The metamorphosis stage that follows the larva is the pupa. In insects that undergo complete metamorphosis, the life cycle runs egg → larva → pupa → adult, and the pupal stage is where virtually all of the body-plan remodeling happens. A caterpillar does not simply grow wings; inside its pupal casing, larval tissues are broken down and rebuilt into an entirely different body, a process so thorough that the animal’s breathing pattern, immune defenses, and even gut bacteria change along the way.
What the Pupa Actually Is
The pupa is a non-feeding, outwardly still stage that sits between the active, eating larva and the reproductive adult. Insects that pass through it are called holometabolous, a group that includes butterflies, moths, beetles, flies, bees, and wasps. During this stage the insect is enclosed in some form of protective shell. In butterflies that shell is a hardened skin called a chrysalis. In many moths and some other insects, the larva first spins a silk cocoon around itself, then forms the pupa inside. Either way, the animal inside is undergoing demolition and reconstruction on a cellular level.
Not all insects have a pupal stage. Grasshoppers, cockroaches, dragonflies, and true bugs go through incomplete metamorphosis: their young, called nymphs, look like small wingless versions of the adult and grow gradually through a series of molts. The pupal stage is unique to the complete-metamorphosis lineage and is one of the reasons that lineage has been so wildly successful, accounting for the vast majority of known insect species.
How Hormones Trigger the Larva-to-Pupa Transition
Two hormones run the show. Juvenile hormone keeps the larva in its larval form, while the steroid hormone 20-hydroxyecdysone pushes the animal toward metamorphosis. As long as juvenile hormone levels are high, ecdysone-driven molts just produce a bigger larva. When juvenile hormone finally drops, the next ecdysone pulse triggers pupation instead of another larval molt.1PubMed Central. Antagonistic actions of juvenile hormone and 20-hydroxyecdysone within the ring gland determine developmental transitions in Drosophila
This hormonal tug-of-war serves a practical purpose: it prevents the larva from metamorphosing too soon. Experiments in fruit flies have shown that adding a juvenile hormone analog blocks premature pupation, essentially giving the larva more time to feed and grow large enough to survive the energy-intensive remodeling ahead.2PubMed. Ecdysone differentially regulates metamorphic timing relative to 20-hydroxyecdysone by antagonizing juvenile hormone in Drosophila melanogaster Downstream of these two master hormones, a cascade of gene regulators determines whether the insect’s cells express larval, pupal, or adult characteristics.3PubMed. Background of Insect Metamorphosis: Numerous Functions of Ecdysteroid
What Happens Inside the Pupa
The inside of a developing pupa is sometimes described as “soup,” and while that is an oversimplification, it captures the spirit of what is going on. Larval organs that the adult will not need are actively dismantled through programmed cell death. In silkworms, the anterior silk glands, which have no function in the adult moth, undergo a two-phase demolition process: first the cells begin to self-digest through autophagy, then apoptosis finishes the job. Research on silkworms found that apoptosis alone is not enough for complete gland breakdown; the cooperation of both cell-death pathways is needed for efficient tissue removal.4PubMed. Some autophagic and apoptotic features of programmed cell death in the anterior silk glands of the silkworm, Bombyx mori
The hormonal signal for this demolition comes from the same ecdysone pulse that initiates pupation. In the fat body, a major metabolic organ, 20-hydroxyecdysone shuts down growth-promoting pathways and flips on the cellular recycling machinery, switching the fat body into breakdown mode.5Philosophical Transactions of the Royal Society B. Cell death during complete metamorphosis
While larval tissues are being torn down, adult structures are being built up from clusters of cells called imaginal discs. These tiny pockets of tissue have been sitting quietly inside the larva since embryonic development, waiting for the hormonal green light. During the pupal stage, one disc becomes a wing, another becomes a leg, another becomes an eye. In fruit flies, wing imaginal discs undergo their final shaping and differentiation during the pupal period to form the functional adult wing and body wall.6Genetics. The wing imaginal disc For leg formation, the timing must be precise: a transcription factor called Broad drives the remodeling of the extracellular matrix and changes in cell shape that allow leg discs to elongate, but this factor has to shut off at exactly the right moment. If it stays on too long, leg development stalls.7PubMed Central. The Broad Transcription Factor Links Hormonal Signaling, Gene Expression, and Cellular Morphogenesis Events During Drosophila Imaginal Disc Development
