To begin with you will only be able to build bridges out of wood, but quickly the Throng will find a more efficient source of energy: Thronglet bones.

1.Wooden Bridge Construction: Thronglets will automatically build a bridge if they are near wood, which is obtained by cutting down trees with an axe. Sometimes they may have competing tasks and needs, so to get them to prioritise bridge-building make sure they are well-cared for, and try moving the wood nearer to a bridge area.

2. "Bug"-based bridge construction: Early on in the game the Throng will provide you with a ‘bug’ in order to harvest a new resource for bridge building. Simply use the bug on Thronglets to unlock their energy and build a bridge out of bones. Both alive and deceased Thronglets can be harvested.