3-D Scans Reveal Caterpillars Turning Into Butterflies

We know this because scientists have dissected lots of pupae, although they’ve mostly trained their scalpels on fruit flies and blowflies. By its nature, such work always destroys the insect that’s being observed. It also only provides a snapshot in time. If you want to work out what happens as metamorphosis progresses, you need to cut open many pupae that you think are at different stages of development.

But now, two teams of scientists have started to captured intimate series of images showing the same caterpillars metamorphosing inside their pupae. Both teams used a technique called micro-CT, in which X-rays capture cross-sections of an object that can be combined into a three-dimensional virtual model.

By dissecting these models rather than the actual insects, the teams could see the structures of specific organs, like the guts or breathing tubes. They could also watch the organs change over time by repeatedly scanning the same chrysalis over many days. And since insects tolerate high doses of radiation, this procedure doesn’t seem to harm them, much less kill them.

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One team analysed the caterpillar of the stunning blue morpho just before it started metamorphosis and a week into the process. They analysed the structure of the tracheae—the network of breathing tubes that carry oxygen throughout the insect’s body. Their work was done with the BBC as part of a documentary on metamorphosis—it was publicised in March but hasn’t been published yet.

The second project had its origins in crime-fighting. Thomas Simonsen from London’s Natural History Museum started using micro-CT to look at the pupae for blowflies. These insects lay their eggs on fresh corpses, whether it’s “someone who has been murdered or a deer in a forest”. They appear so predictably that you can estimate a body’s time of death based on where its blowflies are in their life cycle. This gets trickier once the flies turn into pupae, since those all look the same from the outside. But by scanning their insides using micro-CT, Simonsen hoped to get better estimates for how old they are.