Dead-ant wall protects young spider wasps

When members of one family of wasps, Pompilidae, are adults, they feast on floral nectar. But they’re known as “spider wasps” because they get their start in life by growing on the body of a paralyzed spider.

Within that family, wasps in the genus Deuteragenia construct special nests for those spider-feeding youngsters, with several chambers separated by thin walls of plant material, resin or soil.

The final, outermost chamber is usually empty — except for nests constructed by one species recently discovered in southeast China. These wasps, nicknamed “bone-house wasps,” fill that last chamber with deceased ants, Michael Staab of the University of Freiburg in Germany and colleagues report July 2 in PLOS ONE. They are the only species known to use whole-body ants for constructing a nest.

Staab and colleagues collected 829 nests of cavity-nesting wasps as part of a survey in southeast China. The nests belonged to 18 different species of wasps, but 73 of the nests were slightly different — they had a final chamber filled with ants. And when the scientists reared the young from those nests, they realized they had found a species never before described. They named it Deuteragenia ossarium because the wall of dead ants reminded them of an ossuary, where humans are buried.

The ant walls contained an average of five and up to 13 ants. The wasps weren’t too picky when it came to the species of ants chosen for construction, though: The researchers found nine different ant species — and as many as four different species in one cell — among the 26 chambers they examined. The most common species was Pachycondyla astuta, an abundant, large ant with a powerful sting.

It’s likely that the wall of dead ants helps to deter potential predators, the researchers say. (Most of us would probably be deterred by a wall of dead things, after all.) But that’s not necessarily because of the sight of such a monstrosity. Instead, the volatile compounds emanating from the ants may provide some sort of chemical defense or camouflage for the nests.

The choice of P. astuta may be particularly wise because the wasps wouldn’t need many of the large ants to fill a chamber and because they are common. “Potential predators may have had contact with the species before and therefore avoid the species-specific scent,” Staab and colleagues write.

Considering both the dead ants and the spiders, that’s quite a body count for a species that feasts only on flowers when full grown.