Breathing and Burning Fuel Without Eating
The pupa does not eat, so everything it builds must come from energy reserves the larva packed away. In fruit flies, metabolic rate drops sharply within the first day of pupation, stays low through most of the pupal period, then ramps back up just before the adult emerges, forming a U-shaped curve. The primary fuel powering this entire process is fat, which accounts for more than 80 percent of the pupa’s total energy expenditure.8PubMed. Energetics of metamorphosis in Drosophila melanogaster
Even breathing changes between the larval and pupal stages. Caterpillars breathe continuously and have high metabolic rates, using all their spiracles (the small openings along the body wall that let air in). Once they pupate, they switch to a pattern called discontinuous gas exchange, in which the spiracles open only periodically. Measurements in the ailanthus silkmoth showed that caterpillars and pre-pupae use more than 14 spiracles at once, while pupae rely on only eight to ten.9PubMed. The role of the spiracles in gas exchange during development of Samia cynthia (Lepidoptera, Saturniidae) This more conservative breathing strategy helps the pupa limit water loss during a period when it cannot drink.
How the Pupa Protects Itself
A non-moving, non-fleeing animal is vulnerable, so pupal casings tend to be remarkably well-engineered. Silkworm cocoons, the most studied example, are natural composites with a multi-layered structure that varies in porosity, fiber density, and stiffness from the outer wall to the inner wall.10Oxford Open Materials Science. Bombyx mori silk cocoons: structure, composition, and mechanical properties—a review The outer layers are denser and stiffer for mechanical protection, while the inner layers are softer and more porous, creating a microenvironment for the developing pupa.11PubMed. Silk cocoon (Bombyx mori): multi-layer structure and mechanical properties
Some species go further. The oriental moth incorporates calcium oxalate crystals into its silk, producing a mineralized hard cocoon that is fundamentally different from the soft silk of the domesticated silkworm. Analysis found these cocoons contain hundreds of proteins and mineral crystals bonded to the silk fibers, creating something closer to a shell than a fabric.12PubMed. Insights into the structure and composition of mineralized hard cocoons constructed by the oriental moth, Monema (Cnidocampa) flavescens Walker Beetles and many flies protect their pupae differently, forming a hard case called a puparium from the shed larval skin itself. Butterfly chrysalises, by contrast, are the hardened pupal cuticle with no outer cocoon at all.
Immune Defenses During Pupation
You might expect an animal that is essentially liquefying and rebuilding itself to be defenseless against infection, but pupae maintain at least some immune function. In fruit flies, the ingestion of bacteria during larval and pupal stages activates genes for antimicrobial peptides, small proteins that attack invading microbes. Different bacteria triggered different subsets of these defense molecules across life stages, indicating that the pupal immune system is not simply an inert holdover from the larval one but responds to specific threats.13PubMed. Ingestion of killed bacteria activates antimicrobial peptide genes in Drosophila melanogaster and protects flies from septic infection
The gut microbiome itself, however, may not survive the transition intact. In the burying beetle, researchers found that pupae go through what amounts to a bacteria-free stage, losing most of their gut microbes during pupation and then getting recolonized at emergence with bacteria from the shed larval skin and the pupal chamber walls.14PubMed Central. Gut Microbiota Colonization and Transmission in the Burying Beetle Nicrophorus vespilloides throughout Development This suggests the pupal stage is a kind of microbiological reset: the adult starts with a gut community that may differ from the larva’s.
Can Memories Survive Pupation
The pupal nervous system undergoes extensive rewiring, with neurons being pruned, new neurons growing, and some old neurons dying outright. That raises a question researchers have genuinely investigated: can an insect remember something it learned as a larva after all this neural demolition?
