The Annual Trek of the Christmas Island Crabs

For a few week a year, this tiny island in the Indian Ocean is literally crawling with crabs. Sometime between October and December, cued by seasonal rains and a waning moon, millions of the bright red critters emerge from their burrows in the rainforest floor and begin an epic trek to the coast (pictured above). Scurrying down cliff faces and creeping through towns, they can travel over four miles en masse: "Sometimes there are so many in one place, it looks like a sea of red," says island resident Linda Cash. "When the crabs cross the roads, locals are stopping traffic and scooping them out of harm's way." Once they reach the beach, the crabs mate and the females release their eggs into the surf, where the offspring will grow through the larval stages. One month later, the surviving babies—now just five millimeters long—will crawl onto shore and begin their journey inland, climbing 1,200 feet into the forest. —Catherine DiBenedetto

Subterranean Ant Cities

Just out of sight, beneath the surface of the ground in humid climes of North and South America, leaf-cutter ants are busy building megalopolises. In the construction of a single nest—a massive complex of corridors and chambers, garbage pits and fungal gardens—millions of ants may carry as many as 44 tons of soil aboveground, each load four times the weight of the insects themselves. The mature city can measure more than 700 square feet. Scientists refer to colonies like this as superorganisms because each worker is like a cell programmed to perform a specific task in an elegantly complicated system only Mother Nature could design. —C.D.

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