Although robber spiders belong to a group that includes all round web spiders, they cannot weave their own spider webs.
Build a net, grab a fly, wrap it in silk, and swallow it calmly. This hunting strategy has proven to be very effective, and the round web spider is one of the most successful animal groups. They are found in almost every corner of the world, with more than 3,000 species.
Netting is a fairly complex job. In addition to the many forms of silk and adhesives, the spider also needs to complete a series of precise movements.
But since you can hack into other people's networks and devour the builders, why build your own?
A group of secretly devious spiders known as "robbers" take an evil approach to catching prey. Their hunting strategy is the most prominent in the animal kingdom.
A robber spider of unknown origin (Thanks: Christopher Johnson, Insects Unlock.)
Robber spiders are members of a spider community, which includes all the "round web spiders" — those we're familiar with that weave typical round webs — but they don't weave webs.
In fact, they have lost this ability. They can still produce silk, using it to make egg sacs and wrap their prey. But they are anatomically incapable of webbing. Compared to their closely related taxa, the number of silk "dragon heads" on their private pockets is significantly small.
Instead, they invade the webs of other spiders in order to seduce and kill the unfortunate creators. Gently, they flick the wires on the net, luring the owner closer.
Once the main web spider is close enough, the bandit will move.
First, it wrapped its deceived prey in its two huge front legs. The legs have huge spines on the outside, called "giant bristles," which they use to trap the main web spider in a prison-like basket.
Then, the final step: the robber spider bites its prey and uses its fangs to inject a powerful venom that immediately renders the prey immobile.
This is a powerful hunting technique.
"It's fascinating to watch a robber spider sneak around while waving its long first pair of legs to get close to where the other spiders are." Mark Townley of the University of New Hampshire said. "Although I spent a lot of time feeding robber spiders for our study of silk sacs, I never got tired of seeing them looking for and attacking their prey. This has always been a miracle to watch. They were able to wave their first pair of legs so carefully that I saw them touch their prey spiders so lightly without causing them any reaction, or even noticing. ”
But we don't fully understand how the bandit spider's strategy works.
A wrinkled spider (Larioniodes cornutus) in its web (thanks: Nick Upton/na
In particular, it is unclear why the robber spider would pluck the thread in its owner's web.
It has long been thought that this toggle mimics the vibrations caused by insects that fall into a cobweb. Hence the Latin name for the robber spider: mimetic spider, or "imitator".
Still, not all entomologists agree that this is what this robber spider is doing.
"Native spiders behave completely differently in the face of bandit spiders and their own prey, just as vibrations in a spider's web caused by two sources." Carl Kloock of California State University, Bakersfield, said.
He had another suggestion. "In my opinion, the most likely scenario is that the robber spider is imitating the vibrations of the same kind of spider web invading spiders, and possibly different kinds of spiders." Kloock said. "A spider in its own web needs to defend its web — a valuable resource — against other spiders, and they may try to take over the web to avoid the cost of building their own web, or simply try to steal prey from it."
"These encounters follow a fairly simple pattern in which the spiders signal to each other and then slowly approach each other, usually until the smaller spiders give up and flee the web.
"I think what the robber spiders do is basically send a deceptive signal, disguise themselves as small cobweb invaders, refuse to flee, and attract the native spiders closer and closer until they get into range of attack." Kloock added.
Then there's the problem of the venom of the robber spider, which has evolved to be highly toxic to other spiders, including members of their own race, but not to other animals.
"Once another spider is bitten it stops moving, while the fruit fly may struggle for a few minutes." Daniel Mott of Texas A&M International University said. "Their toxins appear to be specifically targeted at other spiders."
Why did and how did such a strange hunting strategy evolve?
The first problem is that prey spiders are also predators, equipped with fangs and venom. This means they are more dangerous than beetles or flies, and there aren't that many of them.
Second, robber spiders are professional predators. Although they sometimes prey on other prey, their main food source is usually spiders. In contrast, most ring-weaved spiders are omnivorous predators, eating any prey that crashes into the web.
In fact, robber spiders can't even catch other spiders if they don't have webs to lean on.
