Part 19

“Yes, I know. But you have not reported any changes in the creatures themselves. What has been going on in the larva cage has also been going on here in the water, for caddis worms are simply water-caterpillars. You nearly missed something of very great interest.”

Uncle George laid three saucers on the table, and continued:—

“We are going to look into the life of the caddis fly to-day; but before I take them out of the water, I want you to tell me what you have noted about them up to now.”

“They are always climbing up the water weeds,” said Frank.

“They are always adding to their cases,” said Tom.

“Some of them have died,” said Frank.

“What?” said his uncle. “Do you mean these at the bottom of the tank? These are not dead, they are only asleep. Put your hand in, and take some of these out.”

“Their cases are stuck to the pond weeds and to each other,” said Frank, as he lifted a few out, and placed them in a saucer.

“Ah, I have it, Uncle George! They have gone into the pupa state. Is that not so?”

Uncle George nodded.

“Look!” exclaimed Tom. “There are things like earwigs floating on the water.”

“Never mind these just now, Tom,” said his uncle. “I am coming to them by and by. Observe the wonderful cases which the caddis worms have made for themselves.

“Here is one whose case, when we found him, was made of neatly cut pieces of water rush. He has almost doubled the length of his case since then; for see, the front half is made of cut stalks of water weed neatly arranged.”

“He has got a fine collar of green pieces round his neck,” Frank observed.

“Why are they always adding to the length of their cases?” asked Tom.

“Because they are always growing,” said Frank.

“That is the reason,” said Uncle George; “and they grow so fast that they have actually to work hard at building.

“Observe those that make their cases out of tiny shells and stones. They have made free use of the coloured beads and small pieces of coal which we put in.”

“How do they manage to stick these things together?” asked Tom.

“A caddis worm is a busy creature,” his uncle replied. “He does two things besides eating and growing. With those strong pincers, which you see at his head, he saws off pieces of weed.

“Near his mouth he has got a kind of loom for spinning silk. These pieces are stuck together with silk, which is a gluey substance when it first comes from the creature’s body.

“These rough cases are lined with silk.”

“Why does he have to make a house for himself, and carry it about with him?” said Frank.

In answer to this, his uncle took out a few of the active caddis worms, and placed them in a saucer with water. He held one up.

“You see,” he said, “his case is open at both ends. Now, if I tried to get him out from the front, I should never manage it. This is the way to get him out.”

As he spoke, Uncle George pushed the head of a pin into the tail end of the case, and the creature scrambled out at once.

“It is too bad turning you out of your cosy room, Mr Caddis,” Uncle George observed; “but you’ll go back again as soon as you get the chance, won’t you? I want my nephews to understand why you work so very hard.

“Now, Frank, you can answer your own question, I think—‘Why does the caddis worm build a house?’”