Hadrobunus grandis showing its body structure and long legs: one pair of eyes and broadly joined body tagma differentiate it from similar-looking arachnids.
Representatives of each extant suborder can be found on all continents exceptAntarctica.
Well-preserved fossils have been found in the 400-million-year-oldRhynie 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,[6] and, at least in some taxa, has changed little since that time.
Theirphylogenetic position within the Arachnida is disputed; their closest relatives may be camel spiders (Solifugae) or a larger clade comprising horseshoe crabs, Ricinulei, and Arachnopulmonata (scorpions, pseudoscorpions, and Tetrapulmonata).[7][8] Although superficially similar to and often misidentified asspiders (orderAraneae), 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 thecephalothorax. 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.
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 twotagmata, theanteriorcephalothorax orprosoma, and theposterior 10-segmentedabdomen oropisthosoma. 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 singleoval structure. Other differences include the fact that Opiliones have novenom glands in theirchelicerae 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 adorsal shield called thescutum, which in most such species is fused with thecarapace. Some such Opiliones only have this shield in the males. In some species, the two posterior abdominal segments are reduced. Some of them are dividedmedially on the surface to form two plates beside each other. The second pair of legs is longer than the others and function asantennae or feelers. In short-legged species, this may not be obvious.
The feeding apparatus (stomotheca) 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 thepedipalps 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.[9][10] Many cave-adapted species are eyeless, such as the BrazilianCaecobunus termitarum (Grassatores) fromtermite nests,Giupponia chagasi (Gonyleptidae) from caves, most species of Cyphophthalmi, and all species of theGuasiniidae.[11] However, recent work studying the embryonic development of the speciesPhalangium opilio and some Laniatores revealed that harvestman in addition to a pair median eyes also have two sets of vestigial eyes: one median pair (homologous to those ofhorseshoe crabs andsea spiders), and one lateral pair (homologous to facetted eyes of horseshoe crabs and insects).[12] 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 suborderEupnoi (true daddy-longlegs).[12]
A harvestman (a malePhalangium opilio), showing the almost fused arrangement of abdomen and cephalothorax that distinguishes these arachnids fromspiders
Harvestmen have a pair of prosomatic defensivescent glands (ozopores) that secrete a peculiar-smelling fluid when disturbed. In some species, the fluid contains noxiousquinones. They do not havebook lungs, and breathe throughtracheae. A pair ofspiracles 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 thetibia of the legs. They have agonopore on the ventral cephalothorax, and thecopulation is direct asmale Opiliones have a penis, unlike other arachnids. All species layeggs.
Typical body length does not exceed 7 mm (0.28 in), and some species are smaller than 1 mm, although the largest known species,Trogulus torosus (Trogulidae), grows as long as 22 mm (0.87 in).[4] The leg span of many species is much greater than the body length and sometimes exceeds 160 mm (6.3 in) and to 340 mm (13 in) in Southeast Asia.[13] Most species live for a year.
Harvestman eating askink tailProtolophus sp. cleaning its legsA malePhalangium opilio, showing the long legs and thetarsomeres (the many small segments making up the end of each leg)Mites parasitising aharvestmanGregarious behavior in Opiliones
Many species areomnivorous, eating primarily smallinsects and all kinds of plant material andfungi. Some arescavengers, feeding upon dead organisms, bird dung, and otherfecal 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 asgregarines.[4]
Althoughparthenogenetic species do occur, most harvestmenreproduce sexually. Except from smallfossorial species in the suborder Cyphophthalmi, where the males deposit aspermatophore,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.[14] The males of some species offer a secretion (nuptial gift) 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.[15]
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 againstegg-eating females, and cleaning the eggs regularly.[16]Paternal care has evolved at least three times independently: once in the clade Progonyleptoidellinae + Caelopyginae, once in the Gonyleptinae, and once in the Heteropachylinae.[17]Maternal care in opiliones probably evolved due to natural selection, while paternal care appears to be the result of sexual selection.[18] 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 nymphalinstars to reach maturity, with most known species having six instars.[4]
Most species arenocturnal and colored in hues of brown, although a number ofdiurnal 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 theLaniatores, and more than 70,000 in certainEupnoi.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.[4]
Harvestmen clean their legs after eating by drawing each leg in turn through their jaws.