"In the lab, if you put a ring-shaped web-weaving spider into a jar and don't allow it to weave a web, the robber spider won't attack it." Danilo Harms of the University of Hamburg, Germany, said. "It needs a web to catch another spider."
Somehow, the ancestors of the robber spiders lost the ability to weave their own webs and became natural enemies of other spiders.
A robber spider (Ero sp.) in another spider's web (Acknowledgements: Stephen Dalton/naturep
Harms says the most plausible explanation is that it started with theft. The ancestors of robber spiders may have already begun to invade the webs of other spiders to steal insects that their owners had painstakingly captured, poisoned, and stored.
This theft has a vivid name: "theft parasitic phenomenon".
Some primitive bandit spiders may have taken this strategy to the next stage by preying on the master spider itself. Over time, they become more and more specialized in catching other spiders: evolving unusually long front legs, complex web pulling behavior, and venom specifically targeting spiders.
This view has been called the "primitive theft parasitic hypothesis."
Whatever the reason for this outlandish behavior, the bandit spider is very successful. Scientists have officially described more than 160 species that are distributed on every continent except Antarctica.
Egg sac of a robber spider (Ero sp.) (Acknowledgements: Alex Hyde/naturepl.com)
"We know a tiny fraction of the biology of robber spiders, but for most of this species, we know nothing about their life history and behavior." Gustavo Hormiga of George Washington University said. "For example, the fact is that people know almost nothing about the strange South American tropical pirate spider Gelanor genus."
In this genus, the male's whiskered limbs—modified legs, which they use to fertilize females—are twice as long as the male's body. This may allow them to fertilize females over long distances. "In all other spiders, mating requires both parties to be close to each other." Hormiga said. Long-distance mating would be a useful precaution, as bandit spiders are aggressive, have potent venom, and are ready to hunt other arachnids, including their own kind.
But they also have a mild side.
In a study published in the journal Genetic Taxonomy in November 2016, Hormiga and his student Ligia Benavides described five new species. Their study also featured the first report of a female robber spider caring for her cubs.
Mother behavior is common in spiders. Some will only regurgitate their cubs, while others will allow their juveniles to indulge in their carcasses. But the mother's behavior has never been seen in the seemingly "vicious" robber spider.
"In the field, we observed females of lip-shaped mimetic spiders, Annecy van spiders and robber spiders caring for eggs and juveniles. Robber spiders can be good mothers. Benavides said. In some cases, females distribute their eggs evenly among small nets at the bottom of a leaf. But if I move a web or touch the spider, it will quickly collect all the eggs or larvae, create a ball to wrap them around, and then take them away to protect them. ”
The robber spider is clearly a story of its own. However, their habit of imitating their prey to devour another spider — a behavior known as "aggressive imitation" — is not unique to them. It has evolved independently in arachnids at least twice before.
A jumping spider (Portiado scaly spider) (Acknowledgements: Premaphotos/naturepl.com)
There is a genus of jumping spiders called Portia. These spiders also imitate their prey, pulling on the webs of their unfortunate owners, spiders, and then eating them.
Like other jumping spiders, the Portia spider has huge eyes and relies primarily on vision to find prey. In contrast, robber spiders seem to rely more on their sense of touch. In the lab, covering their eyes with paint hardly prevents them from attacking other spiders.
The Robber Spider and the Portia Spider have only recently evolved. However, "pelican spiders" also eat other arachnids — they are so ancient that they date ahead of the evolution of winged insects.
Pelican spiders have nicknames that match their oversized jaws (strictly speaking " spider forelimbs " ) and slender neck. They are officially referred to as "reductive spiders" and sometimes called "assassin spiders".
They use one chelation horn to pierce their prey and another horn to inject venom into the suspended, pierced spider.
Pelican spiders often feed on other spiders — unlike mimetic spiders that evolved from web-weaver spiders. According to Harms, they were first found in fossil samples from 1854, living samples found in Madagascar earlier than 1881.
"So, if you had a longer radial distribution than insects, what would you eat?" Probably other spiders. Harms said. That's why they have such strange shapes. ”
Curiously, for some spiders, enjoying the flesh and blood of their spider-like close relatives is a great way to make a living.
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Original from: http://www.bbc.com/earth/story/20170608-pirate-spiders-make-a-living-by-preying-on-other-spiders