Vicki Lewis, age 11, of Louisville, Kentucky, for her question:

What are apterous insects?

Naturally we try to keep bugs out of the basement. But some types seem determined to share our homes. Most of them stay hidden in quiet crevices. When they hear footsteps or see a light, they scuttle out of sight. These household pests often include scaly little silverfish and frisky little bristletails. Both these types are classified as apterous insects.

Apterous means "without wings" and the sub class Apterygota belongs to a rather strange group of wingless insects. Scientists suspect that they resemble ancestral insects that appeared about 300 million years ago. They are said to be primitive be¬cause their relatives developed fancy wings and all sorts of other advanced features. The apterous insects did not do this. What's more, they even retained features that went out of style in the rest of the insect world more than a million years ago.

So far, about 15,000 species have been identified. They are smallish creatures that wear drab or pasty colors to match the secret places where they live. Various species are found all over the world and some can live where more advanced insects would perish. One of them claims to be the Antarctic ice field's only native insect. Others live in the Arctic and on mountain glaciers. Some live on the surface of quiet ponds and tidal pools. All of them like moisture and shun the daylight. Various types swarm in the world's soils, especially in the tropics.

Apterous insects come in very different types that do not seem to be closely related. Scientists subdivide them into groups, but they do not agree where all of them belong. The ones we are most likely to notice are the silverfishes that sneak into our homes. They are whiskery little creatures about an inch long. During the day, they hide in dark crevices. At night they hunt around for food but when you switch on a light they scuttle back into hiding.

Sometimes our homes are invaded by bristletails. You can recognize one of these because he travels in jerky leaps. His remarkable tail is jointed so that he can bend it down and let it go like a spring. Several bristletail species prefer to live on the water. Hundreds of them crowd together to form a little raft about an inch wide. It is a lively little raft because this one and that one keeps springing into the air and dropping back.

Some scientists classify the silverfish and bristletails in the same order and some give them separate orders. Two other orders include a vast variety of apterous insects that live in the soil. They are tiny midgets and some are eyeless as well as wingless.

Apterous insects hatch from eggs and grow by molting. But, unlike other insects, they keep on molting after they are fully grown. Many species have whiskery tails and antennas and most of them have skinny legs for fast scuttling. Unlike other insects, they have little stubs which may be signs that their ancestors had extra legs.