A social spider is a spider species whose individuals form relatively long-lasting aggregations. Whereas most spiders are solitary and even aggressive toward other members of their own species, some hundreds of species in several families show a tendency to live in groups, often referred to as colonies.
Most species of social spiders live in the tropical regions of the world where size and density of their prey – insects – is highest. But several species reach into the eastern United States and other temperate areas. By building a communal web, it is thought that the spiders approximately maximize total biomass capture per spider. [1] Having a larger web and multiple spiders to work together to subdue prey allows them to prey on larger organisms than would be possible if they led a solitary existence. The colonies can grow large enough to take down birds and bats, as well as very large insects.
Living in a colony also has another major benefit for spiders: cooperative nest maintenance. Nest maintenance does not rely solely on an individual in a colony setting and thus saves on a per-capita investment in maintaining silk structures. Predator defense is also increased in a colony with a large web and multiple individuals analogous to schools of fish or herds of mammals. [2]
Social spiders exhibit varying levels of sociality, of which there are six defined. Agnarsson et al. estimate that spiders as a whole have independently evolved sociality 18 or 19 times. [3] Most of these social spiders broadly fit into the quasi-social definition of sociality, meaning they show cooperative brood care, use the same nest (web), and have some amount of generational overlap. Several permutations of social behavior exist amongst the 23 species of spider considered to be quasi-social out of some 45,000 known species of spider. [4] These 23 species are phylogenetically scattered in 11 genera across eight widely separated families. [3]
The level of sociality often varies between species (interspecies) but can vary within a species (intraspecies) as well. Intraspecific variation is generally habitat dependant, where some populations within a species show all the characteristics of quasi-sociality, yet a population a mile away may be largely solitary [5] because they inhabit a different environment. This facultative sociality allows them to survive periods of sub-optimal conditions, when sustaining large aggregations is not feasible. Some of these aggregations can contain as many as 50,000 individuals as in the case of Anelosimus eximius (in the family Theridiidae). [6] The genus Anelosimus has a strong tendency towards sociality: all known American species are social, and species in Madagascar are at least somewhat social. [7]
Members of other species in the same family but several different genera have independently developed social behavior. For example, although Theridion nigroannulatum belongs to a genus with no other social species, T. nigroannulatum build colonies that may contain several thousand individuals that co-operate in prey capture and share food. [8] Other communal spiders include several Philoponella species (family Uloboridae), Agelena consociata (family Agelenidae) and Mallos gregalis (family Dictynidae). [9] Social predatory spiders need to defend their prey against kleptoparasites ("thieves"), and larger colonies are more successful in this. [10] The herbivorous spider Bagheera kiplingi lives in small colonies which helps to protect eggs and spiderlings. [11] Even widow spiders (genus Latrodectus ), which are notoriously aggressive and cannibalistic, have formed small colonies in captivity, sharing webs and feeding together. [12]
A few species, such as Anelosimus eximius , are also known to have reproductive skew, where some of the females reproduce and others do not. [13] Even though these spiders cooperate by caring for each other's young, cooperating in prey capture and sharing food, they are not considered eusocial as they do not have defined castes. Any female has the potential to reproduce, therefore differences in the reproductive success is due to competition for scarce food resources rather than the existence of physically distinct castes. [14]
Many more species of spider are considered to be more sub-social than quasi-social – meaning they lack fixed or complex social organization. These species may only display social behaviors as a seasonal venture and have an obligate solitary phase. Some other species will establish territories within the colony and can even have discrete webs, narrowly connected to other webs within the colony (such is the case with Leucauge spp. [15] ). This is not a fully cooperative behavior as there is little to no cooperative nest maintenance or brood care occurring. The subsocial species, however, appear to be crucial for the evolution of sociality in spiders. Recent theory suggests that social spiders evolved along a restricted pathway through solitary subsocial ancestors. [3] [16] It has been shown that the subsocial spiders of genus Stegodyphus tolerate a low level of inbreeding with low inbreeding depression, suggesting a possible stepping stone towards the fully inbred mating system found in social spiders. [17] [18]
Several social spiders including Parasteatoda wau and Anelosimus eximius also swarm in an analogous way to the eusocial ants, bees and wasps. [5] These species disperse and establish new colonies by means of synchronized emigrations of adult and sub-adult females. After courtship and copulation, but prior to oviposition, many females will emigrate to a new nesting site and deposit their eggs, forming a new colony. In this way social spiders are also extremely inbred, as there is limited migration of males or juveniles to different colonies, forcing the offspring to mate with one another decreasing genetic variation within the colonies. [5] Occasionally males will emigrate with females or will emigrate from one colony to another but this is a rare event and has not been studied sufficiently to quantify for any social spider species. Females can also out-number males as much as 10:1 in many species; this too acts as a genetic bottleneck and further decreases the genetic variation of the species.
