Kathryn Hanley, age 9, of San Francisco, California, for her question:

Are flies needed in the balance of nature?

Naturally we detest the pesky flies that ruin our summer days and evenings outdoors. They infest our trash cans and we have to screen doors and windows to keep them out of our homes. But suppose we wiped out all the zillions of flies in the whole world. The frogs who eat fly meat would starve, so would many toads, lizards and other small creatures. We also would lose the pretty kinglets and many songbirds that feed on flies. And this is only part of the sad story.

Balancing is a tricky thing, even in small items. It is hard to balance on one foot and harder to balance two weights on a see saw. But the balance of nature is the trickiest thing in the whole world. It is so complicated that nobody really understands how the billions of things all fit together. It seems that large and small living things need each other to keep going. Certainly the busy flies are needed in this complicated balance of nature.

Nothing much is upset when we swat one pesky fly. But there are hundreds of different flies and zillions of each type. Altogether they weigh millions of tons. This is not counting the fly eggs and young fly maggots. Most flies never live long enough to buzz around in the air. They become food to feed countless animals. As the young fly maggots grow, they help to decay the garbage, so that tiny mites and bacteria can change it into rich soil to feed the plants.

If we wiped out all the flies, many frogs and toads, bats and birds would go hungry. Many snakes, raccoons and birds would starve because they feed on small animals that eat flies. Some fish would go hungry and so would many larger animals that feed on fish. In fact, if there were no flies in the world, most of the wild animals would suffer a food shortage and many would starve to death.

And this is not the worst of it. Those grubby maggots that breed in our trash are needed to renew the soil with chemical plant foods. We need the green plant world to provide our fresh breathable oxygen. We also need its vegetables, fruits and salads. The plant world also feeds the farm animals that give us meat and dairy products. Every year, zillions of flies are used and reused, cycled and recycled throughout the world. They are needed to keep the balance of nature going.

True    the pesky critters even annoy the cows. But a cow just swishes her tail to brush them away. She does not try to wipe all of them out. If she did, the soil might be poorer and the grasses thinner. Bossy the cow would be hungry  ¬and a hungry cow has less milk and ice cream to give us. So, let's just swat the few flies that annoy us    and leave the rest of them to perform their duties in the complicated, world wide balance of nature.