They draw the edges of their new world slowly, by consensus, spreading in a thousand different directions until one of them leads somewhere useful. The return journey is the message; food here, come and take a look. If there's no return journey then that's a message too: enemies, emptiness, here be dragons.
The rain has changed the contours that they mapped last night, but they wouldn't have remembered them anyway. Each moonlit landscape is new, the stones and scraps and rain or heat appearing like the beginning of another world. They move over hills, around oceans, sometimes; through brush, briar, slowly. Each explorer has seventy nights before she dies, and sometimes a whole night passes in futile search. The individual failures don't matter, as long as somebody gets somewhere, and returns to call the others on.
Sometimes branches get in the way, or stray boots, or a blown plastic bag. Sometimes there are deserts and cliff-edges and buildings, too huge to be a hindrance: their size makes them irrelevant, like the curve of the earth, just another surface to brush with a thousand explorers who spread from the centre, the legs and antennae that carry and guide each one.
The last drops of water have fallen from the trees, but the ground is still thick and muddy. Lucky, then, that the first explorers are only a few stones and diversionary twigs from the nest before they find themselves mounting huge apples, already half dismantled by their daytime rivals. Soon the scattered paths converge, not on the most provably efficient route but on something that works, well enough; something that the ants can follow and return from, confirming its efficacy, as longer and less successful journeys fade away in the wake of unreturning pioneers.
Even the more distant explorers should return eventually, but some of them get trapped, or killed, or just keep on going. One of them follows the canyon between two concrete slabs, then runs up against a vast body of water on the other side. She circles it, still heading outward while her sisters carry their apple back to the nest. Something's fallen apart inside her, the indication that it's time to head back, that out here it's too far from home to be worth the journey even if she does stumble across a serendipitous island.
She reaches out further, and further, along twigs, ignoring the faint trails of previous explorers. Where do they lead, these paths that stretch out in the wrong direction? Not home, certainly. They trace the borders of new seas and stranger islands, exploratory and aimless, by puddles and gravestones and trees and enormous flowers. She stumbles across some crumbs but doesn't turn around, moving brokenly onward, and she draws out purposeless lines that will define the very furthest edges of the world, at least until daylight.