Karen Elsner, age l3, of Albany, N.Y., for her question:

Why do silkworms spin their silk?

The adult silkworm is a moth and like all the moths he lives through a cycle of four different stages. H£ begins life as an egg that hatches into a hungry larva. This greedy grub is a silkworm caterpillar, and this fussy fellow insists upon a daily diet of freshly picked mulberry leaves. As he eats he grows too big for his tightly fitted skin. A larger skin grows underneath it and he soon bursts his old skin and sheds it.

The growing grub molts several skins before he reaches his full size. Then he is ready to change to stage three of his life cycle and rest for a while as a sleeping pupa. This is when he Uses his spinnerets to spin a thread of fl.ne8t silk. As the thread emerges, he wriggles his body and weaves it around himself in figures of eight. It takes about a half mile of thread to weave his soft cocoon. The silken blanket is made to protect him and keep him warm while his body changes into an adult winged moth.