be taken in. The stomotheca is formed by extensions from the pedipalps and the first pair of legs.

Harvestmen have a single pair of eyes in the middle of their heads, oriented sideways. However, there are eyeless species (for example the Brazilian Caecobunus termitarum (Grassatores) from termite nests, Giupponia chagasi (Gonyleptidae) from caves, and all species of Guasiniidae) (Pinto-da-Rocha and Kury 2003).

A harvestman (a male Phalangium opilio), showing the almost fused arrangement of abdomen and cephalothorax that distinguishes these arachnids from spiders.

Harvestmen have a pair of prosomatic defensive scent glands (ozopores) that secrete a peculiar smelling fluid when disturbed, confirmed in some species to contain noxious quinones. Harvestmen do not have silk glands and do not possess venom glands, posing absolutely no danger to humans (see below).

Harvestmen do not have book lungs, and breathe through tracheae only. Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located, one opening on each side. (Spiracles are small openings on the surface that lead to the respiratory system.) In more active species, spiracles are also found upon the tibia of the legs.

Harvestmen have a gonopore on the ventral cephalothorax, and copulation is direct as the male has a penis (while the female has an ovipositor). All species lay eggs. Most species live for a year.

The legs continue to twitch after they are detached. This is because there are "pacemakers" located in the ends of the first long segment (femur) of their legs. These pacemakers send signals via the nerves to the muscles to extend the leg and then the leg relaxes between signals. While some harvestman's legs will twitch for a minute, other kinds have been recorded to twitch for up to an hour. The twitching has been hypothesized as a means to keep the attention of a predator while the harvestman escapes (Pinto-da-Rocha et al. 2007).

The former scientific name for Opiliones was Phalangida and this name still often appears in the literature. The common name "daddy longlegs" also is used for the crane fly (Tipulidae) and the cellar spider (Pholcidae) (Crawford 2005).

Behavior, diet, and reproduction

Harvestman eating a skink tail

Many species of harvestmen 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. This broad range is quite unusual in arachnids, which are usually 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. Also unlike most other arachnids, harvestmen do not have a sucking stomach and a filtering mechanism, but ingest small particles of their food, thus making them vulnerable to internal parasites, such as gregarines (Pinto-da-Rocha et al. 2007).

Although parthenogenetic species do occur, most harvestmen reproduce sexually. Mating involves direct copulation, rather than the deposition of a spermatophore. 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.

The females lay eggs shortly after mating, or up to months later. Some species build nests for this purpose. A unique feature of harvestmen is that in some species the male is solely responsible for guarding the eggs resulting from multiple partners, often against egg-eating females, and subjecting the eggs to regular cleaning. The eggs can hatch anytime after the first 20 days, up to almost half a year after being laid. Harvestmen need from four to eight nymphal stages to reach maturity, with six the most common (Pinto-da-Rocha et al. 2007).

Harvestmen are mostly nocturnal and colored in hues of brown, although there are a number of diurnal species that have vivid patterns in yellow, green, and black with varied reddish and blackish mottling and reticulation.

To deal with predators such as birds, mammals, amphibians, and spiders, some species glue debris onto their body, and many play dead when disturbed. Many species can detach their legs, which keep on moving to confuse predators. Very long-legged species vibrate their body ("bobbing"), probably also to confuse. This is similar to the behavior of the similar looking but unrelated daddy longlegs spider, which vibrates wildly in its web when touched. Scent glands emit substances that can deter larger predators, but are also effective against ants (Pinto-da-Rocha et al. 2007).

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 can count up to 200 animals in the Laniatores, but more than 70,000 in certain Eupnoi. This behavior may be a strategy against climatic odds, but also against predators, combining the effect of scent secretions, and reducing the probability of each individual of being eaten (Pinto-da-Rocha et al. 2007).

Endangered status

Some troglobitic (cave dwelling) Opiliones are considered endangered if their home caves are in or near cities where pollution and development of the land can alter the cave habitat. Others species are threatened by the invasion of non-native fire ants.

