There’s a new bridge in the Netherlands designed for pedestrians. And bats.

The bat bridge, as it’s colloquially called, arcs over the Vlotwatering river in the Dutch town of Monster. (Yes, Halloween is this weekend. No, we can’t make this stuff up.) Next Architects, which runs its studio out of Amsterdam, came up with the design when the local town of Westland held a competition seeking ecologically sound infrastructure ideas for its Poelzone waterfront. Next proposed a 160-foot-long bridge that accommodates pedestrians and cyclists while providing a much-needed place for bats to roost.

Seen from above or at ground level, the Vlotwatering bridge is a curvy, tasteful structure fashioned from ruddy bricks and planks of wood. Zoom in a bit, and you can see where it’s pulling double duty: slats are spaced exactly wide enough for bats to get in. Within the bridge, the nocturnal creatures find three types of habitat: an enclosed area for cold winter months, crevices along the sides for roosting in springtime, and room to hang beneath the bridge during the summer.

Next Architects

The enclosed spaces are made of thick slabs of concrete. Bart Reuser, the partner in charge of the project at Next, says the idea came from the World War II-era bunkers that dot the coastline. Their monolithic concrete walls provide excellent insulation against the cold, and bats have taken to roosting in them. Reuser likens them to the thick walls of a historic church. “In the summertime the church is cool, and in the winter time it’s warm—not because it’s heated, but because of its massive walls,” he says.

Bats tend to roost in attics, where heat collects as it rises from lower rooms. But as the government tightens energy efficiency requirements, that’s less and less the case, leaving bats with fewer places to roost.

To ensure that the bridge would provide suitable refuge for the winged mammals of Monster, Reuser and his team consulted with Herman Limpens, a bat expert at the Dutch Mammal Society. His input informed design decisions like the spacing between the wooden planks—wide enough let bats in, but narrow enough to keep predators out—and the thickness of the concrete, which averages 29.5 inches thick.

It this strikes you as a lot of time and money for some creepy nocturnal creatures, consider this: Bats eat enormous quantities of insects, some of which can be harmful to crops, animals, and humans. Without a safe place to nest, bats can’t survive or populate. Displace too many and you risk tilting things in the bugs' favor. Making room for bats, then, is more than inter-species politesse—it's a great way of keeping insects under control.

This is why cities like Monster incorporate roosting spaces into their bridges. The Congress Avenue bridge in Austin, Texas, is another well-known example of this kind of eco-infrastructure. It is home to some 1.5 million Mexican free-tailed bats, and Bat Conservation International estimates they eat 10,000- to 20,000-pounds of insects a night. That's right: The payoff of a bat-friendly bridge can be measured in bug-tonnage.