Have you ever wondered how a worm-like striped creature that crawls on many legs can transform itself into the 6 legged velvet winged beauty that is a butterfly? By what magic does this transformation take place? That magic is known as complete metamorphosis but exactly what is going on when we say that multi-syllabic word?
Like most gardeners, I love butterflies.
I plant host and nectar plants hoping they will visit my garden. I study my field guides learning their names and like most folk on DG, worry about plummeting numbers of species in the news. But the thing that fascinates me most about butterflies is metamorphosis, that bit of biological magic that transforms a striped thing that crawls with many legs into a beautiful aerial creature that floats above our blossoms like stained glass windows that can fly.
Curiosity calls; turns out there are two types of metamorphosis. There's theholometabolous (complete) and the hemimetabolous, (partial), the former being the type that butterflies exhibit. Partial metamorphosis and is exhibited by grasshoppers and other insects whose younger stages are called nymphs. In this type there is no pupal stage. By contrast, butterfly larvae, have a pupal or resting stage before transforming to an adult. Perhaps the most noticeable difference in the two types of transformation is that with the partial type, the 1st instar and succeeding ones do not look very different from the final adult stage. With butterflies, the tiny caterpillar and succeeding instars bear no resemblance to the adult butterfly. From a tiny egg to a tiny caterpillar which could be loosely translated eating machine; the worm-looking thing with many legs, outgrows his skin and sheds it around 5 times before it's considered an adult caterpillar. This adult caterpillar is triggered by some unknown signal to hang itself upside down on a twig or other handy attachment by a strong silk thread. It then forms on the inside of its body a chrysalis which splits the caterpillar skin and sheds it one last time. The chrysalis hangs there for some length of time while on the inside, all that caterpillar protoplasm is re-sorted and used to build a thing with wings that looks nothing like a caterpillar.
All this takes place inside the small vessel under the direction of DNA. The result is a fully formed often large butterfly folded neatly in the space once occupied by the caterpillar. It's mind boggling! A caterpillar looks very different from a butterfly. Is it possible that it just looks that way but isn't? Do the legs on a caterpillar somehow get recycled as legs on a butterfly? The body of a butterfly looks a bit similar to the caterpillar so is that part re-cycled somehow? But what gets re-purposed to make those beautiful wings? And, last but not least, HOW?
It appears my suppositions are not even close! Simple answer is, it's complicated. When the caterpillar stops eating and suspends itself in a place hopefully hidden from predators, it sheds its skin, (called molting) for the last time. Instead of another caterpillar skin underneath the old, a harder more protective skin called a pupa is formed.Then a process called histolysistakes place where the caterpillar digests itself almost completely. It uses the same digestive juices to do this as it once used to digest its food. Some parts are not digested. These are cells the caterpillar has always had in its body but never used for any purpose until now. These are called histoblasts and it is in these clusters of cells that the DNA blueprint for the adult butterfly resides. The cells direct the digestion of the caterpillar body and the building of the butterfly body from the material left. This material contains all the nutrients that went into building the caterpillar from its food. The same process used to turn caterpillar food into caterpillar tissue during growth, is the same biochemical process used to build the butterfly.
Wait, there's more. In other living things, hormones do a lot of the work when it comes to making biological change happen. This is true for metamorphosis in butterflies. There are three main hormones that precipitate changes. First is Prothoracicotropin or PTTH which might be thought of as somewhat analogous to our pituitary hormone in that its main role is to tap other hormones into action. It acts on the prothoracic glands in the caterpillar to produce a steroid-like hormone called ecdysone which is responsible for the molt at each instar.
PTTH also acts on thecorpora alatain the caterpill's brain to produce Juvenile Hormone. Scientists have found that when large amounts of juvenile hormone (JH) are present, the caterpillar will molt when it outgrows its skin. When the amount decreases, the drop in the level of hormone causes the caterpillar to pupate and when the flow of juvenile hormone completely stops, it causes the adult to emerge. This information helps my understanding but metamorphosis still seems obscure.The image at left is aMonarch(Danaus plexippus) shared by DG member 'TomH3787'.
I guess I've always fantasized about somehow being able to peek inside. Maybe that would satisfy my curiosity. Voila! British Scientists at the University of Manchester have done CT scans on living pupas to essentially peek inside and observe this amazing feat. Click onthis linkto take you to their website and watch videos of both the black and white scan and the colored model of what they saw.
As Dolly Parton would say, Ain't it amazin'.?