All spiders are predators, but most of them are small and have rudimentary defences against larger animals that in turn prey on them. Spiders have thus evolved a range of predatory behaviours that, at the same time, allow them to evade the threat of predation – and some of the most effective strategies involve deceiving ants.
Wolf in ant’s clothing
Over 300 species of spiders are known to mimic the outward appearance of ants, a phenomenon called myrmecomorphy. Aggressively territorial, ants are typically avoided by several predators and are thus the perfect creatures to impersonate. Most ant-mimicking spiders have a “false waist” and are covered with reflective hairs to simulate the shiny, three-segmented bodies of ants. They have coloured patches around their eyes to make their simple eyes look more like an ant’s compound eyes. The spiders also behave like ants by waving their front pair of legs near their heads like antennae, and adopting an erratic zig-zag pattern of movement that is more akin to that of ants than spiders.
There are two reasons why a spider would want to mimic an ant: to eat them, or to avoid being eaten.
The first reason, called “aggressive ant-mimicry”, is a rare but intriguing phenomenon, and is employed by spiders to deceive their prey, which in most cases is the ant itself. Ants make for dangerous prey; they have strong jaws, poisonous stings, and chemical defenses, and can launch forceful collective attacks. Aggressive ant-mimicking spiders thus prefer to attack their victims while they are alone, as an army of ants would probably overpower the spider. And after killing the ant, the spider also has to ensure that other ants do not attack it while it carries the corpse to its nest.
Aggressive crab spiders typically jump on a lone unsuspecting ant and bite it. Then, in order to avoid encounters with other ants, the spider and its victim fall away on a safety line made of the spider’s silk while the venom takes effect. Others, like the ant-mimicking ground spider, use the body of their dead prey as a shield, holding it up between themselves and any other challenging ants. This tricks attacking ants into believing that the spider is just another ant, carrying a dead nest-mate away from their nest.
At the other end of the spectrum is “Batesian mimicry”, a tactic used by spiders to deceive their predators. Batesian ant-mimics dishonestly advertise the unpalatable characteristics of ants as their own, thus deterring those predators that have an innate aversion to ants.
The best-known example of a Batesian mimic is the jumping spider, which is regularly preyed upon by both ants and other, larger jumping spiders. In one study, when an ant, an ant-mimicking jumping spider, and a non-ant-mimicking jumping spider were simultaneously presented to a larger jumping spider predator, it was most often the non-ant-mimicking jumping spiders that were attacked, suggesting that mimicry was an effective strategy to avoid being eaten.
A Batesian ant-mimicking jumping spider looks and behaves like an ant by waving its “antennae” and moving erratically. By doing this, it avoids being eaten by other ant-fearing jumping spiders.
Keep your enemies close
Some jumping spiders are also the prey of the far more vicious spitting spiders. The latter are so named because they catch their prey from a distance by spitting a liquid that contains both venom and spider silk from their fangs. In less than a second, the silk hardens on contact and restrains the prey, allowing the venom to take effect. The spider then bites its entangled victim and begins to wraps more silk around its body before carrying it back to its nest for feeding.
To shield themselves from the lethal grips of these predators, jumping spiders turn to weaver ants for protection, presumably because the social and territorial nature of these tiny insects make them attractive defenders. Here, instead of mimicking the ant, the spider simply takes advantage of its close proximity. Spitting spiders typically build their webs above the nests of jumping spiders, carefully positioning them to ensure a direct target. It turns out, however, that if the jumping spider’s nest is built near that of a weaver ant, the spitting spider stays away because it is repelled by olfactory cues released by the ant.
But as if being targets of their predators isn’t enough of a problem, the jumping spiders are also the food of choice for their own bodyguards! Therefore, jumping spiders have evolved to develop a defense strategy that protects them from the weaver ants as well. To do this, jumping spiders build an “ant-proof” lair, weaving an abnormally tough nest that is difficult for ants to tear open. The nest also has silk flaps that serve as swinging doors; and while the resident spider can enter and leave the nest by raising these flaps, the ants seldom attempt to maneuver this obstacle.
How is hunting behaviour represented in the spider’s brain?
The evolution of predatory strategies in jumping spiders has been well studied, but efforts to understand the neurological underpinnings of their behaviour have, until very recently, been unsuccessful. This is because their internal cavity is filled with a pressurized fluid that enables them to jump swiftly but causes their bodies to explode if surgically probed. Most research has thus been limited to neurophysiological recordings from their eyes. However, last month, scientists from Cornell University, published the first successful recordings from the brains of these creatures.
Instead of making large incisions that are typical of arachnid experiments, Gil Menda and colleagues devised a way to create a small opening, not much wider than a strand of hair, thus preventing the large-scale fluid loss seen in previous experiments. In fact, the wound was so small that it activated the spider’s clotting mechanisms, self-healed and curbed even the slightest loss of fluid.
The researchers recorded neuronal activity while presenting the spider with images of flies (prey) and other jumping spiders (either prey or rivals) to study how hunting behaviour was represented in the brain. The parts of the brain that process visual information showed a burst of electrical activity when spiders were shown images of flies, their natural prey.
As a control experiment, the researches presented images of jumbled up flies, and found that this did not elicit a response in the brain, suggesting that it is the entire fly, and not just random parts of the fly, that conveys information to the spider. The images of the other jumping spiders did not elicit a strong response either, perhaps because the neuron from which this activity was recorded showed a preferential response to flies alone. The authors suggest that other neurons that were not attached to the recording electrode may have responded to the spiders.
While these findings are certainly preliminary, they open up doors for understanding the neural correlates of spiders’ remarkable predatory behaviours.
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