Species include: [19]
Theridiidae, also known as the tangle-web spiders, cobweb spiders and comb-footed spiders, is a large family of araneomorph spiders first described by Carl Jakob Sundevall in 1833. This diverse, globally distributed family includes over 3,000 species in 124 genera, and is the most common arthropod found in human dwellings throughout the world.
Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies.
Spider behavior refers to the range of behaviors and activities performed by spiders. Spiders are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms which is reflected in their large diversity of behavior.
Anelosimus is a cosmopolitan genus of cobweb spiders (Theridiidae), currently containing 74 species. Anelosimus is a key group in the study of sociality and its evolution in spiders. It contains species spanning the spectrum from solitary to highly social (quasisocial), with eight quasisocial species, far more than any other spider genus. Among these is the South American social species Anelosimus eximius, among the best studied social spider species.
Anelosimus studiosus is a subsocial tangle web spider or theridiid spider living in both North America and South America. In 2012, genetic analysis revealed a previously identified species, A. tungurahua, is in fact the same species as A. studiosus.
Anelosimus eximius is a species of social spider in the genus Anelosimus, native to the Lesser Antilles and the area from Panama to Argentina. Colonies can comprise several thousand individuals.
Agelena consociata is a social species of funnel web spider that occurs in tropical forests in West Africa and lives in colonies of one to several hundred individuals. This species is found in rainforest habitats in Gabon. It favors dense forests along creeks where colonies can build huge complex webs.
Anelosimus monskenyensis is a small species of spider in the family Theridiidae. It has only been found on Mount Kenya, the location from which its name originates. Males range in size from 1.80 to 1.90 millimetres, while females range from 1.90 to 2.55 millimetres. Specimens were shades of brown and white, although color variation is not known. They may kleptoparasitically inhabit the webs of agelenid spiders, although this behavior is not found in other Anelosimus species. Agnarsson and Zhang find it more probable that A. monskenyensis is a social or semi-social spider.
Anelosimus oritoyacu is a species of tangle-web spider found in Ecuador and Mexico at altitudes from 1,800 to 2,000 metres. It is subsocial, although it has some features which distinguish it from other social or subsocial spiders in the genus. It has long-lived nest sites, unlike the social spider Anelosimus eximius which has more transitory nest sites, and its webs do not have aerial threads found in other social and sub-social species. It has a female-biased sex ratio, which is indicative of social behavior, although its sex ratio is smaller than other social species. It was first identified as distinct from Anelosimus studiosus in 2006 by Ingi Agnarsson. It is named for Oritoyacu, Ecuador, where the type specimen was collected.
Anelosimus potmosbi is a species of spider found in Papua New Guinea. It is found along the coast near Port Moresby. It is solitary, despite the sociality commonly found in the genus Anelosimus. The total length of individuals is approximate 2 to 3 millimetres, and it can be distinguished from other species by the genitalia: the male has an elongated corkscrew embolus, while the female has a simple copulatory duct trajectory. It is named for the name of Port Moresby in the Tok Pisin language.
