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Fly Metamorphosis is a Beautiful Nightmare

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Like the beloved butterfly, a house fly goes through an incredible metamorphosis. To make its grand entry into the world, it deploys a specialized, fluid-filled balloon on its head called the ptilinum (till-EYE-num) to break open its pupal casing—and free itself to buzz around your kitchen.


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Coming of age is awkward, messy, challenging. Even for a house fly.

See this pulsing sac on the fly’s head?

It’s a specialized organ that the fly uses just once … to break free from its pupal casing.


Whew. What a headache!

It all started a few weeks back, when mom and dad had a brief but spirited courtship.

Which resulted in these delicate, oblong eggs. And from the eggs: maggots.


The little squirmers dig right in wherever mom laid them!

After chowing down for a few days, this chonker is now five times its original size.

What it wants now is privacy.

Over the next four days or so, the maggot’s ivory exoskeleton hardens …

“Ughhh, I’m going to my room.”

… and darkens to a deep mahogany.

The fly is now a pupa.

See these holes? This is how it gets oxygen … and that air just happens to flow through its butt.

While things may look quiet on the outside, there’s a lot happening beneath the surface.

This is a blowfly – a house fly’s cousin.

Scientists at the Natural History Museum in London X-rayed the pupa to reveal the transformation behind its opaque shell.

See them go from chubby tube to a complex creature with legs and wings and big eyes?

The once blobby maggot has totally transformed.

But the cozy casing that protected it now holds it captive.

That’s why this inflatable organ, called the ptilinum, is essential.

The fly pumps it full of clear-colored insect blood called hemolymph – and pops the lid off its casing.

Once the head’s out, the struggle continues.

Next come the front legs, abdomen and back legs.

The process of freeing itself is called eclosion.

But not everyone makes it: Some get stuck right in the middle. Forever.

The ones that do survive pull the ptilinum into their head.

All that remains is the ptilinal suture. It’s a scar the adult fly will carry around as a reminder of the heroic effort it made to start its life on the outside.

Now what to do with all these new body parts?

Its wings extend from crumpled to elegant as they fill with hemolymph.

This insect has now realized its full potential – to fly off into your house and drive you mad.

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