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Zoologger: Keep freeloaders happy with rotting corpses

Zoologger: Keep freeloaders happy with rotting corpses

12:25 21 April 2010 by Colin Barras

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Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Species: Nephila clavipes

Habitat: Areas of high humidity and relatively open space, south-east US and South America

Pretty name, not-so-pretty domestic arrangements. The golden orb-weaving spider Nephila clavipes has the unpleasant habit of weaving rotting insect carcasses into its web.

What possesses it? The gruesome behaviour isn't unique to Nephila: many spiders add cadavers to their web designs. Some studies suggest the whiff of rotting carrion is designed to lure insects to their doom, whereas others conclude that the corpses ward off predators.

There might be a third motive for Nephila's gothic design tastes. Yann Hnaut at the research institute ECOSUR in Chetumal, Mexico, and colleagues found it when they studied webs on the edge of a coffee plantation in southern Mexico. They chose a 200-metre-long fence dotted with Nephila webs and removed both the spiders and their webs from the first 100 metres. Next they erected 16 sticky insect traps at spider-web height at regular intervals along the whole length of the fence. The next day they catalogued their insect haul.

Drawn to the dead

The results firmly vindicated the insect attraction hypothesis. The traps in the web-free stretch had each captured eight insects on average, while those erected among the fetid stench of the webs had caught an average of 23 insects each.

The team found five insect families were especially prevalent in the traps near the spider webs – and three of those families love the taste of rotting insect cadavers.

Mystery solved? Not quite: insects in the traps near webs were on average much smaller than those in the web-free section. In fact, the decay-loving insects caught near webs are so small – all with a body shorter than 2.5 millimetres – that Nephila finds them too fiddly to eat.

So why attract them at all, and why go to the trouble of weaving them into your web? Some spiders regularly eat their webs, including any tiny morsels caught in it, before building a new one – but not Nephila.

It seems Nephila is stocking its web with food for a freeloader. Argyrodes, the dewdrop spider, hangs out on Nephila webs and tries to steal the host's catch. Hnaut and team conclude that the whole corpse-weaving rigmarole is designed to feed Argyrodes. Nephila, they argue, goes to these extraordinary lengths to keep its unwanted web mates happy so that it can enjoy any larger insects that fall onto its web in peace.

Journal reference: Journal of Arachnology, DOI: 10.1636/t08-72.1

Read previous Zoologger columns: Robin Hood meets his underwater match, The mud creature that lives without oxygen, Magneto-bat steers by a built-in compass, The world's most promiscuous… snail, Pregnant males are pro-choice for abortion, Mummy, can I have some more carrion soup?, The largest arthropod to prowl the land, Fireproofing tips from the great bowerbird, What turns a tadpole into a killer?.