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The Curious Case of the Spiders in the Ant-Acacia Mutualisms  

spider and ants on acacia

In Panama, a team of entomologists has found that two species of orb-weaver spiders, Eustala oblonga and Eustala illicita, live in acacia plants that are defended by ant colonies. The spiders’ webs intercept a variety of insects, including some potential acacia pests but also some ants alates snatched during mating flights. However, the catch of ants does not seem to have a serious negative impact on the ant colony’s health. “In a sense,” says John D. Styrsky, Ph.D., of the University of Lynchburg in Virginia, “natural selection has come up with a way for the spiders to ‘hack’ the mutualism for their own benefit without inflicting a cost and thereby exerting selection pressure on the mutualism. (Photo by John D. Styrsky, Ph.D.)

By Ed Ricciuti

Ants with a serious sting protect acacia trees from herbivorous enemies in return for room and board: At first impression, this might seem like an arthropodan protection racket, but it’s actually a wonderfully sweet deal for both parties. It’s even more of a bargain for certain orb-weaving spiders that freeload off the arrangement, creating a triangular relationship unraveled by researchers working in the steamy forests of central Panama and described in a new report published in August in the Journal of Insect Science.

Ed Ricciuti

Ed Ricciuti

The interaction between ant and acacia, in which two species benefit from an arrangement between them, is called mutualism. The ants, whose sting is horrendously painful, create a second layer of defense besides the thorns acacias brandish against creatures that might munch on them. The covenant gives the ants a place for their colony to live and provides food in the form of sugars, fats, and proteins from structures on the leaves.

Enter the spiders Eustala oblonga and Eustala illicita: small and inconspicuous enough to hide from ants during the day, crouching against leaves, stems, and thorns while deploying their webs among the leaves by night. The ant patrols and the acacia itself shield the spiders from predators such as wasps, assassin bugs, stinkbugs, anoles, and birds. The thorny tree also provides a platform for the spiders’ webs and attracts some of the insects on which they feed.

The study showed that the webs constructed by the spiders intercept a variety of insects, including a small proportion of potential acacia pests, but also the winged, sexual forms of the ants (alates), snatched during mating flights. Surprisingly, the researchers found, the catch of ants during their nuptial flights does not seem to have a serious negative impact on the ant colony’s health.

“In a sense,” says study leader John D. Styrsky, Ph.D., associate professor of biology at the University of Lynchburg in Virginia, “natural selection has come up with a way for the spiders to ‘hack’ the mutualism for their own benefit without inflicting a cost and thereby exerting selection pressure on the mutualism. While there are many examples of third-party exploiters of mutualisms, they typically exploit a reward traded between mutualist species rather than a service, and usually at some cost to at least one of the mutualist species.”

Eustala oblonga spider on Acacia melanocerus

In Panama, a team of entomologists has found that two species of orb-weaver spiders, Eustala oblonga (shown here, in Acacia melanocerus)and Eustala illicita, live in acacia plants that are defended by ant colonies. The spiders’ webs intercept a variety of insects, including some potential acacia pests but also some ants alates snatched during mating flights. However, the catch of ants does not seem to have a serious negative impact on the ant colony’s health. “In a sense,” says John D. Styrsky, Ph.D., of the University of Lynchburg in Virginia, “natural selection has come up with a way for the spiders to ‘hack’ the mutualism for their own benefit without inflicting a cost and thereby exerting selection pressure on the mutualism. (Photo by John D. Styrsky, Ph.D.)

The findings in the paper hint at the subtle complexities involved in ecological relationships between different organisms. “Unlike any other species in the orb weaver family, these spiders are apparently myrmecophilic [living among ants] and host-plant specific, meaning that they are adapted to live among ants on specific ant-defended acacia species.”

Styrsky, who has been studying ant-acacia mutualism for more than a decade, worked with his wife, Jennifer Nesbitt Styrsky, Ph.D., assistant professor of environmental science at the University of Lynchburg, and student Anna Ledin on both sides of the continental divide of the Americas, which reaches its low point of 85 feet in Panama. Each slope has its own pairing of ant-acacia species. In the understory of lowland tropical moist forest that characterizes the eastern, Caribbean-Atlantic slope of the divide in central Panama, the acacia Vachellia melanocerus shelters the ant Pseudomyrmex satanicus. Only 18 miles away, on the Pacific side of the divide, dry forest contains a V. colinsii–P. spinicola partnership. Eustala oblonga, in turn, lives in association with V. melanocerus and P. satanicus, while E. illicita resides with V. colinsii and P. spinicola.

The rainforest site was particularly remote, in Soberania National Park, barely reachable by a track that challenges even a high-suspension four-wheel-drive vehicle. Negotiating the track in the rain, says Styrsky, “is just like it is depicted in Jurassic Park—chaotic and muddy and you keep expecting a T. rex head to pop out of the forest.” To make their observations, the researchers crept among acacia trees while fending off swarms of deer flies, among the enormous buttress roots of trees that towered over a canopy resounding with the calls of howler monkeys.

Says Styrsky, “Working with the acacia–acacia ant mutualism is challenging also because pain from the ants’ stings is intense. P. satanicus is an apt name. The spiders are pretty small, so you have to contort your body around to fit under and between acacia branches to get to them and their webs. The ants will often jump (or fall, but I think it is intentional) onto you. One or two will release alarm pheromones, which then triggers all of the ants to release them in a big puff. You can just smell it, but before you have time to react, the ants start to nail you. The pain is an intense burning sensation that you feel down into your bones, particularly if the stings are near the joints of your fingers.”

Styrsky and his team collected the insects caught in the spider webs and sticky cards. Proportionately more plant eaters were caught by the cards on V. collinsii than on its rainforest counterpart. The team counted more herbivorous insects on the cards than in the webs, but the take by the spiders was still considerable. The snaring of alates by spider webs was significant, accounting for 23 percent of all insects in E. oblonga webs and half of all in those of E. illicita. The much higher density of acacias in the dry forest compared to those in the rainforest may account for the difference, say the researchers. Differences in mating behavior may also have an impact.

“Alates of monogynous Pseudomyrmex species, such as P. spinicola, engage in two-way traffic around their host acacias as males both disperse from and fly to acacias in response to sex pheromones produced by virgin queens, thereby potentially doubling their chances of being captured in E. illicita webs,” the researchers write. “Both male and female alates of polygynous Pseudomyrmex species such as P. satanicus, however, disperse from their host acacias to mate elsewhere.”

The research by Styrsky and his team focuses on only a few species—ones not so obvious in the spectacular world of tropical forests. At the same time, it probes and clarifies another piece in the immense puzzle of this vast ecosystem, which is rapidly dwindling due to human activity.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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