All troglobitic species (of all animal taxa) are considered to be at least threatened in Brazil. There are four species of Opiliones in the Brazilian National List for endangered species, all of them cave-dwelling species. Giupponia chagasi (Pérez & Kury, 2002, Iandumoema uai Pinto-da-Rocha, 1996, Pachylospeleus strinatii Šilhavý, 1974, and Spaeleoleptes spaeleus H. Soares, 1966).

Several opiliones in Argentina appear to be vulnerable, if not endangered. These include Pachyloidellus fulvigranulatus (Mello-Leitão, 1930), which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba), and Pachyloides borellii (Roewer, 1925) is in rainforest patches in North West Argentina, which are in an area being dramatically altered by humans. The cave living Picunchenops spelaeus (Maury, 1988) is apparently endangered through human action. So far no harvestman has been included in any kind of a Red List in Argentina and therefore they receive no protection.

Maiorerus randoi (Rambla, 1993) has only been found in one cave in the Canary Islands. It is included in the Catálogo Nacional de especies amenazadas (National catalog of threatened species) from the Spanish government.

Texella reddelli (Goodnight & Goodnight, 1967) and Texella reyesi (Ubick & Briggs, 1992) are listed as endangered species in the United States. Both are from caves in central Texas. Texella cokendolpheri (Ubick & Briggs, 1992) from a cave in central Texas and Calicina minor (Briggs & Hom, 1966), Microcina edgewoodensis (Briggs & Ubick, 1989), Microcina homi (Briggs & Ubick, 1989), Microcina jungi (Briggs & Ubick, 1989), Microcina leei Briggs & Ubick 1989, Microcina lumi (Briggs & Ubick, 1989), and Microcina tiburona (Briggs & Hom, 1966) from around springs and other restricted habitats of central California are being considered for listing as endangered species, but as yet receive no protection.

Misconception

An urban legend claims that the harvestman is the most venomous animal in the world, but possesses fangs too short or a mouth too round and small to bite a human and therefore is not dangerous (Crawford 2005). (The same myth applies to the cellar spider, which is also called a daddy longlegs.) This is untrue on several counts. None of the known species have venom glands or fangs, instead having chelicerae (OIDG 2005). The size of its mouth varies by species, but even those with relatively large jaws hardly ever bite humans or other large creatures, even in self-defense.

Research

Harvestmen are a scientifically much neglected group. Description of new taxa has always been dependent on the activity of a few dedicated taxonomists. Carl Friedrich Roewer described about a third (2,260) of today's known species from the 1910s to the 1950s, and published the landmark systematic work Die Weberknechte der Erde (Harvestmen of the World) in 1923, with descriptions of all species known to that time. Other important taxonomists in this field include Eugène Simon, Tord Tamerlan Teodor Thorell, William Sørensen, and Zac Jewell around the turn of the twentieth century, and later Cândido Firmino de Mello-Leitão and Reginald Frederick Lawrence. Since 1980, study of the biology and ecology of harvestmen has intensified, especially in South America (Pinto-da-Rocha et al. 2007).

Phylogeny and systematics

Harvestmen are very old arachnids. Fossils from the Devonian, 400 million years ago, already show characteristics like tracheae and sexual organs, proving that the group has lived on land since that time. They are probably closely related to the scorpions, pseudoscorpions, and solifuges; these four orders form the clade Domopod. The Opiliones have remained almost unchanged morphologically over a long period (Pinto-da-Rocha et al. 2007). Well-preserved fossils have been found in the 400-million year old Rhynie cherts of Scotland, which look surprisingly modern, indicating that the basic structure of the harvestmen has not changed much since then.

As of 2006, over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000 (Pinto-da-Rocha et al. 2007). The order Opiliones can be divided in four suborders: Cyphophthalmi (Simon, 1879), Eupnoi (Hansen & Sørensen, 1904), Dyspnoi (Hansen & Sørensen, 1904), and Laniatores (Thorell, 1876). Cyphophthalmi are one of the two lineages of harvestmen; the other, containing the Laniatores, Dyspnoi and Eupnoi, is also called Phalangida.

