The weird and wonderful metamorphosis of the butterfly Franziska Bauer
In order to become a butterfly,
a caterpillar’s body dissolves
almost completely
and is rebuilt from its own juices.
As inconvenient and even downright
dangerous as this process sounds,
it’s actually quite common.
Butterflies are just a few of
the 800,000 insect species
that transition from larvae to adults
through complete metamorphosis.
That’s approximately 85% of insects
and 70% of all known animal species.
But how exactly does
a caterpillar become a butterfly?
When a caterpillar hatches
from an egg,
it has none of a butterfly’s
physical traits.
It does have groups of cells
inside its body called imaginal discs
that will eventually
become its butterfly parts.
For now, these cells remain dormant.
Juvenile hormone suppresses
their activity,
and prevents the caterpillar
from beginning metamorphosis too early.
Immediately after hatching,
the caterpillar begins to feed,
building up fat until its rigid skin,
called a cuticle, becomes too tight.
At this point, a hormone called ecdysone
triggers the cuticle to shed, or molt.
As the caterpillar grows,
it usually molts four times.
Then, when it’s nice and plump,
the caterpillar’s levels
of juvenile hormone drop,
which triggers it to stop
eating and moving.
A final burst of ecdysone
prompts the caterpillar’s cells
to begin to self-destruct.
Soon, the muscles, fat, and other tissues
are almost entirely liquefied,
though the imaginal discs stay intact
and begin to grow.
At the same time, a second skin layer
called the pupal cuticle
forms underneath the first.
One more molt exposes
the hard exterior of the pupa.
Besides the imaginal discs,
only a few tissues are spared,
including parts of the respiratory system,
the heart,
some abdominal muscles,
and the mushroom bodies of the brain.
The caterpillar juice then fuels
the development of the imaginal discs
into eyes,
antennae,
legs,
wings,
genitalia,
and other body parts.
Once its new body is built,
the insect molts one last time,
shedding the pupal cuticle.
From there, it’s free to fly away
a new butterfly.
Even after such a dramatic transformation,
the butterfly does retain some memories
from its caterpillar days.
It’s likely the mushroom bodies
of the brain
carry important knowledge from the
caterpillar over to the adult butterfly.
How did such an involved developmental
process come to be?
We don’t know for sure.
The leading theory is that the caterpillar
is actually a drawn-out version
of a life stage that takes place
inside the egg for some other insects.
According to this hypothesis,
over millions of years,
the larvae evolved the ability
to eat and live outside the egg.
Regardless of how complete
metamorphosis originated,
it’s become part of the life cycles of
a dizzying number of insect species.
Still, plenty of species get along
perfectly well
with a simpler developmental process.
What survival advantages might
complete metamorphosis provide
to make up for the added hassle?
For one, it keeps larvae and adults
from competing for the same habitats
and food sources.
And while the pupa may seem vulnerable,
this immobile stage
can actually be a good way
to pass parts of the year
when food is scarce.
To us, a butterfly’s metamorphosis
might sound as fantastic
as a phoenix rising from its ashes.
But these transformations are taking place
all around us, all the time.
From the Hercules beetle
to the honey bee
to the garden ant,
countless squishy larvae dissolve
and emerge
as armored, aerodynamic,
and nimble adults.