In six further days they moulted a third time. Instead of dingy, hairy grubs, they were now hairless caterpillars, yellowish-white in colour, and with round black heads. Their fourth and last moult took place in another week. By this time they were over an inch in length. They now began to eat the lettuce leaves ravenously, and grew very fast. For ten days they ate much and grew rapidly. Then they seemed to get tired of feeding, and began to crawl lazily up the sides of their cage. [Illustration: Silk Moth.] 1. Eggs. 2. Larva. 3. Silk Cocoon. 4. Chrysalis. “They are full grown now, boys,” said Uncle George, “and will soon begin to spin their silken cocoons. “They are now about three inches long, and very different from the tiny hairy grubs which came out of the eggs over a month ago.” “We have been comparing drawings of them with drawings we made of the caterpillar of the white cabbage butterfly,” said Frank, “and we find that they are very much alike.” “That is good,” said Uncle George. “They are, indeed, very much alike. The body is made up of thirteen segments, and they have the same number of legs. They differ from the cabbage caterpillar, however, in size and colour. Notice the little horn which sticks up out of the twelfth segment. Your cabbage caterpillar did not have that. If you look closely you will notice a long silk thread issuing from the head of one of these silk-spinning caterpillars. This one has just begun to make his cocoon; and, in order to prevent his threads becoming entangled with those of the other caterpillars, I will place this small paper cone over him.” Next day Uncle George covered several of the caterpillars with paper cones. On the roof of the cage the boys saw one caterpillar surrounded with a flimsy web of silk of a golden-yellow colour. As they watched the creature turning its head round and round inside the silky veil, their uncle told them that the creature was spinning silk at the rate of about a foot per minute. In three days each caterpillar had surrounded itself with a dense silken cocoon as large as a walnut. Then Uncle George took one of the cocoons, and, cutting it open, showed the boys the brown, shining chrysalis inside. He took another cocoon, and, having dipped it into warm water, began to wind the silken thread on to a piece of card. Both Frank and Tom were amazed at the length of this silken thread. They were more surprised when they were told that each caterpillar produces a silk thread more than half a mile long. “Tell us, please, Uncle George,” said Frank, “why the creature spins all this silk, and how it makes it?” “You are asking a difficult question,” said his uncle, “but I will tell you something about it. [Illustration] S. Silk Fibres. W. Wool. C. Cotton. (All highly magnified.)