Anelosimus eximius

A group of A. eximius in a communal web.

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]

Mature colonies consist of a mixture of adult and immature individuals of both sexes. The gender ratio is skewed towards females, as males only make up between 5% and 22% of a given colony's population. Reproduction occurs throughout the year. Young are fed by regurgitation; females will feed young other than their own.[1]

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.[4]

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]

Anelosimus eximius are notable for favoring large prey items, which make up a much larger portion of their diet compared to solitary or less social species. A. eximius colonies are recorded to primarily capture prey in the 10-20 mm range with peaks reaching up to 50 mm or more, while solitary species from the same locations rarely take prey exceeding 10 mm in size. In a comparative study, large prey items such as wasps, bees, coleopterans, lepidopterans, heteropterans, blattodeans and orthopterans were found to make up 74.8% of the diet of A. eximius but only 22.5% of the diet of Anelosimus jucundus, a related but less social species. Smaller prey is often ignored by the spiders and is typically able to escape from the web. A. eximius webs typically makes use of non-sticky "knockdown strands" up to several meters long which are strung across open flyways through which larger flying insects tend to move. Insects that collide with these strands fall into the web below, where their struggling movements attract the spiders. Large groups of spiders will swarm captured insects; larger prey and animals that struggle longer will typically attract more spiders. Groups of 30-50 spiders or more can be observed attacking a single large insect.[5] Large prey items are afterwards fed on collectively by multiple individual spiders.[1]

A. eximius webs do not capture large numbers 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.[6][7]

Colony distribution is known to occur in two manners: budding and dispersal. Budding occurs when an established colony splits into two or more independent webs, either due to being broken by falling debris or other accidents or due to a number of spiders establishing a secondary web in its vicinity. Dispersal occurs when a large number of mated females leave the colony and disperse individually, each creating a solitary web. These solitary females may be joined by other dispersing individuals, typically other females whose attempts to establish a new colony have failed. Newly-founded webs have high failure rates, but are more likely to survive if joined by other females. New colonies with multiple founders tend to be encountered near established webs, while single-founder webs are usually more distant. Males and juveniles do not leave their natal webs during dispersal events.[1]

These patterns of distribution lead to A. eximius colonies often occurring in aggregations of two to forty distinct webs, which may be separated from other such clusters by distances ranging from a few meters to several kilometers. Colonies within a single cluster tend to be derived from budding events, while distinct clusters may originally become established through dispersal founder events. Neighboring colonies are typically in close proximity, and may share the same knockdown threads or be in physical contract. Gene flow among colonies within a single cluster is high. Gene flow is low to absent among distinct clusters, as individual spiders do not typically move between unrelated colonies. It is however possible that new colonies might be established by a number of unrelated females. Consequently, populations of A. eximius exist as mosaics consisting of groups of geographically and genetically distinct colony clusters.[1]