I have no idea of the life cycle of butterflies were as scientifically known then as they are today, (at some point in history people thought that swallows hibernated at the bottom of lakes and ponds instead of migrating to Africa as we now know they do) so if Cadfael's knowledge is far too advanced I apologise! Also the butterfly I described is the peacock and the caterpillars are for the peacock, comma and small tortoiseshell.

Also, as it would seem that people don't know this and aren't passing it onto their children – don't touch hairy caterpillars! They can cause nasty rashes and even allergic reactions! Oak Precessionary Moth caterpillars (bad news – they can cause really bad reactions) have invaded my village this year and I was shocked to discover that people didn't know what I had been taught from a very young age. Yes, the UK is relatively safe from a wildlife point of view but there are still things that can cause you to end up in hospital!

"Giles, are you ready to go?" Hugh peered round the open door into the shadows of the herbarium in the grounds of Shrewsbury Abbey.

"Not yet," said his young son, eager gaze fixed onto the bunch of nettles in the corner of the wooden hut. "This one's hatching!" Giles bounced on the spot, using the counter in front of him to push off with his arms, attention unwavering from the plants. His movement caused the jars further along the bench to shake and clink together.

"Calm down, Giles," said Hugh, crossing to him and placing a soft hand on his son's head. Once he was still Hugh crouched down to Giles' level and smiled. "Which one is hatching?"

A small hand pointed past the stems of the nettles to the wooden battens beyond that made up the wall of the hut. There, underneath one of the battens was a small, yellow pupa, no longer than from tip to the first knuckle of his index finger. And it was shaking, a dark slit running down one side, a mass inside bulging out of the opening in the hard case.

Giles looked at his father for the first time, eyes wide in wonder and mouth open in a tooth showing grin.

"Can we stay?" he asked. "Pleease," he whined, "just until this one's hatched." Hugh glanced at the late summer sunlight streaming through the open doorway then back at Giles' pleading eyes. He could not resist those eyes.

"As long as there is light remaining to get home we will. And remember what Brother Cadfael said, they are not hatching, they are emerging. And what do they emerge from?"

"Pupa!" came the enthusiastic reply.

It was warm in the herbarium, but not as hot as it had been at midday so the pair were comfortable as the bulging mass inside the pupa fought its way out. It didn't matter to Hugh that his knees were starting to ache as he knelt on the dirt floor next to his son, chin resting on folded arms on the bench. He was relishing the rare moment he got to spend with his son. Usually he was too tired for much play once he got home, being deputy sheriff kept him busy, sometimes keeping him away from home until after Giles' bedtime. This quiet moment he would keep in his heart to warm him in the freezing winter months to come.

The mass escaped from its case and crawled up to the top of the wooden batten. Now it looked like the butterfly it was, though its wings were crumpled and dark with damp. It took awhile for the butterfly's wings to unfurl, but the pair didn't get bored, watching the variety of caterpillars munch their way through the nettle leaves that Cadfael had harvested for them that morning. There were black caterpillars, mostly staying in a group on one of the stalks surrounded by silvery webs; smaller green caterpillars with black heads, also sheltering amongst a web; and black and white caterpillars with orange spikes dotted around the plants on their own. Hugh was glad that his son was acting on Cadfael's advice, all the caterpillars were covered in fine hairs which, according to the monk, were not to be touched or he would end up in the infirmary covered in itchy and painful rashes. Normally he would have his hands on everything but this time he was keeping a respectful distance.

Suddenly the butterfly moved, flicking open brightly coloured wings: a deep, vivid red with blue and white eyes at the bottom of its wings. Then the colour disappeared, then appeared again. Giles was mesmerised at this display.

With one last flap of its wings the butterfly shot up into the air. It flew above their heads, circled the room and settled on the cork of a bottle on one of the many shelves around the room. Giles chased after it. He tried to reach up and touch it but it was too high and even jumping didn't get him any closer to it.

"Nooo," whined Giles at the butterfly, "you need to go outside. You can't stay in here." He jumped again, waving his hands. The butterfly didn't budge. "Papa," he called to Hugh. "How do we get it out? It should be free now."

Hugh slowly climbed to his feet, knees aching from the long time on the floor. He moved Giles away from the butterfly and gently caught it in cupped hands. Fragile wings fluttered frantically between his hands.

He ushered his son outside and the pair stood in the golden sunlight

"Hold your hands out."

Hugh covered Giles's hands with his own, the butterfly trapped between them. Once the fluttering under his palms had stilled he withdrew them. The butterfly sat neatly on Giles' hands, wings flicking open and closed, flashing the red and the dark eyes.

"Wow," breathed Giles. He stood stock still, eyes never wavering as they hadn't as he watched this butterfly emerge from its chrysalis.

With a last fluttering of wings the butterfly suddenly swept off into the sky, into the evening sunset. Giles's eyes never left it.

"Bye bye butterfly," he whispered.