Typically, the lion is known for hunting in packs, using coordinated movements to ambush its unfortunate prey.
But a new study shows how a spider species called Anelosimus eximius similarly traps its prey in packs using special web vibrations.
This spider species, which is native to South America, lives in large, elevated, non-stick webs and can suddenly swarm over insects caught in them.
They collectively do this using a tricky double attack involving moving as one and then staying in place to pick up the vibrations emanating from their webs.
The researchers used a dead fly connected to a vibration generator and in contact with the web to trigger a collective hunt for the spiders.
A. EXIMIUS: SOCIAL HUNTING FOR SPIDERS IN PACKS
Anelosimus eximius is a colonial social spider native to South America. Colonies consist of thousands of spiders.
This species performs tasks common to the entire colony, including web maintenance and construction, brood care, defense and attack on prey when insects fall into their trap.
Moreover, spiders cooperate in capturing captured insects using coordinated movements.
Spiders change their behavior to suit the situation or prey.
Source: Animal Diversity Network.
The study was led by Raphael Janson, researcher at the French National Center for Scientific Research in Paris (CNRS), and published in the Proceedings of the National Academy of Sciences.
The researchers used a dead fly connected to a vibration generator and in contact with the web to trigger a collective hunt for spiders in French Guiana.
“When prey is caught in the web, it causes the spiders to move,” Jeanson told Live Science.
“But after a while, they all stop for a few milliseconds before starting to move again.”
Social spiders are notable for living together in large colonies, cooperating to capture prey, sharing parental responsibilities, and rarely leaving their basket-shaped nests.
Of the 50,000 known spider species, about 20 have developed a permanent social life characterized by such cooperation.
Among these cooperative spiders, A. eximius may be one of two species that hunt in “packs” to help them catch insects, including moths and grasshoppers.
Colonies of A. eximius can contain several thousand individuals of all ages coexisting peacefully in giant webs that often measure several cubic meters in size.
Anelosimus eximius (pictured in French Guiana) is a species of social spider. Colonies are made up of thousands of spiders
SOCIAL BENEFITS
It is believed that there are many benefits to spiders associated with size reduction.
Smaller body sizes mean that each spider requires fewer resources to mature, resulting in less competition for limited resources within the colony. Thus, one large insect can go a long way, feeding many inhabitants of the group.
Second, although smaller body sizes are accompanied by the production of fewer eggs per female, social spiders invest more in quality than quantity of offspring, producing far fewer but significantly more eggs than their non-social contemporaries.
In addition, the ability to mature at a smaller size provides some level of flexibility in response to unpredictable environmental conditions.
Source: University of Portsmouth.
“The nests are usually composed of a basket-shaped horizontal silk web and a network of vertical threads connected to vegetation, which are used to intercept flying prey,” the authors say.
“Massive and rapid accumulation of spiders on prey is all the more important because the web of A. eximius is not sticky, and there is a high risk that the prey will escape before the spiders grab it.”
It is already known that spiders of this species cooperate in capturing insects caught in the web using coordinated movements.
By combining field studies and simulations, the research team determined the activities involved in synchronizing these movements.
A. eximius spiders approach prey in two phases depending on the vibration of the web – they approach their resisting prey in unison or, alternatively, stand as one, the team found.
“A person’s decision to move depends on the relative intensity of the vibrations emitted by the prey and the moving spiders,” their article states.
“This rule allows the group to quickly adapt to any change in the size of the prey or the number of spiders involved in the hunt.”
Synchronization involves modulating each spider’s behavior according to the relative intensity of the prey’s signals compared to those of other spiders.
In other words, spiders remain motionless in the web when vibrations emitted by other spiders mask vibrations from prey.
Colony of social spiders (Anelosimus eximius) in French Guiana. Social spiders are notable for living together in large colonies, cooperating to capture prey, sharing parental responsibilities, and rarely leaving their basket-shaped nests.
“It’s a bit like when you’re in a room with people chatting,” Jeanson said.
This coordination increases the spiders’ ability to detect prey and optimizes their hunting performance.
Thus, they can catch prey hundreds of times their own size.
Interestingly, the more sociable the spiders are, the smaller they tend to be, which means you’re unlikely to find giant spiders that hunt in packs.
According to Animal Diversity Web, female A. eximius spiders range in size from 0.17 to 0.23 inches (4.4 to 6 mm), with males likely even smaller.
Although smaller body sizes are accompanied by the production of fewer eggs per female, social spiders invest more in quality than quantity of offspring.
In other words, they produce far fewer but significantly more eggs than their non-social contemporaries, according to researchers at the University of Portsmouth.
SPIDERS THAT HUNTER IN PACKS CAN HAVE MORE THAN 22 TIMES THEIR SIZE, BUT MORE LIKELY REMAIN SMALL, RESEARCH SHOWS
A 2020 study found that spiders that hunt in packs can take down prey more than 22 times their size, while single arachnids tend to attack insects that are half their size.
Tiny social web spiders known as Anelosimus eximius live by the thousands in webs up to 25 feet long suspended from South American rainforest trees.
Known for eating larger prey, they can prey on insects the size of a giant four and a half inch tall grasshopper, swarming over it before it can escape.
A. eximius is one of 33 species studied by University of Portsmouth researchers who convinced them that “social” spiders are more likely to stay small.
Social spiders tend to live in large, towering webs and swarm over infested insects (pictured)
They found that while group arachnids preyed on prey two and a half times their size on average, solitary spiders preferred to eat creatures about half their size.
The researchers made the discovery by comparing the body size ratio of each species to the average size of their prey.
Eight species of velvet spiders of the genus Stegodyphus found in Africa, Israel and India were studied together with 25 species of the genus Anelosimus from America.
Spiders in the Anelosimus group ranged from 0.07 to 0.2 inches while those of Stegodyphus ranged from 0.3 to 0.9 inches, with social spiders generally being much smaller.
Zoology Senior Lecturer Lena Grinstead said her findings show that working together reduces the need for spiders to grow.
“Our argument is that when these spiders start to cooperate in capturing prey, they can still catch the same very large prey even though they start to mature at a smaller body size,” she said.
“And having a smaller body size carries a lot of overall fitness benefits when you live in a crowded group.”
Dr. Grinstead said a smaller insect would mean a larger insect could feed more group members and fewer eggs would be produced, resulting in higher quality offspring.
She added that the mortality rate for social species is decreasing as they build larger, more sheltered nests and don’t have to take risks traveling to find a mate.
She called her theory “the prey-predator size ratio hypothesis”.
“When spiders develop the ability to co-trap prey, we see a beneficial increase in the body size ratio of prey to predator,” she said.
“This increase can be achieved either by capturing larger prey, as the classical theory suggests, or by reducing predator body size, as we see this has happened in some social spiders.”