听力原文： Do you understand what insectivorous plants are? Insectivorous plants are plants that trap and assimilate insects in order to sustain life. All insectivorous plants have roots, so they don't get all of their nutrients from the insects they take in. Insectivorous plants tend to live in soil that lacks enough nitrogen for them to exist, so they consume their insect victims in order to get the nitrogen they need. There're many types of insectivorous plants in the world — perhaps 500 known species of insectivorous plants worldwide. Of these types of insectivorous plants, we're going to look at one of the better known ones in depth. The insectivorous plant that we're going to discuss in detail is the Venus flytrap, which is native to North America. It catches insects by suddenly snapping the ends of one of its trap leaves around an insect. A Venus flytrap has a number of trap leaves and that each trap leaf is divided into two parts. Inside the two parts of the trap leaf are three sensory bristles, which resemble tiny hairs. When an insect touches the bristles inside a trap leaf, the two surfaces of the leaf shut instantaneously, and the insect is trapped inside the parts of the leaf. The Venus flytrap then discharges a digestive liquid into the leaf in order to assimilate the insect and obtain the nitrogen the plant needs in order to survive. What is the topic of this passage?