Keuie Cox, Age 9, Of Tucson, Ariz., for her question:

Why do silkworms eat only certain leaves?

It is fun to keep a few silkworms and watch them develop. For a while, the hungry little caterpillars munch away day and night. You would think that such famished fellows would eat every green leaf they can find. But not at all silkworms are very fussy about their food, and if it is not just right they starve. We eat a varied diet of meat and vegetables, because this is the kind of food our bodies can digest. We could not get enough nourishment from a diet of green leaves. A little silkworm gets all the nourishment he needs frost green leaves, and this is the only kind of food he can eat. Mary other caterpillars feed on green leaves, but they are not as fussy as the true silkworm caterpillar.

The caterpillar of the white cabbage butterfly prefers to dine on cabbage leaves. But he lives in the wild, and his favorite food may be scarce. Then he settles for a second choice. He will eat the leaves of almost any mustard type plant.

The caterpillar of the lovely luna moth is a chubby green fellow. He likes the leaves of the sweet gum tree. But he will also feed on walnut, hickory or persimmon leaves. About 100 years ago, a big brown silk moth was brought from china, and its children still live in a few eastern states. They live in the wilds and prefer to dine on ailanthus leaves. But when their first choice of food is scarce, they sett1e for a second choice. The true silkworm is also a native of china. And its wild ancestors may have eaten different kinds of mulberry leaves  though we are not surf.

We do know that the true silkworm has been pampered for at least 5000 years. He has come to depend upon a lot of tender care. He has becomee so tame and help1ess that it is not likely that he could survive in the wild. Generations of silkworms have been given their favorite food, and they never had to take second choice. Perhaps all this pampering has spoiled the silkworm. For he can eat only the leaves of a certain mulberry tree  and his food must be served just right or he refuses to eat it. The leaves must be freshly picked each day, dried and slightly wilted. The fussy fellow dies of starvation if he is not given exactly the right kind of food.

The true silkworm is the caterpillar of a smallish, brown moth. As he eats, he grows and four times sheds his skin for a bigger one. After 30 days, he spins a long silken thread winding it around and around himself. He plans to sleep inside his cocoon for two weeks while he changes into a winged moth. But the silk is taken before he can nibble his way out. When it is unwound, the silken thread is about half a mile long.