Anelosimus eximius is a species of spider that is also known as “social spider” because it socializes with its fellow spiders and builds colossal spider webs that have the potential of reaching up to 25 feet (7.6m) in height and 5ft (1.5m) in width. According to the Journal of Arachnology, this Brazilian spider species is a very cooperative and group-living spider.

Social spiders in their spider webs

Among the thousands of species of spiders (40,000 arachnids), there are only about 23 species that live in social groups. Anelosimus eximius is most commonly found in South American countries from Panama to Argentina. The massive spider webs that are created by these spiders can contain spiders in excess of 50,000 with females outnumbering men 10 to 1.

Below picture illustrates what we mean when we say that these species create massive spider webs

A. eximius was first discovered more than a century ago by a French arachnologist named Eugène Simon. An interesting advantage that these social spiders acquire by building giant webs is that they can capture prey which can be much larger in size than what a solitary spider would have been able to catch. These communal spiders work together to build, maintain and clean their webs.

Sometimes, as many as 40 colonies can be found within a distance of 1 km. To put that into perspective, check out the below image

In the below picture, a colonized spider web was discovered in a North Texas park in 2007. The people in the below picture-Mike McCord, left, and Freddie Gowin- the Lake Tawokoni State Park rangers, monitor a giant communal spider web at the park.

Researchers predict that the two major factors that might have led to spiders setting up communities are:

1. Heavy rain- The areas inhabited by social spiders are prone to heavy rains so creating and maintaining mammoth webs is inevitable that leads to spiders living together
2. Social spiders tend to feed on heavy prey which cannot be done alone, therefore, the need of being social comes into picture

Also, In 2013 in Brazil, these spiders reportedly fell from the sky in large quantities

In 2013 in Santo Antonio da Platina, Brazil, these spider webs housing large quantities of spiders were eradicated by strong winds that detached the webs from their anchors, thereby, carrying the spiders and their ruined home to new sites. This event led to a “spider rain” in which people in Santo Antonio da Platina observed spiders raining from the sky.

Within the built colonies, roles are segregated for all the spiders-males and females alike. Some work as “warriors”-those who act as predators and attack the prey-while some work to clean and maintain the webs. Social spiders are likely to have evolved from subsocial ancestors via the subsocial route. 

It has been suggested that the transition from subsocial to social living requires a change in three behavioural traits: from premating dispersal(wherein juveniles are dispersed from the colonies before breeding) to postmating dispersal (wherein juveniles are dispersed from the colonies after breeding), from outbreeding to inbreeding mating system, and from maternal care to cooperative breeding. (1, 2, 3)