新托福听写训练 第4期（在线收听）

As you all know, Dr.Nelson's assignment was for each of us to find a little scientific puzzle to research and solve. Well, mine was why don't spiders get stuck in their own webs--a pretty good question if you ask me. Someone even read a thesis on it I'm sure. There are actually several answers depending on the spider. Let's start with one common type of spider. These spiders usually stay in the middle of their webs and don't put any glue-like material there. But they do leave a thin strand of sticky material on the outer part of the web. This material is made of compounds that draw water molecules out of the air. When it draws enough water, it does what water normally does. It forms little drops. The result is sticky little globs, little balls of glue-like material. When the spider steps into one of the little glob by accident as it sometimes does, it is as human beings step on a piece of chewing gum. It is just inconvenience. But when a fly flies into the spider's web, it hits about 50 of the globs in order to make stick tightly to the web. What's more, because the sticky material is in the glob instead of in the uniform coat, the spider uses less of it. By doing this, the spider saves energy and that means it has to catch fewer flies. It is not of economy really. The spiders do what is easiest for spiders.