Predators of harvestmen include a variety of animals, including some mammals,[19][20] amphibians, and other arachnids like spiders[21][22] and scorpions.[23] Opiliones display a variety of primary and secondary defences against predation,[24] ranging from morphological traits such as body armour to behavioral responses to chemical secretions.[25][26] Some of these defences have been attributed and restricted to specific groups of harvestmen.[27]
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.[28] Darker colourations and patterns function ascamouflage when they remain motionless.[29] Within the genusLeiobunum are multiple species with cryptic colouration that changes over ontogeny to match themicrohabitat used at each life stage.[27][30] Many species have also been able to camouflage their bodies by covering with secretions and debris from theleaf litter found in their environments.[27][31] Some hard-bodied harvestmen haveepizoiccyanobacteria andliverworts growing on their bodies that suggest potential benefits for camouflage against large backgrounds to avoid detection bydiurnal predators.[32][33]
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.[27][34][35] 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 producetranslucid secretions have orange markings on theircarapaces. This may have an aposematic role by mimicking the colouration of glandular emissions of two other quinone-producing species.[34] Mimicry (Müllerian mimicry) occurring between Brazilian harvestmen that resemble others could be explained byconvergent evolution.[27]
Secondary defences allow for harvestmen to escape and survive from a predator after direct or indirect contact, includingthanatosis,freezing, bobbing,autotomy, fleeing,stridulation,retaliation and chemical secretions.
Some animals respond to attacks by simulating an apparent death to avoid either detection or further attacks.[36] Arachnids such as spiders practise this mechanism when threatened or even to avoid being eaten by female spiders after mating.[37][38] Thanatosis is used as a second line of defence when detected by a potential predator and is commonly observed within theDyspnoi andLaniatores suborders,[35] with individuals becoming rigid with legs either retracted or stretched.[39][40][41][42]
Freezing – or the complete halt of movement – has been documented in the family Sclerosomatidae.[43] While this can mean an increased likelihood of immediate survival, it also leads to reduced food and water intake.[44]
To deflect attacks and enhance escape, long-legged species – commonly known as daddy long-legs – from theEupnoi suborder, use two mechanisms. One is bobbing, for which these particular individuals bounce their bodies. It potentially serves to confuse and deflect any identification of the exact location of their bodies.[27][44][45][46] This can be a deceiving mechanism to avoid predation when they are in a large aggregation of individuals, which are all trembling at the same time.[27][47] Cellar spiders (Pholcidae) that are commonly mistaken for daddy long-legs (Opiliones) also exhibit this behavior when their webs are disturbed or even during courtship.[48]
Autotomy is the voluntary amputation of an appendage and is employed to escape when restrained by a predator.[49][50][51][52] Eupnoi individuals, more specifically sclerosomatid harvestmen, commonly use this strategy in response to being captured.[47][53][54] This strategy can be costly because harvestmen do not regenerate their legs,[27] and leg loss reduces locomotion, speed, climbing ability, sensory perception, food detection, and territoriality.[47][54][53][55]
Autotomised legs provide a further defence from predators because they can twitch for 60 seconds to an hour after detachment.[51] This can also potentially serve as deflection from an attack and deceive a predator from attacking the animal. It has been shown to be successful against ants and spiders.[40]
The legs continue to twitch after they are detached because 'pacemakers' are 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 twitch for a minute, others have been recorded to twitch up to an hour. The twitching has been hypothesised to function as an evolutionary advantage by keeping the attention of a predator while the harvestman escapes.[4]
Individuals that are able to detect potential threats can flee rapidly from attack. This is seen with multiple long-legged species in theLeiobunum clade that either drop and run, or drop and remain motionless.[56] This is also seen when disturbing an aggregation of multiple individuals, where they all scatter.[27][47]
Multiple species within the Laniatores and Dyspnoi possessstridulating organs, which are used asintraspecific communication and have also been shown to be used as a second line of defense when restrained by a predator.[35]
Armored harvestmen in Laniatores can often use their modified morphology as weapons.[21][57][58] Many have spines on their pedipalps, back legs, or bodies.[27][59] By pinching with their chelicerae and pedipalps, they can cause harm to a potential predator.[21] Also this has been proven to increase survival against recluse spiders by causing injury, allowing the harvestman to escape from predation.[58]
