The biological mechanisms of metamorphosis that make butterflies and moths
It’s funny how many kids’ early exposure to biology involves the metamorphosis of caterpillars and butterflies, since the exact process of of these transformations is kind of a mysterious, black box of an event. We know the Hungry Hungry caterpillar eats a lot, then makes a coc- *ahem,* a chrysalis and then… poof! Magic! We’ve got a butterfly! Even after helping my second grader raise some butterflies from their larval stage, and seeing our neighborhood dotted with Tussock moth cocoons each spring, metamorphosis is just hard to relate to as a mammal. Fortunately, even knowing some of the nuts-and-bolts of what’s happening in a cocoon doesn’t really make it any less fantastic.
My kids apparently hadn’t dug into this process too much in their heads. My four-year-old explained that involved the caterpillar turning into “a black thing,” which somehow yielded a moth or butterfly. My second grader didn’t want to expose herself with a wrong answer, but she did note some similarities between the tube-shape of a caterpillar’s body and the shape of their adult abdomen and thorax. Maybe caterpillars just moved anatomy around, giving it small adjustments as needed, then adding wings?
The truth is a bit gloppier than that, although it’s obviously well managed enough to create amazing, delicate structures like the color-bending scales of a butterfly’s wings. Once inside a cocoon or chrysalis, the caterpillar essentially liquefies the bulk of its body so that the proteins can be recycled into new anatomy. Certain tissues, like muscle, are broken up but kept somewhat recognizable, where they can be reshaped or reallocated into something fit for an adult insect. Other anatomy is built upon structures called imaginal discs, which are a sort of scaffolding to anchor things like eyes and antennae onto.
Importantly, nerve tissue may be preserved enough that adult moths and butterflies can retain larval memories. Experiments with caterpillars conditioned to avoid certain smells found that those lessons were retained after metamorphosis. Some species of caterpillars even carry around the beginnings of anatomy like wings inside their exoskeletons as well, so it’s not like the entire bug is pureed in this process.
Peeking at the growth process
Knowing this isn’t just hard for amateur entomologists either. For a long time, the best data available was from cutting open cocoons and chrysalises. This not only interrupted the process, but also required matching progress and bodies of various specimens to infer how things were actually growing. More detailed data is now available thanks to micro-CT scans, which can create 3D models of a single specimen at different phases of development. The resulting views include detail and reveal patterns and trends, such as how tracheae bend and shift from meet the needs of the emerging butterfly. Which is basically what we were all thinking as kids, right?
Source: How Does a Caterpillar Turn into a Butterfly? by Ferris Jabr, Scientific American