June 24, 2009 -- Tarantulas and black widows may cause human Miss Muffets to get off their tuffets, but new research shows many spiders themselves run for their lives if they encounter Myrmarachne melanotarsa, a gregarious jumping spider that pretends to be an ant.
Three new studies on this unusual spider reveal how it looks, acts and hangs around ants -- even forming mini colony-type gangs to foil its own predators. Most spiders are afraid of ants and this ant mimic for good reason.
"Ants are very dangerous to arthropods," project leader Ximena Nelson told Discovery News. "They are social and can mount a strong response if alerted to potential danger, and they have strong mandibles and are extremely lethal to many spiders."
"Many ants also contain formic acid, which they can use for defense by squirting it on potential predators, causing considerable harm," said Nelson, a researcher at both the University of Canterbury and the Center for the Integrative Study of Animal Behavior at Macquarie University.
Spiders that make the mistake of putting an ant in their mouths, she added, often spit them out immediately, suggesting that "ants don't taste good either."
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In one study, accepted for publication in the journal Animal Behavior, Nelson and co-author Robert Jackson observed the ant mimic spider in western Kenya. They staged encounters between it and potential predators, such as other jumping spiders.
The would-be predators did sometimes attack and eat the spider that looks like an ant, indicating it is palatable to consumers.
They avoided the ant mimics, however, when they were in a gang-like group, probably because this formation made them look even more like ants, which march in lines and live in colonies. Such cooperation and collective mimicry is extremely rare in spiders, since arachnids more often live a solitary existence.
Additional research determined M. melanotarsa is so feared that female spiders from other species will flee when they see it, even abandoning their broods, which the fake ant then consumes as an easy snack. Still other new research has found that the faux ant chooses spiders as prey in preference to other insects.
Nelson believes this spider evolved its ant-like ways over a long period of evolution "in which each morph that resembled ants more was selected for and morphs that did not resemble ants were selected against."
The deception works better if the fake behaves like an ant too, so natural selection appears to have also selected for spiders within this species that act like ants. The end result is a spider that is the spitting image of an ant.
Paula Cushing, president of the American Arachnological Society and curator of invertebrate zoology at the Denver Museum of Nature & Science, told Discovery News that insects and spiders that mimic ants "are well protected from their own potential predators," since "ants are often unpalatable prey for arthropod predators or are not attractive as prey due to their aggressive behavior towards intruders and their ability to sting and bite animals that attack them."
"Nelson and Jackson's experimental studies have gone a long way towards supporting the hypothesis that myrmecomorphy (resembling an ant) is evolutionarily adaptive in that it protects the myrmecomorph against predation," Cushing added.
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