After a while, Yagharek nodded slowly.

“It will soon be able to fly,” he said quietly.

“Not necessarily, old son. Not everything with a chrysalis gets wings.”

“You do not know what it will be?”

“That, Yag, is the only reason I’ve still got the damn thing. Wretched curiosity. Won’t let me go.” Isaac smiled. The truth was he felt a certain nervousness, seeing the bizarre thing finally perform the action he had been waiting for since he had first seen it. He watched it cover itself in a strange, fastidious inversion of cleanliness. It was quick. The bright, mottled colours of its pelt went misty with the first layer of fibres, then quickly disappeared from view.

Yagharek’s interest in the creature was short-lived. He replaced the wooden framework which hid his deformity onto his shoulders, and covered it with his cloak.

“I will take my leave, Grimnebulin,” he said. Isaac looked up from where the caterpillar held his attention.

“Right! Righto, Yag. I’ll get a move on with the…uh…engine. I know by now not to ask when I’ll see you, right? You’ll drop in when the time’s right.” He shook his head.

Yagharek was already at the bottom of the stairs. He turned once, briefly, and saluted Isaac, and then he left.

Isaac waved back. He was lost in thought, his hand remaining in the air for several seconds after Yagharek had gone. Eventually, he closed it with a soft clap and turned back to the caterpillar’s cage.

Its coat of wet threads was drying fast. The tail end was already stiff and immobile. It constrained the grub’s undulations, forcing it to perform more and more claustrophobic acrobatics in its attempt to cover itself. Isaac pulled his chair over in front of the cage to watch its efforts. He took notes.

A part of him told him that he was being intellectually dissolute, that he should compose himself and focus on the matter in hand. But it was a small part, and it whispered to him without confidence. Almost dutifully. There was, after all, nothing that was going to stop Isaac from taking the opportunity to watch this extraordinary phenomenon. He settled into his chair comfortably, pulled over a magnifying lens.

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…”

“Yeah, well, that’s partly why I’m showing you. Just to say keep your eyes out for it opening. You can help me pin it inside a case.” The two men grinned.

From below came a series of bangs, like water fighting its way through obstreperous plumbing. There was a faint hiss of pistons. Isaac and David stared at each other, nonplussed for a moment.

“Sounds like the cleaner’s gearing up to some serious action,” said David.

In the short, stubby byways of copper and brass that were the construct’s brain, a welter of new data and instructions clattered violently. Transmitted by pistons and screws and innumerable valves, the grots and gobs of intelligence bottlenecked in the limited space. Infinitesimal jolts of energy burst through tiny, finely engineered steamhammers. In the centre of the brain was a box crammed with rank upon rank of minuscule on-off switches that puttered up and down at great and increasing speed. Each switch was a steam-powered synapse, pushing buttons and pulling levers in intensely complicated combinations.

The construct jerked.

Deep in the construct’s intelligence engine circulated the peculiar solipsistic loop of data that constituted the virus, born where a minute flywheel had skittered momentarily. As the steam coursed through the brainpan with increasing speed and power, the virus’s useless set of queries went round and round in an autistic circuit, opening and shutting the same valves, switching the same switches in the same order.

But this time the virus was nurtured. Fed. The programmes that the repairman had loaded into the construct’s analytical engine sent extraordinary instructions coursing throughout the crafted tubework cerebellum. The valves flapped and the switches buzzed in staccato tremors, all seemingly too fast to be anything but random motion. And yet in those abrupt sequences of numerical code, the rude little virus was mutated and evolved.

Encoded information welled up within those limited hissing neurones, fed into the recursive idiocy of the virus and spun out from it skeins of new data. The virus flowered. The moronic motor of its basic, mute circuit sped up, flung blossoms of newborn viral code spiralling away from it with a kind of binary centrifugal force, into every part of the processor.

Each of these subsidiary viral circuits repeated the process until instructions and data and self-generated programmes were flooding every pathway of that limited calculating engine.

The construct stood in the corner, shaking and whirring very slightly.

In what had been an insignificant corner of its valved mind, the original virus, the original combination of rogue data and meaningless reference that had affected the construct’s ability to sweep floors, still revolved. It was the same, but transformed. No longer a destructive end, it had become a means, a generator, a motive power.