Anelosimus terraincognita is a species of spider discovered in the collection of the Rijksmuseum van Natuurlijke Historie, with no associated information as to its collector or location of discovery. Males have a corkscrew-shaped embolus, which is a characteristic unique to Australasian species within the genus Anelosimus. It is known only from the holotype specimen, which has a total length of 2.2 millimetres (0.087 in). It is named for the cartographic Latin phrase terra incognita, meaning unknown land.
Anelosimus pomio is a species of tangle-web spider found in Papua New Guinea. It was first collected in 2009 by Ingi Agnarsson, and identified by the same in 2012. It was collected from small-leaved mangrove trees adjoining a beach. It is 3 to 4 millimetres in length, and can be distinguished from other species in its genus by the shape of the embolus. The embolus looks similar to that of Anelosimus chonganicus and Anelosimus membranaceus: It forms a corkscrew shape with fewer turns than A. chongnicus and the turns are closer to the base than A. membranaceus. It is presumed to be a solitary spider, although there are limited data. Its name is derived from the village of Pomio, in East New Britain Province, near where it was collected.
Anelosimus eidur is a species of tangle-web spider found in Papua New Guinea. Its habitat is high elevation scrub forest, in Southern Highlands Province and Enga Province. It has a total length of 2.75 to 4 millimetres, with the females being larger than the males. It can be identified by its unique genitalia, particularly the spiraling embolus in the males. The web structure is similar to social and sub-social species, leading to its tentative identification as subsocial. The species is named Eiður Francis, the son of Ingi Agnarsson who first identified the species in 2012.
Anelosimus luckyi is a species of spider found in Papua New Guinea. It is known only from the holotype specimen, found by Andrea Lucky in 2009 and after whom the species is named. It was discovered in Western Province at an elevation of 1,587 metres (5,207 ft). It has a distinctive embolus, which differentiates it from other species. The sociality of the species is not known.
Anelosimus bali is a species of spider found in Bali, Indonesia, after which the species is named. It is a coastal species, found in small-leaved mangrove trees along the beach. The holotype is female and 2.9 millimetres (0.11 in) long. No male specimens have been identified, and the social structure of the species is not known. It was first identified in 2012 by Ingi Agnarsson.
Anelosimus pratchetti is a species of tangle-web spider found in New South Wales, Australia. Initial field observations indicate it is a subsocial spider. It lives in low elevation environments, including beachfront mangrove forests. It was identified by Ingi Agnarsson in 2012, who named the species after Terry Pratchett, whom Agnarsson described as "a comic genius".
Anelosimus jabaquara is a species of spider found in subtropical, humid, lowland forests in Brazil. Anelosimus jabaquara was first described by Herbert W. Levi in 1956. These spiders cooperate to spin and repair the colonial web, capture prey, and care for the brood. Colony size is small, and the sex ratio is biased towards females.
Leticia Avilés is an Ecuadoran evolutionary biologist and ecologist who studies the evolution of social behavior and the evolution of life history traits in metapopulations. Her methods include a combination of theory and empirical work, the latter using social spiders as a model system. Her research on these organisms has addressed questions such as why some spiders live in groups, why do they exhibit highly female-biased sex ratios, and why have they evolved a system where individuals remain in the natal nest to mate from generation to generation.
Stegodyphus sarasinorum, also known as the Indian cooperative spider, is a species of velvet spider of the family Eresidae. It is native to India, Sri Lanka, Nepal, and Myanmar. This spider is a social spider that exhibits communal predation and feeding, where individuals live in large cooperatively built colonies with a nest or retreat constructed of silk woven using leaves, twigs, and food carcasses, and a sheet web for prey capture.
Stegodyphus dumicola, commonly known as the African social spider, is a species of spider of the family Eresidae, or the velvet spider family. It is native to Central and southern Africa. This spider is one of three Stegodyphus spiders that lives a social lifestyle. This spider has been studied living in large natal colonies in large, unkempt webs. Each colony is composed mainly of females, where a minority act as reproducers, and a majority remain childless and take care of the young. Males live a shorter lifespan, during which they will largely remain in the natal nest. Females are known for extreme allomaternal care, since all females – even unmated virgin ones – will take care of the young until they are eventually consumed by the brood.
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