How Do Caterpillars Turn Into Butterflies During Metamorphosis?

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How do caterpillars turn into butterflies during metamorphosis? The answer is a hormone switch. The first source states that “Insect larvae metamorphose to winged and reproductive adults either directly (hemimetaboly) or through an intermediary pupal stage (holometaboly).” Butterflies belong to the group that passes through the pupal stage, and the timing of the change is set by juvenile hormone and a molting hormone working together. This article walks through that mechanism, using two PMC sources, and it marks where the sources do not reach.

Quick answer

• Caterpillars are larvae of an insect group that goes through a pupal stage. The first source notes that “larvae of holometabolous insects including flies, butterflies or beetles can differ dramatically from the adults.”
• Juvenile hormone holds the larva back. The source states that “juvenile hormone (JH) prevents metamorphosis until a larva has attained an appropriate phase of development.”
• Metamorphosis starts when juvenile hormone falls. The source says that “At an appropriate stage, a natural drop of JH secretion permits metamorphosis.”
• Ecdysone works with juvenile hormone. According to the second source, the two hormones “synergistically regulate the timely onset of metamorphosis during insect metamorphosis.”

Two kinds of insect development

Insects do not all change the same way. The first source explains that “Hemimetabolous insects such as grasshoppers, true bugs or cockroaches develop from larvae (also called nymphs) that resemble adults, possess externally growing wing pads, and metamorphose during the final molt by acquiring perfect wings and genitalia.” The contrast is with the group that includes butterflies. The same source states that “larvae of holometabolous insects including flies, butterflies or beetles can differ dramatically from the adults.”

The source describes the holometabolous route as passing through “an intermediary pupal stage,” which is the stage that separates the larval form from the adult form. The caterpillar is the larval form in that route.

Juvenile hormone as a brake

Juvenile hormone is the key signal that keeps a larva in its larval form. The first source explains that “In both hemimetabolous and holometabolous insects, the developmental switch between juvenile and adult forms depends on juvenile hormone (JH), a sesquiterpenoid produced by the corpora allata gland.” When JH is present, the larva does not become an adult. The source explains that “The presence of JH in pre-final larval instars ensures that the next molt, promoted by ecdysteroids, produces another, only a larger larva.”

The timing is what matters. A larva that has not yet reached the right stage goes through another molt that makes only a larger larva. The first source phrases the rule as “At an appropriate stage, a natural drop of JH secretion permits metamorphosis.”

The falling hormone and the pupal stage

A drop in juvenile hormone is the switch the first source describes. The second source states the balance more fully: “JH inhibits metamorphosis by antagonizing 20E signaling and inhibiting ecdysone synthesis.”

So the larva is held in place by juvenile hormone, and the molting hormone is restrained at the same time. When JH falls, the restraint is lifted. The second source notes that “the JH-20E synergistic regulation of metamorphosis involves three key genes.” Those genes are Kr-h1, Broad Complex and E93.

Ecdysone and the molt

Ecdysone is the molting hormone. The second source states that “In insect larvae, 20-hydroxyecdysone (20E) is well known to act as a molting hormone to induce molting and metamorphosis.” Juvenile hormone works against that molt until its level falls, which is the balance described in the previous section.

The first source gives an example from a true bug, where the final molt “produces an adult possessing external genitals and wings with a specific color pattern.” That example comes from a different species, so it illustrates the process rather than the caterpillar itself.

The receptor and its target genes

The juvenile hormone signal is received inside the cell. The second source says that “JH signals through its putative intracellular receptor, the bHLH-PAS protein Methoprene-tolerant (Met), originally identified in the fruit fly Drosophila melanogaster.” The first source describes the next step: “In response to JH, Met regulates expression of transcription factor genes.”

One of those target genes is Kr-h1. The second source describes it as “a key factor antagonizing insect metamorphosis.” The first source reports what happens when it is lost: “loss of Kr-h1 also elicits precocious metamorphosis of beetle larvae.”

The first source also notes that the hormone signal is shared across insect groups. It states that “transduction of the anti-metamorphic JH signal relies on the common-core elements, Met and Kr-h1.” That sharing is part of why the mechanism is useful as a general model for insect development.

Intracellular and membrane signals

Juvenile hormone also acts outside the nucleus. The second source explains that “JH regulates multiple physiological processes during insect development via two distinct pathways: intracellular signaling and membrane signaling.” The intracellular pathway is the one described above, through Met and its target genes. The membrane pathway is a separate route, and the source reports that JH “rapidly induces physiological responses across multiple insect species.”

The membrane route adds to the picture, but the excerpts used here do not connect it to the timing of the final molt. The second source is a review of the pathways, not a full account of the caterpillar’s timing.

For a different insect question, the article on how moths find night-blooming flowers covers adult behavior rather than development. A second related read, how plants defend against insect herbivores, covers a different insect interaction.

How we chose

Both sources are PMC articles that opened directly with HTTP 200 on the check date. The first has the title “Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects,” which matches the metamorphosis question. The second has the title “Advances in the study of juvenile hormone signaling,” which covers the signaling pathways. Neither is a retail page, and each was read as text before any statement was drawn from it.

What we could not verify

The sources explain the hormone mechanism, but they do not describe the caterpillar-to-butterfly change in detail for a butterfly species. The first source’s experiments are on a true bug, and the second source is a review that draws on several insects. The result is a general account of insect metamorphosis, not a butterfly-specific timeline.

What the sources do not say

The first source focuses on “the hemimetabolous firebug, Pyrrhocoris apterus,” and the second source is a review. The first source also states that “our knowledge on the molecular mode of JH action almost exclusively derives from studies in holometabolans,” so the holometabolous evidence comes mainly from other insect orders. The timing of the caterpillar’s own molts is therefore not settled by these two papers.

Frequently asked questions

Why does a caterpillar not become a butterfly right away?

The first source states that “juvenile hormone (JH) prevents metamorphosis until a larva has attained an appropriate phase of development.” The larva has to reach the right stage before the hormone signal falls.

What is the difference between a caterpillar and a butterfly?

The first source notes that “larvae of holometabolous insects including flies, butterflies or beetles can differ dramatically from the adults.” The caterpillar is the larval form, and the source places the adult after the pupal stage.

Which hormone makes the caterpillar molt?

The second source says that “20-hydroxyecdysone (20E) is well known to act as a molting hormone to induce molting and metamorphosis.” Juvenile hormone works against that molt until its level falls.

Do all insects go through a pupal stage?

No. The first source says that “Insect larvae metamorphose to winged and reproductive adults either directly (hemimetaboly) or through an intermediary pupal stage (holometaboly).” The first route is the direct one.

Is the same gene involved in every insect?

The first source reports a shared core: “transduction of the anti-metamorphic JH signal relies on the common-core elements, Met and Kr-h1.” The source also reports differences, noting that “BR-C mRNA expression was neither as strongly dependent on Met at the L4 instar” as Kr-h1 was.

Sources

• “Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects.” https://pmc.ncbi.nlm.nih.gov/articles/PMC3234286/ (HTTP 200, opened 2026-10-28)
• “Advances in the study of juvenile hormone signaling.” https://pmc.ncbi.nlm.nih.gov/articles/PMC13126561/ (HTTP 200, opened 2026-10-28)