What happens inside a cocoon?

If you were an alien arriving on Earth who was shown a caterpillar and then a butterfly, you probably wouldn’t think they were one and the same.

How exactly does it happen? How does this this squishy, multi-legged, soft thing become a flying insect with an exoskeleton?

It’s all a question of metamorphosis – in particular, insect metamorphosis.

Luna moth, Actias luna, emerging from silk cocoon (link to insect metamorphosis on Wikipedia)
Luna moth, Actias luna, emerging from a silk cocoon (Taken by Shawn Hanrahan)

Now there are two types of insect metamorphosis:

  1. Incomplete metabolism, or hemimotabolism, as seen in mantises, cicadas and cockroaches. This is incomplete because the insects don’t completely change form. We have the egg, from that hatches the nymph and it in turn develops into the adult or imago. But essentially it’s a bit like us – we have a smaller version of the adult that gets bigger.
  2. Complete metamorphosis, or holometabolism. This process is truly fascinating.

In holometabolism you have four stages embryo – larva – pupa – imago.

The embryo is what develops in the egg. When the egg hatches we have the larva. Then once the larva has had it’s full and stored enough energy for the forthcoming process, it will build a cocoon or, in the case of butterflies, a chrysalis.

Inside… essentially the pupa digests itself! A lot of the caterpillar’s old body dies, and then is consumed by “digestive juices” in a process called histolysis (see Gordon’s Lepidoptera page).

Not all the tissue is destroyed however, as some of the insect’s old tissue passes on to its new self, the amount that does varying between different insects.

There is one particular sort of tissue that remains: in a number of places in the insect’s body are collections of special formative cells, which have played no part in the insect’s larval life, and have stayed hidden or protected during this partial death. Each of these groups of cells is called an imaginal bud or a histoblast.

The job of these histoblasts is to supervise the building of a new body out of the soup that the insect’s digestive juices have made of the old larval body. This they do using the same biochemical processes that all insects use to turn their food into part of their bodies. This rebuilding process is called histogenesis. During this time the insect is very vulnerable because it cannot run away .

With so much change going on can it truly be the same creature? Well in fact it is – it even retains memories from the larval stage.

Some researchers at Georgetown Uni in Washington conducted some experiments, where they associated odors with negative stimuli at various stages of the pupa’s development (Blackiston DJ, Silva Casey E, Weiss MR 2008, “Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar?”, PLoS ONE 3(3): e1736).

“We have demonstrated that M. sexta larvae can learn to associate odor cues with an aversive stimulus, and that this memory persists undiminished across two larval molts, as well as into adulthood. The behavior represents true associative learning, not chemical legacy, and, as far as we know, provides the first definitive demonstration that associative memory survives metamorphosis in Lepidoptera. Furthermore, the results from our differential timing of larval training are consistent with the idea that retention of memory could be due to the persistence into adulthood of intact larval synaptic connections.”

The vast differences between the young and older versions of an animal may well give an evolutionary advantage. By being so different, they fit into different and, importantly, non-competing niches in the environment and ecosystem.

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3 thoughts on “What happens inside a cocoon?

  1. Beetles (Order Coleoptera) are not hemimetabolous! They are holometabolous insects, exactly like Lepidoptera, Hymenoptera, Diptera, Mecoptera and few others. The ones with incomplete metamorphosis are Hemiptera, Orthoptera etc.

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