In a chalet on the Alps which had no obvious draughts, I noticed spider threads spanning horizontally from wall to wall. How did they do it?
• Air currents carry a silk line from a spider's spinneret until it reaches a solid object. Because of its sticky nature, the line attaches to whatever it encounters. Draughts or convection currents sufficient to carry the line would exist even in a room that was hermetically sealed. For example, windows allow sunlight to penetrate, heating a patch of floor. Air in contact with the floor would warm and expand. Being less dense, it would rise, to be replaced by cooler, denser air, initiating the required air movement.
The spider scurries along the first line - called a bridge line - spinning a stronger second thread. It continues making return journeys until the line is sufficiently strong. The rest of the web follows.