Complex sentence: The vertical strands act as trip wires to initially alert the spider to the presence of prey or threats, and also snare and haul prey into the air, when weaker horizontal strands that hold them down, known as guy lines, break when prey thrash around.
Split into simpler sentences: The vertical strands act as trip wires to initially alert the spider to the presence of prey or threats. They also snare and haul prey into the air when weaker horizontal strands that hold them down, known as guy lines, break when prey thrash around.