Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Loxosceles gaucho
Habitat: Human-made or disturbed natural environments in Brazil and Tunisia
In a fight between an armoured soldier and an unarmed opponent, who would win? When it comes to the recluse spider the answer might surprise you. Using its wits and speed, it can kill and eat one of its toughest prey: another arachnid, the armoured harvestman.
Harvestmen have a hard exoskeleton that protects them against several spiders, which are their main predators alongside birds and amphibians. Rodrigo Willemart from University of São Paulo, Brazil, and his colleagues found that even large predator spiders have difficulty piercing this armour.
For the attacker to have a half-decent chance, it needs to pin its prey between its fangs. "But this rarely happens, and usually both fangs slide on the surface of the harvestman's body," says Willemart.
Spiders have been seen attacking and eating harvestmen in the wild, but there are so few observations that harvestmen are generally left off lists of their frequent meals.
Dressed like Batman
"Harvestmen are somehow successful in avoiding predation from spiders, and it is well known many harvestman species exhibit several lines of defence," says Glauco Machado, also from University of São Paulo.
Those defences range from feigning death, which puts off most predators that feed on live prey, to releasing chemical irritants to repel attackers.
But Willemart and his colleagues found that armoured harvestmen seldom go for the chemical option, presumably because these compounds are costly to make. Instead, they rely on their exoskeleton for protection. But this backfires when recluse spiders exploit flaws in the armour's design.
Machado likens it to a Batman suit. When Batman asked for a more flexible suit, his business manager, Mr Fox, told him that more movement would imply more exposure to weapons such as knives, he says. "Of course, this lesson is relevant not only for the prey, but also for the predators," says Machado.
Willemart and his team found carcasses of harvestmen in the webs of recluse spiders in Brazil. These successful hunters were then brought to the lab, where 31 out of 38 spiders found a way to kill and then eat the harvestmen offered. So how do they do it?
Judo manoeuvre
The recluse spider carefully approaches and repeatedly feels out the harvestman with its own legs, looking for weak areas. The recluse spider can outrun any escape attempt, and may then do what Willemart likens to a judo move, pinning the harvestman's back to the ground. Finally, it delivers the death blow: a series of poisonous bites in the exact areas not shielded by armour.
"Recluse spiders are exceptional in that they do not try to pierce through the armour. They simply avoid it and bite the soft parts of the harvestman," says Willemart.
Although we now know how recluse spiders prey on harvestmen, much remains to be discovered.
"Do they use this strategy for all arthropod prey?" asks Eileen Hebets at the University of Nebraska, Lincoln. "Is this targeted biting behaviour learned? Is it innate?"
If this is a learned behaviour, Hebets suggests researchers can explore how they learn it in the first place and remember it for future use. She says spiders are great subjects for lab study of mechanisms of complex behaviours in animals, as they are easy to catch and exhibit complex behaviour themselves.
Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2014.12.025
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