Bone-House Wasp Protects Nest With Ant Corpses

Female Deuteragenia ossarium / Staab et al. PLOS ONE 2014
Janet Fang 03 Jul 2014, 07:51

What’s that lining your chamber door? ‘Tis ants corpses, and nothing more. Scientists investigating wasp nests stumbled upon some that had an entryway filled with dead ants. The wasp behind this never-before-seen nest protection strategy has been identified as a new species of spider hunters, now dubbed the "bone-house wasp," Deuteragenia ossarium.

Spider wasps use many tactics to protect their nests, such as digging holes in wood or occupying pre-existing cavities. Previous work revealed how their nests contain several chambers for eggs (called brood cells) separated by thin walls of plant debris, resin, or soil. The female makes the nest, lays the eggs, and provides provisions for her future little larvae -- usually a paralyzed spider she had already eaten “nonessential” parts from. (Each developing larvae gets its own immobilized, but still living, spider.) Afterwards, she constructs an outermost vestibule, or entry compartment, and then she’s gone. 

To investigate protection strategies used by cavity-nesting wasps, an international team led by Michael Staab from the University of Freiburg in Germany laid out trap nests, consisting of a plastic tube filled with grasses, in the Gutianshan National Nature Reserve in Southeast China. From these, they collected 829 nests with 1,929 brood cells belonging to 18 wasp species.

In 73 of the nests, they discovered an outer vestibular cell filled with dead ants. This nesting behavior has never been seen in the animal kingdom before, and after the larvae from these ant-lined nests matured (and chewed their way out), the team realized that these all belong to a single unknown species of spider wasp. So they named the new species after graveyard bone-houses, or ossuaries, where human remains are deposited and covered. It’s described in PLOS ONE this week. 

Above is an overview of the nest and its contents. Individual brood cells are separated by thin walls of soil material (A), and the nest is closed by a vestibular cell (B) filled with dead ants in otherwise great condition (C). 

The dead ants in the barricade belong to nine different species. Large and with a powerful sting, Pachycondyla astute was the most popular one, found in 42 percent of the nests. Because the ants (up to 13 in a single chamber) look fresh, the wasps likely hunt live ones rather than collect dead ants. 

The researchers also found lower parasitism rates in bone-house nests (3 percent), compared with similar cavity-nesting wasps (16.5 percent) -- suggesting that the corpse-plugged nests are less vulnerable to natural enemies, like flies and other wasps. Chemical cues emanating from the dead ants used as nest-building material likely play a role in defense, either by deterring others or by camouflaging odors. “If we’re able to synthesize them, we want to do behavioral studies,” Staab tells Slate. A male is pictured to the right. 

He adds in a news release: "Our discovery demonstrates in an impressive way, what fascinating strategies of offspring-protection have evolved in the animal kingdom.” 

Images: Bernhard Schurian (top, bottom) & Merten Ehmig, Michael Staab (middle) from Staab et al. PLOS ONE 2014

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