Nancy Noble, age 10, of Charleston, West Virginia, for her question:
Why is it so hard to swat a fly?
Even the most careful aim with a swatter often fails to whack the fly. You may swat at ten of the bitsy pests and miss them all. But do not feel bitter or blame your aim. A fly is born with built in gadgets that just about guarantee his escape from fly swatters.
A fly may be about a quarter of an inch long and, in flight, his gauzy wings measure no more than half an inch from tip to tip. It takes about a thousand flies just like him to weigh one ounce. But many a great white hunter can bring down a charging rhinoceros and fail to swat a fly. Most likely he used the wrong kind of swatter and aimed too far ahead. The best weapon for the job has a long, liable handle and a swatting end that covers a long, wide area of several square inches. The best place to aim is half an inch behind the spot where the fly happens to be sitting. And the whack must be fast, very fast.
Mr. Fly takes off in a split second and you might think that he has eyes in the back of his head to warn him that a swatter is on the way. If you look at him under a magnifying glass, you may discover that this is almost true. Those two round bumps on his forehead are eyes. They are special compound eyes made from dozens of boxy little reflectors. They are placed to show a large area of the scenery, above and below and around the sides.
In addition to his big round compound eyes, the fly has three small simple eyes. They are not exactly in the back of his head. But they are on the top. The three of them form the corners of a triangle between and above the big compound eyes. The fly's vision is not sharp or clear. But with five eyes in his head he does not miss much of what goes on around him. He is sure to notice any suspicious movement of light and shadow in time to make a getaway.
You have no doubt watched him sitting there, quietly preening himself or carefully combing his hairy legs. The next moment he is airborne and flying across the room. He has, of course, six legs for the take off and he uses them to spring up as he opens his wings. However, a fast camera reveals a weakness in his speedy operation. The fly's legs are made so that the take off jump must go in a backward direction. When you aim the swatter directly at him, he has time to escape and you hit the place where he was. But if you aim about a half an inch behind the spot where he is, you have a chance to whack him during take off as he makes that jump backward. There are, however, several backward directions and you cannot be sure which one he will take. This is why the swatting end of your weapon should be big enough to cover a sizeable area.
Flies are pests that carry germs of all sorts on their sticky feet. We should do all we can to keep them out of the house and also prevent them from breeding in trash cans and garbage dumps. They can multiply at a staggering rate. They breed from April to September and during this season, one pair of flies can produce more than five billion offspring. Most of this horde, however, becomes food for birds and other animals. But there are enough survivors to endanger human health. When you swat a fly successfully, you prevent the birth of billions of future flies.