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 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]}

Anelosimus eximius work together to capture large prey which allows them to handle increased colony sizes. This puzzles many scientists because, due to laws of nature like surface-to-volume-ratio, the surface area of the webs of most social spider colonies do not increase as quickly as the number of occupants in the nest, thus, the members of these colonies will obtain very little nutrients. The scarcity of nutrients makes it impossible for these species to survive in larger colonies. It is due to the ability of Anelosimus eximius to sufficiently work together, to increase their web size, that they are able to obtain enough nutrients to sustain their colonies. 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]

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. ^[5]

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. ^[6]

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