Harvestmen are well known for being chemically protected. They exude strongly odored secretions from their scent glands, calledozopores,[27][29][34][41][60] that act as a shield against predators; this is the most effective defense they use which creates a strong and unpleasant taste.[57] InCyphophthalmi the scent glands releasenaphthoquinones, chloro-naphthoquinones and aliphaticmethyl ketones,Insidiatores use nitrogen-containing substances,terpenes,aliphaticketones, andphenolics, whileGrassatores usealkylated phenolics andbenzoquinones, andPalpatores use substances like naphthoquinones, methyl- and ethyl-ketones.[61] These secretions have successfully protected the harvestmen against wandering spiders (Ctenidae),[21][22] wolf spiders (Lycosidae) andFormica exsectoides ants.[26] However, these chemical irritants are not able to prevent four species of harvestmen being preyed upon by the black scorpionBothriurus bonariensis (Bothriuridae).[23] These secretions contain multiple volatile compounds that vary among individuals and clades.[62][63][64]
Alltroglobitic species (of all animal taxa) are considered to be at least threatened inBrazil. Four species of Opiliones are on the Brazilian national list of endangered species, all of them cave-dwelling:Giupponia chagasi,Iandumoema uai,Pachylospeleus strinatii andSpaeleoleptes spaeleus.
Several Opiliones in Argentina appear to be vulnerable, if not endangered. These includePachyloidellus fulvigranulatus, which is found only on top ofCerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) andPachyloides borellii is in rainforest patches in northwest Argentina which are in an area being dramatically destroyed by humans. The cave-livingPicunchenops spelaeus is apparently endangered through human action. So far, no harvestman has been included in any kind of a Red List in Argentina, so they receive no protection.
Maiorerus randoi has only been found in one cave in theCanary Islands. It is included in the Catálogo Nacional de especies amenazadas (National catalog of threatened species) from theSpanish government.
Texella reddelli andTexella reyesi are listed as endangered species in the United States. Both are from caves in centralTexas.Texella cokendolpheri from a cave in central Texas andCalicina minor,Microcina edgewoodensis,Microcina homi,Microcina jungi,Microcina leei,Microcina lumi, andMicrocina tiburona from around springs and other restricted habitats of centralCalifornia are being considered for listing as endangered species, but as yet receive no protection.
Chelate (pincer-like) chelicerae typical of harvestmen (200× magnification); these chelicerae are homologous to chelicerae that take the form of fangs in spiders orchelae in theSolifugae.Opilio canestrinii chelicerae shown in context to the rest of the body.
Anurban legend claims that the harvestman is the mostvenomous animal in the world;[65] however, it possesses fangs too short or a mouth too round and small to bite a human, rendering it harmless (the same myth applies toPholcus phalangioides and thecrane fly, which are both also called a "daddy longlegs").[66] None of the known species of harvestmen have venom glands; theirchelicerae are not hollowed fangs but grasping claws that are typically very small and not strong enough to break human skin.
Harvestmen are a scientifically 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 workDie 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:
Harvestmen are ancientarachnids. Fossils from theDevonianRhynie chert, 410 million years ago, already show characteristics like tracheae and sexual organs, indicating that the group has lived on land since that time. Despite being similar in appearance to, and often confused with, spiders, they are probably closely related to thescorpions,pseudoscorpions, andsolifuges; these four orders form the cladeDromopoda. The Opiliones have remained almost unchanged morphologically over a long period.[4][6] Indeed, one species discovered in China,Mesobunus martensi, fossilized by fine-grained volcanic ash around 165 million years ago, is hardly discernible from modern-day harvestmen and has been placed in the extant familySclerosomatidae.[67][68]
The interfamilial relationships within Opiliones are not yet fully resolved, although significant strides have been made in recent years to determine these relationships. The following list is a compilation of interfamilial relationships recovered from several recent phylogenetic studies, although the placement and even monophyly of several taxa are still in question.[69][70][71][72][73]
Despite their long history, few harvestman fossils are known. This is mainly due to their delicate body structure and terrestrial habitat, making them unlikely to be found in sediments. As a consequence, most known fossils have been preserved withinamber.
