听力原文： Our topic today is about fleas that plague all of us who have cats and dogs in our homes. You may be surprised to find out that there are about 1,500 species of fleas, and that each species has its own preferred host. Fleas are, in fact, quite adept at finding their hosts. In an experiment, 270 fleas from rabbits were marked and then let loose in a large meadow. After a few days, a full half of the fleas had found rabbits to live on. Fleas also have a very interesting flexible diet. They can go for many months without eating if they have to, and they will change hosts in order to get more food, even if they don't like the taste of their new host as much as the old! Their jumping ability is incredible, too. You may know this from your own experience. Some fleas can jump as high as 13 inches, which is quite a lot for such a small creature. This jumping ability comes from their relatively large muscular legs that are equipped with resilin, a kind of natural rubber. The elastic properties of this resilin have not yet been equaled in any artificial substance. Though the fleas on your cat or dog are not dangerous to you, some fleas carry diseases. It is the rat flea that is responsible for carrying some fatal plagues. These fleas have been in part responsible for the Great Plague of Europe in 500 A.D. that spread from Ethiopia to Byzantium to Europe. They are also part of the cause of Black Death in China and Europe. (30)