Relationship within suborders

Cyphophthalmi

The Cyphophthalmi have been divided into two infraorders, Temperophthalmi (including the superfamily Sironoidea, with the families Sironidae, Troglosironidae, and Pettalidae) and Tropicophthalmi (with the superfamilies Stylocelloidea and its single family Stylocellidae, and Ogoveoidea, including Ogoveidae and Neogoveidae). However, recent studies suggest that the Sironidae, Neogoveidae, and Ogoveidae are not monophyletic, while the Pettalidae and Stylocellidae are. The division into Temperophthalmi and Tropicophthalmi is not supported, with Troglosironidae and Neogoveidae probably forming a monophyletic group. The Pettalidae are possibly the sister group to all other Cyphophthalmi.

While most Cyphophthalmi are blind, eyes do occur in several groups. Many Stylocellidae, and some Pettalidae bear eyes near or on the ozophores, as opposed to most harvestmen, which have eyes located on top. The eyes of Stylocellidae could have evolved from the lateral eyes of other arachnids, which have been lost in all other harvestmen. Regardless of their origin, it is thought that eyes were lost several times in Cyphophthalmi. Spermatophores, which normally do not occur in harvestmen, but in several other arachnids, are present in some Sironidae and Stylocellidae (Giribet and Kury 2007).

Eupnoi

Male Phalangium opilio

The Eupnoi are currently divided into two superfamilies, the Caddoidea and Phalangioidea. The Phalangioidea are assumed to be monophyletic, although only the families Phalangiidae and Sclerosomatidae have been studied; the Caddoidea have not been studied at all in this regard. The limits of families and subfamilies in Eupnoi are uncertain in many cases, and are in urgent need of further study (Giribet and Kury 2007).

Dyspnoi

Troguloidea

Nipponopsalididae




Nemastomatidae




Dicranolasmatidae



Trogulidae




(after Giribet & Kury 2007)

The sabaconid Sabacon cavicolens

The Dyspnoi are probably the best studied harvestman group regarding phylogeny. They are considered to be clearly monophyletic, and divided into two superfamilies. The relationship of the superfamily Ischyropsalidoidea, comprised of the families Ceratolasmatidae, Ischyropsalididae, and Sabaconidae, has been investigated in detail. It is not clear whether Ceratolasmatidae and Sabaconidae are each monophyletic, as the ceratolasmatid Hesperonemastoma groups with the sabaconid Taracus in molecular analyses. All other families are grouped under Troguloidea (Giribet and Kury 2007).

Laniatores

The triaenonychid Fumontana deprehendor

There is not yet a proposed phylogeny for the whole group of Laniatores, although some families have been researched in this regard. The Laniatores are currently divided into two infraorders, the "Insidiatores" (Loman, 1900) and the Grassatores (Kury, 2002). However, Insidiatores is probably paraphyletic. It consists of the two superfamilies Travunioidea and Triaenonychoidea, with the latter closer to the Grassatores. Alternatively, the Pentanychidae, which currently reside in Travunioidea, could be the sister group to all other Laniatores.

The Grassatores are traditionally divided into the Samooidea, Assamioidea, Gonyleptoidea, Phalangodoidea, and Zalmoxoidea. Several of these groups are not monophyletic. Molecular analyses relying on nuclear ribosomal genes support monophyly of Gonyleptidae, Cosmetidae (both Gonyleptoidea), Stygnopsidae (currently Assamioidea), and Phalangodidae. The Phalangodidae and Oncopodidae may not form a monophyletic group, thus rendering the Phalangodoidea obsolete. The families of the obsolete Assamioidea have been moved to other groups: Assamiidae and Stygnopsidae are now Gonyleptoidea, Epedanidae reside within their own superfamily Epedanoidea, and the "Pyramidopidae" are possibly related to Phalangodidae (Giribet and Kury 2007).

The family Stygophalangiidae (1 species, Stygophalangium karamani) from underground waters in Macedonia is sometimes misplaced in the Phalangioidea. It is not a harvestman.

References

External links

All links retrieved February 20, 2015.


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