The oldest known harvestman, from the 410-million-year-old Devonian Rhynie chert, displayed almost all the characteristics of modern species, placing the origin of harvestmen in theSilurian, or even earlier. A recent molecular study of Opiliones, however, dated the origin of the order at about 473 million years ago (Mya), during the Ordovician.[74]
No fossils of theCyphophthalmi orLaniatores much older than 50 million years are known, despite the former presenting abasal clade, and the latter having probably diverged from the Dyspnoi more than 300 Mya.
Naturally, most finds are from comparatively recent times. More than 20 fossil species are known from theCenozoic, three from theMesozoic,[68] and at least seven from thePaleozoic.[75]
The 410-million-year-oldEophalangium sheari is known from two specimens, one a female, the other a male. The female bears anovipositor and is about 10 mm (0.39 in) long, whilst the male had a discernable penis. Whether both specimens belong to the same species is not definitely known. They have long legs,tracheae, and no median eyes. Together with the 305-million-year-oldHastocularis argus, it forms the suborderTetrophthalmi, which was though to form the sister group to Cyphophthalmi.[5][76] However, recent reanalysis of harvestman phylogeny has shown thatE. sheari andH. argus are in fact members of the suborderEupnoi, after it was discovered that living daddy-longlegs have the same arrangement of eyes as the fossils.[12]
Brigantibunum listoni from East Kirkton nearEdinburgh in Scotland is almost 340 million years old. Its placement is rather uncertain, apart from it being a harvestman.
From about 300 Mya, several finds are from theCoal Measures of North America and Europe.[5][6] While the two describedNemastomoides species are currently grouped as Dyspnoi, they look more like Eupnoi.
Kustarachne tenuipes was shown in 2004 to be a harvestman, after residing for almost one hundred years in its own arachnid order, the "Kustarachnida".
Some fossils from thePermian are possibly harvestmen, but these are not well preserved.
etc. Bartel et al, 2023 report "These new records bring the total number of Burmese amber laniatorean species to ten"
Halitherses grimaldii, a long-legged Dyspnoi with large eyes, was found inBurmese amber dating from approximately 100 Mya. It has been suggested that this may be related to the Ortholasmatinae (Nemastomatidae).[77]
Currently, no fossil harvestmen are known from theTriassic. So far, they are also absent from theLower CretaceousCrato Formation of Brazil, aLagerstätte that has yielded many other terrestrial arachnids. An unnamed long-legged harvestman was reported from the Early Cretaceous ofKoonwarra,Victoria, Australia, which may be a Eupnoi.[citation needed]
The Swedish naturalist and arachnologistCarl Jakob Sundevall (1801–1875) honored the naturalistMartin Lister (1638–1712) by adopting Lister's termOpiliones for this order, from Latinōpiliō (“shepherd”), because these arachnids were known in Lister's days as "shepherd spiders"; Lister characterized three species from England (although not formally describing them, being apre-Linnaean work).[78]
In England, the Opiliones are often called "harvestmen" or "harvest spiders".Folk-etymological explanations for this are numerous, such as that they appear duringharvest season, or because of a superstitious belief that if one is killed there will be a bad harvest that year,[79] but these are unfounded. More likely, as in other European languages which call them a word meaning “cutter” or “scyther”, the original explanation is that their oddly shaped legs look like tinysickles orscythes.[citation needed] The alternative name "shepherd spiders" is sometimes attributed to the assumption that Englishmen knew of the shepherds in theLandes region of France who traditionally usedstilts to better observe their wandering flocks from a distance, and so were reminded of them by the long-legged arachnid,[80] but is much more likely just an extension of this agricultural imagery, with the farmer's implement changed to ashepherd's crook rather than a reaping tool. Compare similar developments in theDutch words for Opiliones:hooiwagen (literally meaning “hay-wagon”) and in southern dialectskoewachter (literally “cowherd”, one who herds cattle).
English speakers may colloquially refer to species of Opiliones as "daddy longlegs" or "granddaddy longlegs". However, this name is also used for two other distantly related groups ofarthropods: the crane flies of the superfamilyTipuloidea, as well as thecellar spiders of the family Pholcidae (which may be distinguished as "daddy long-leg spiders"), because of their similar appearance.[citation needed]
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Adriano Kury: National Museum of Rio de JaneiroClassification of Opiliones—A synoptic taxonomic arrangement of the order Opiliones, down to family-group level, including some photos of the families
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