Image copyright Eric Carle Image caption Its 50 years since The Very Hungry Caterpillar was first published

"In the light of the moon a little egg lay on a leaf."

It's probably one of the most memorable opening lines in children's literature, and this month The Very Hungry Caterpillar is celebrating its 50th anniversary.

The best-selling book which tells the story of a ravenous caterpillar eating its way through the week was first published in June 1969 and has since been translated into 62 languages.

Eric Carle, the New York born creator of the simple but everlasting tale, began writing children's books in his late 30s.

From his home in the Florida Keys, Carle told the BBC why he thinks the story has endured for five decades.

"For many years, my publisher and editor and I did not know the reason for The Very Hungry Caterpillar being so popular.

"But over time, I've come to feel that it is a book of hope. And it is this hopeful feeling that has made it a book readers of all ages enjoy and remember. For this I am very touched.

"I have been sent many letters and drawings from children. Some letters are profound.

"My all-time favourite was, 'Our teacher, Ms Smith, made us read all your books. Will you ever retire?'"

Image copyright Jim Gipe Image caption "It is a book of hope" says Eric Carle the creator of the The Very Hungry Caterpillar

Born in upstate New York to German immigrant parents, he moved as a child with his family to Stuttgart in the mid-1930s because his mother was homesick, only for his father to end up drafted into the army during World War Two and get taken prisoner by the Russians.

It was experiencing the trauma of war that gave Carle a unique way of seeing the world through the eyes of a child.

He eventually moved back to New York and worked as a graphic designer and then at an advertising agency, before collaborating on his first picture book 'Brown Bear, Brown Bear, What do you see?'

The first version of The Very Hungry Caterpillar was called A Week With Willi Worm, inspired after Carle used a hole punch to create circles into a stack of paper.

It gave him the idea of a bookworm but Carle says his editor wasn't keen on a worm so various other animals were discussed. Finally they settled on a caterpillar and butterfly.

The finished book is just 224 words long.

Image copyright Eric Carle

No US printers would publish the book because of the differently shaped pages with the holes, but eventually a printing company in Japan was found. Penguin books say more than 50 million copies have since been sold worldwide with one book bought every 15 seconds.

Dr Michelle Martin, from the University of Washington's Information School, who specialises in children's literature, told the BBC she thinks the book still resonates because there are "so many different layers to it".

"It's simple in the way it talks about days of the week, different foods, and different states of being. It's also visually stunning.

"Children take to his books because of the collages and the colours. It is a book with information, but it not only educates, it entertains.

"It also teaches math concepts, he ate through 3 oranges, but he was still hungry."

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It's not just the book that remains popular, there are animated versions of the story, various merchandise spin-offs and a stage production that to date has been watched by a million children.

There is also The Eric Carle Museum of Picture Book Art which was opened in 2002 in Amherst, Massachusetts, and in April a species of spider was named after Eric Carle because of its resemblance to his famous caterpillar.

Image caption The Very Hungry Caterpillar show has been seen by a million children across the world

"Describing the book's timeless qualities, Lara Hancock, the publisher for Puffin Picture Books, says: "It's a story of transformation and hope and is as relevant now as it was 50 years ago.

"Eric's artwork is so inimitably itself that it defies aging. Everything else in the world changes, but children's curiosity about the world around them and interacting with it does not.

"The book's split pages with the die-cut holes provide an immersive experience that remains as enticing and immersive as it was to its first readers in 1969."

Fans of the hungry caterpillar from across the globe have been reacting on social media to the greedy insect's 50th birthday celebration. On Facebook mum Vivien Tan in Singapore posted a picture of a specially made quilt she made:

"Happy 50th birthday Hungry Caterpillar! Eric Carle, your book is so inspirational and has accompanied both my kids through their childhood."

Another commentator posted: "I can't imagine my life as a teacher or mother of 2 without this beautiful book. Thank you for your amazing ability to connect with others through your art and soul. You are a treasure."

A Twitter user said he marked the occasion by eating the same foods the hungry caterpillar enjoyed.

"To celebrate, I ate one piece of chocolate cake, one ice-cream cone, one pickle, one slice of Swiss cheese, one slice of salami, one lollipop, one piece of cherry pie, one sausage, one cupcake, and one slice of watermelon," he posted.