Anelosimus eximius

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.[1]

Anelosimus eximius
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Infraorder: Araneomorphae
Family: Theridiidae
Genus: Anelosimus
Species:
A. eximius
Binomial name
Anelosimus eximius

Anelosimus eximius are classified as a social spider species because they engage in shared brood care and cooperate to capture prey within their web, which allows them to capture prey much larger than a single individual would be able to.[2][3]

Their webs do not capture a lot of prey, but the prey that are caught are significantly larger than most prey captured in the webs of other individual social or antisocial spider species. Thus, their techniques provide more nutrients than other social spider colonies may obtain. These techniques are most efficient in Anelosimus eximius colonies of about 1,000 individuals.[4][5]

The sociality of Anelosimus eximius aids in the increased fitness of the species. One potential cost of sociality in Anelosimus eximius is that they produce fewer egg sacs. However, each egg sac holds more individual offspring than most arachnid egg sacs would normally hold. Thus, the benefits seem to outweigh the costs.[6]

It is difficult to explain how sociality has evolved from a typically solitary animal. One trait that has facilitated this shift is the lack of discrimination against foreign offspring. It has also been questioned whether the alloparental behavior of Anelosimus eximius was an ancestral trait or if the species had to overcome discrimination in order to gain their trait of sociality. Through studies on social and sub-social species that observed reactions to foreign offspring, scientists discovered that the species did not need to overcome discrimination; both sub-social and social species of arachnids showed no discrimination towards foreign offspring.[3]

A. eximius is preyed upon by Zatypota parasitoid wasps, the larva of which feed on the spider's hemolymph and induce the spider to move away and spin an abnormal web, serving as a cocoon for the pupating insect to feed on the spider and emerge. The process is particularly noteworthy because the actions induced do not seem to be part of the spider's typical behaviour repertoire.[7]

References
  1. JSTOR: Journal of Arachnology, Vol. 14, No. 2 (Summer, 1986), pp. 201-217
  2. Choe,..., ed. by Jae C.; Crespi, Bernard J. (1997). The evolution of social behavior in insects and arachnids (1. publ. ed.). Cambridge: Cambridge university press. ISBN 9780521589772.CS1 maint: extra text: authors list (link)
  3. "Indiscriminate care of offspring predates the evolution of sociality in alloparenting social spiders", 2013, Samuk K, Aviles L.
  4. "Spiders Who Eat Together, Stay Together -- And Form Enormous Colony Sizes", 2008, ScienceDaily
  5. "Meet the spiders that have formed armies 50000 strong", 22 January 2016, Jason G. Goldman, BBC
  6. Avilés, L; Tufiño, P (1998). "Colony size and individual fitness in the social spider Anelosimus eximius". Am Nat. 152 (3): 403–18. doi:10.1086/286178. PMID 18811448. S2CID 4816076.
  7. "Behavioural modification of a social spider by a parasitoid wasp", 2018, Fernandez-Fournier P, Straus S.


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