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Feature

Spider Cities of Africa

Sociability gives spiders protection from the dangerous world.

Associate Professor Robert R. Jackson

The popular image of spiders is that they are solitary and often cannibalistic, very anti-social in their behaviour. Sociality is rare among the more than 4,000 species in the largest spider family, the Salticidae or jumping spider, yet recent work in East Africa has revealed some fascinating aspects to their social lives.

Jumping spiders' behaviour is complex and flexible enough to rival that of a mammal. My studies on the shores of Lake Victoria (one site in Entebbe, Uganda, and one in Kenya at Mbita Point) reveals another way in which this family is exceptional, with several species having remarkable and unique forms of sociality.

Six species of jumping spider were found regularly to live in colonies: three species of Menemerus, two species of Afraflacilla and one species of Myrmarachne. Colonies or nest complexes are continuous sheets of silk embedded with individual nests. The spiders use their nests as havens, but go out on foraging expeditions on the surface of the colony and in the neighbouring environment.

Hundreds or even thousands of jumping spiders may occupy a single colony, which are located on walls of buildings, surfaces of boulders and tree trunks, and under bark. Any combination of the six species may be in a single colony. Many web-building spiders from other families also live in large web complexes, and jumping spider colonies are often adjacent to, or even inside, these webs.

Insect life, especially midges locally known as "lake flies", is present in unimaginable numbers on Lake Victoria. In the late afternoon, lake fly swarms flood the shoreline, sometimes literally blocking out the sun and coating every surface, including spider colonies, with a seething mass of six-legged spider food. Lake Victoria's generous bounty must be critical in permitting large and numerous jumping spider colonies to form; the largest were within a few hundred meters of the lakeshore.

However, these colonies are not simply the incidental consequence of spiders living together in enormous numbers. Instead, as demonstrated by experiments, the spiders actively seek out the company of members of their own, as well as other jumping spider, species, and join existing colonies or form new ones in preference to living alone.

Jumping spiders generally prefer prey of a similar size to themselves and, for the social species (with body lengths 3-5 mm), lake flies are the perfect size. The astronomical biomass of insects translates into extraordinary numbers of spiders, available for araneophagic (spider-eating) arthropods. Field studies combined with laboratory experiments suggest these predators are a critical factor why jumping spiders are living together.

The Eaters and the Eaten

Although most jumping spiders are insectivorous, araneophagy is also common. Portia uses web invasion and aggressive mimicry signalling to catch spiders, and Portia africana and Portia schultzi prey on web-building spiders at Entebbe and Mbita Point. Holcolaethis has a flattened body and cryptic markings and is primarily an ambush predator. It sits on tree trunks waiting for unsuspecting spiders to walk by. A new discovery is that P. africana and Holcolaethis, especially juveniles, enter jumping spider nest colonies and prey on residents.

Some insects also prey on the social jumping spiders. There are reduviids ("assassin bugs") that specialise on jumping spiders, and many ant species, abundant around Lake Victoria, eat the spiders and especially their eggs. Besides ants, spiders that mimic ants are another problem for the social jumping spiders. Myrmarachne is a large, primarily tropical, jumping spider genus renowned for looking and walking like an ant. Besides preying on insects, many Myrmarachne species also eat the eggs of other jumping spiders by raiding the nests of brooding females. There are a large number of Myrmarachne species around Lake Victoria, and one species is even more ant-like, by being social. This small, red Myrmarachne is almost always found living together in colonies. They tend to join colonies of the other five social species where they are unwelcome guests, and for good reason: the ant-mimic, like the ant it resembles, raids nests and eats eggs. Experiments have shown that the other social jumping spiders tend to avoid joining colonies that have become infested with Myrmarachne, and residents will abandon their homes when infestation levels becomes high.

Juvenile Portia africana and Holcolaethis and the assassin bugs capture jumping spiders by ambush. They position themselves near the openings of nests and lunge at spiders as they leave. Most nests are more or less tubular in shape and have two openings. Portia africana juveniles are themselves social. They seek out each other's company and gather within the colonies of the five primary social species where they capture and feed on juveniles of Afraflacilla and Menemerus, but not Myrmarachne. Sometimes they feed together on the same prey, although capture never appears to be a communal effort.

Death by Numbers

Assassin bugs tend to aggregate on jumping spider nests and feed together on the spiders they capture. Being about 10-12 mm in body length, the adult assassin bugs are effective at taking jumping spiders of almost any size. Aggregating assassin bugs present formidable problems to solitary jumping spiders and ones in small complexes, because the bugs quickly cover both doors of a nest, leaving the spider no recourse but to run the gauntlet. In large colonies the spatial arrangement of individual nests tends to be complex, and the assassin bugs appear to have no particular aptitude for discerning which door goes with which nest. Often, even if many assassin bugs have infested a large complex, a resident spider will have a door free.

The master practitioners of using numbers to overwhelm jumping spiders are the ants. Ants get entangled in silk, and nest silk helps keep eggs safe, but only to a degree. When large numbers of ants act in concert, ants entangled in the silk become non-sticky platforms from which other ants mount their attack. Once the silk barricade is breached, the cause is lost and the resident usually abandons her brood and flees, if she can. Myrmarachne also mount mass attacks on nests, but have little trouble getting through nest silk. There is no evidence that a group of Myrmarachne could physically overpower the resident spider, but ant-mimicry appears to be their secret weapon. Resident Menemerus and Afraflacilla tend to panic and leave when the nest is breached regardless of whether it is by the dangerous ants or by the not-so-dangerous ant mimics.

Numerous factors may have influenced the evolution of sociality in jumping spiders, but one important adaptive advantage stands out from the present study: anti-predator defence. Experiments have shown that an individual social jumping spider living in a colony tends to be safer than living alone, and larger colonies are safer than smaller ones.

The way this works is different from what is more typically associated with many of the social insects. Wasps and ants, for example, mount communal attacks on predators invading their nests. For the social jumping spiders, a large colony is a communally-built fortress. The ant-deterring silk barrier built by many jumping spiders is more formidable than the barrier a solitary spider or a small group could build. Even when ambush predators are in the neighbourhood, the intricate multi-layered structure of the colony may create more paths by which a spider can move in and out of its nest without being nabbed. Also, in large colonies there is safety in numbers and an individual spider is less likely to be singled out by the ambush predators.

Robert Jackson is senior lecturer in zoology at the University of Canterbury.