It's spring, and there’s a buzz in the air. Everywhere you look, new life is blooming. Flowers bloom, insects buzz and trees are unfurling shiny, new leaves. Beneath their shade, dangling from a leaf on a fine silk thread, a chrysalis sways and spins in the warm morning breeze.

Inside, a butterfly is about to emerge. It has spent the last few days metamorphosing from a caterpillar – hidden from our gaze, its whole body restructuring itself for a new phase of life. But what actually happens inside a chrysalis?

Monarch butterfly chrysalis

During the process of pupation, insects such as this Monarch butterfly are at risk from predation and cold weather. Photo: iStock

Pupa, chrysalis or cocoon?

Pupa (or pupae if plural) refers to a hard casing within which a larval insect metamorphoses into an adult. Not all insects go through this process. Those that do are the holometabolic insects. These are the true flies, beetles, Lepidoptera (butterflies and moths) and Hymenoptera (wasps, ants and bees).

Read more

A chrysalis refers specifically to the pupae of butterflies, which usually hang from trees, attached by a thin thread. Other insect pupae may have different names, such as the tumblers of mosquitos.

A cocoon, on the other hand, is not really a pupa, it’s a silk case that is sometimes spun around the pupa. Depending on the species this might be soft or hard and may be covered with debris as a form of camouflage.

Swallowtail butterfly chrysalis

Inside the chrysalis, the swallowtail butterfly caterpillar has broken down, but it has not simply turned to soup as once thought. Photo: iStock

What happens inside the pupa?

You might have heard that insects turn into goo or soup inside the pupa. This was thought to be the case for some time but we now know that it isn’t true. Or at least, not entirely true.

Safe within the pupa, our caterpillar undergoes a remodelling. Groups of cells called imaginal discs act like blueprints for adult structures. As some tissues are broken down into component cells (a bit like a soup), the imaginal discs repurpose them, forming the legs, eyes and wings of our soon–to–be butterfly. Other key structures, however, don’t break down; the gut changes in length but remains in situ, as do the trachea (the channels that allow insects to breath) and some neural connections.

During this process the caterpillar does not need to eat and can still breathe as the trachea remain open through the pupa. As the neural connections remain intact too, the adult butterfly will remember the experiences of the caterpillar and what threats to be watchful for.

Read more

Where can you find a pupa?

For the most part, pupae don’t move. While all these changes take place they stay right where they were formed. For some species that means hanging from a tree, others are tucked under a leaf or buried in soil near the plants they ate as larvae. Some pupae do move, though. They can shake or wobble and sometimes make a buzzing or hissing sound. This is a defence mechanism intended to scare off potential predators or parasites if the pupa is disturbed.

Monarch butterfly emergence

The monarch butterfly emerges, slowly drying its wings. Photo: iStock

How long do insects pupate for?

The time of emergence can vary with species. Mosquitos tend to emerge in the evening and moths might emerge during the day or at night depending on when they would usually be active. After a few days, if everything has gone well and the pupa was undisturbed by predators or cold weather, our adult butterfly is fully formed and about to emerge in the morning sun. It cuts its way free using little hooks at the base of its wings and perches on the now empty chrysalis. Unfolding its new wings, it fans them so that they dry. When it’s ready it will let go of the chrysalis and take its first flight.

Learn more about amazing insects by reading Ross Piper's column in every issue of the RSPB magazine Nature's Home. Become a member today to get your copy.