托福听力
Questions 39 through 41. Listen to part of a talk in a biology class. Before moving on to a new topic, I want to finish up our unit on a rag nit, by looking at what may seem a very unusual aspect of spider behavior--a species where the young spiders actually consume the body of their mother. Unlike most other spiders this species lays one and only one clutch of forty eggs in a life time. The young spiders hatch in mid-spring or early summer inside a nest of eucalyptus leaves. Their mother spends the warm summer months bringing home large insects, often ten times their weight for meals. The catch is always significantly more than her young spiders can eat. So the mother fattens herself up on this extra prey, and stores the nutrients in her extra unfertilized eggs. As the weather turns colder, there are fewer insect prey hunts. That’s when the nutrients stored in those extra eggs begin to seep into the mother’s blood stream. So when there are no more insects to feed to the young spiders, they attach themselves to the mother’s leg joints and draw nourishment by sucking the nutrient-rich blood. After several weeks, the mother is depleted of all nutrients and diets. But then how do the young get nourishment? They start to feed on one another Now if you recall our discussion of Darwin, you’ll see the evolutionary value of this. Only the strongest spiders of the clutch will survive this cannibalism. And the mother spider will ensure that her genes have an increased chance of survival through future generations.