Metamorphosis is an amazing process. A caterpillar appears to magically turn into a butterfly. Think of it – a worm-like critter that spends its time constantly eating leaves and growing somehow becomes an insect with relatively huge wings that mostly eats nectar from flowers. The two stages look completely different and have completely different diets and lifestyles. How does this happen? It’s a difficult question to answer because the metamorphosis is very well concealed within a butterfly’s chrysalis, or cocoon. But recently a research group at the University of Manchester in the UK and their collaborators used a complex imaging technique to let us observe the insides of living chrysalises, and see what happens as a caterpillar becomes a butterfly.
One of the most impressive and surprising findings of this study is that the system of tubes that the butterfly uses to breathe and get oxygen to its cells is pretty much completely formed by the first day of life as a chrysalis. (This system is called the tracheal system — it’s shown in blue in the image above.) It doesn’t hatch for about another two weeks, but the butterfly is already ready to breathe when it does! Changes in other parts, such as the midgut, were more expected – for example, the midgut has to drastically change and shrink, as the nectar-slurping butterfly has a very different diet from, and eats a lot less than, the rapidly-growing, leaf-devouring caterpillar. Combining the images they took over time of the developing chrysalises, the researchers made a fascinating video (published in the paper). So much cellular rearrangement is going on in such a tiny space!
Previously, studies have used techniques to explore chrysalises that, by necessity, destroyed the chrysalises, such as dissection and staining the chrysalises with various dyes to investigate them under a microscope. In the current study, the researchers used a technique called high-resolution X-ray computed tomography (CT), which allowed them to take these images of the living chrysalises with minimally damaging them. (The method exposes the chrysalises to some radiation, but it is minimal and the relatively radiation-resistant butterflies that emerged from the chrysalises appeared normal.) This means they could follow the changes in a single chrysalis over the entire metamorphosis process without worrying that they were just seeing variation between the chrysalises they observed.
While people have marveled at the mysteries of metamorphosis for centuries, it’s amazing to live in a time when we have the ability to really solve some of them. And, as our tools continue to improve, we can make better and better discoveries.
For further reading:
- Tristan Lowe, Russell J. Garwood, Thomas J. Simonsen, Robert S. Bradley, and Philip J. Withers’s article “Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis” at the Journal of the Royal Society
- John R. Meyer’s webpage “Insect Physiology: Respiratory System” at NC State University
- The High Resolution X-ray CT Facility’s webpage “About High-Resolution X-ray CT” at the University of Texas at Austin