It took a little over two hours for the caterpillar to cover itself completely in a moist chrysalis. The most complicated manoeuvre was at the head itself. The grub had to spit itself a kind of collar, then allow it to dry a little before bunching itself up within its swaddling, making itself shorter and fatter for a few moments while it wove a lid, closing itself in. It pushed against it slowly, ensuring its strength, then exuded more of the cement-filaments until its head was completely covered, invisible.

For a few minutes the organic shroud quivered, expanding and contracting in response to the movements within. The white covering became brittle as he watched, changed colour to a drab nacre. It pendulumed very gently as minute air currents disturbed it, but its substance had hardened, and the motion of the grub within could no longer be discerned.

Isaac sat back and scrawled on the paper. Yagharek was almost certainly right about the thing having wings, he thought. The gently moving organic sac was like a textbook drawing of a butterfly or moth chrysalis, only vastly bigger.

Outside the light became thicker as the shadows lengthened.

The suspended cocoon had been motionless for more than half an hour when the door opened, startling Isaac to his feet.

“Anyone up there?” yelled David.

Isaac leaned over the railings and greeted him.

“Some chap came and dealt with the construct, David. Said you just had to stoke it up a bit and switch it on, said it should work.”

“Good stuff. I’m sick of the rubbish. We get all yours, as well. Would that be deliberate?” David grinned.

“Why no,” replied Isaac, ostentatiously shovelling dust and crumbs through the gaps in the railings with his foot. David laughed and wandered out of his sight. Isaac heard a metallic thud as David gave the construct an affectionate clout.

“I am also to tell you that your cleaner is a ‘lovely old thing,’ “ said Isaac formally. They both laughed. Isaac came and sat halfway down the stairs. He saw David shovelling some pellets of concentrated coke into the construct’s little boiler, an efficient triple-exchange model. David slammed shut and bolted the hatch. He reached up to the top of the construct’s head and pulled the little lever into an on position.

There was a hiss and a little whine as steam was pushed through thin pipes, slowly powering up the construct’s analytical engine. The cleaner jerked spastically and settled back against the wall.

“That should warm up in a little while,” said David with satisfaction, shoving his hands in his pockets. “What have you been up to, ’Zaac?”

“Come up here,” answered Isaac. “I want to show you something.”

When David saw the suspended cocoon he laughed briefly, and put his hands on his hips.

“Jabber!” he said. “It’s enormous! When that thing hatches I’m running for cover . . .”