In tobacco hornworm moths, the answer is yes. Caterpillars trained to avoid a specific odor retained that avoidance as adults, even after the massive nervous system remodeling of pupation. Control experiments ruled out chemical traces or simple familiarity; the moths showed genuine associative memory that had crossed the larval-adult divide. The researchers concluded that the retention is consistent with some larval synaptic connections surviving intact through metamorphosis.15PLoS ONE. Retention of Memory through Metamorphosis: Can a Moth Remember What It Learned As a Caterpillar? This finding is striking because it implies that even though much of the nervous system is rebuilt, the specific circuits encoding learned associations can be preserved.16PubMed Central. The stability of memories during brain remodeling: A perspective
Pupal Diapause and Pausing the Clock
Not every pupa proceeds straight to adulthood. Many species can enter diapause, a state of developmental arrest that lets the pupa survive unfavorable conditions like winter cold or summer drought. Diapause is not simply the pupa slowing down because it is cold; it is a genetically programmed halt that is triggered by environmental cues the larva experienced weeks or months earlier, such as shortening day length.
During pupal diapause, metabolism is actively suppressed. In the cotton bollworm, a major agricultural pest, researchers identified a signaling pathway that reprograms metabolism in diapause-destined pupae, coordinating the downregulation of a key enzyme involved in making glucose from non-sugar precursors.17Physiological Entomology. TGFβRI modulates metabolic reprogramming via the aPKC-PEPCK axis during pupal diapause in the cotton bollworm, Helicoverpa armigera In practical terms, this means a diapausing pupa burns through its fat reserves far more slowly than one developing straight through, allowing it to survive months of winter underground. Some species can remain in pupal diapause for more than a year, and a few desert moths are thought to stay dormant for several years until rains trigger emergence.
How the Adult Breaks Out
The final act of the pupal stage is eclosion, when the fully formed adult emerges from the pupal case. This is not a passive event. A surge of eclosion hormone triggers behaviors that crack the pupal shell, and then a second hormone, bursicon, takes over. Bursicon causes the cuticle of the new adult to harden and darken, a process called sclerotization and tanning. Without bursicon signaling, the adult cuticle stays soft and pale; in honeybees, knocking down bursicon genes resulted in thinner, poorly tanned cuticle and impaired emergence from the pupal stage.18PLOS ONE. RNAi-Mediated Functional Analysis of Bursicon Genes Related to Adult Cuticle Formation and Tanning in the Honeybee, Apis mellifera
Wing expansion is another bursicon-driven event. In the tobacco hornworm moth, bursicon is released from the abdominal nerve cord after eclosion and makes the wing cuticle extensible, allowing the crumpled wings to inflate with hemolymph (insect blood) and then stiffen into their final flight-ready shape.19Journal of Experimental Biology. Control of Cuticle Extensibility in the Wings of Adult Manduca at the Time of Eclosion: Effects of Eclosion Hormone And Bursicon In fruit flies, bursicon signaling through its receptor is also required earlier, during the prepupal phase, to properly form the pupal case itself.20PubMed Central. The essential role of bursicon during Drosophila development So the same hormonal system bookends the pupal period, helping to build the casing at the start and harden the adult at the end.
Where Did the Pupa Come From Evolutionarily
The origin of the pupal stage is one of the more contested questions in insect evolution. The most widely supported idea is that the pupa evolved from the final juvenile stage of an ancestor that went through incomplete metamorphosis, where nymphs gradually become adults. Under this view, ancestral nymphal stages were compressed into the simplified larval stages we see today, and the last pre-adult nymphal stage was modified into the pupa. Recent work on the hormonal control of metamorphosis has bolstered this hypothesis: the pattern of juvenile hormone signaling that defines the pupal stage maps neatly onto what happens in the final nymphal stage of incompletely metamorphosing insects.21PubMed Central. Where did the pupa come from? The timing of juvenile hormone signalling supports homology between stages of hemimetabolous and holometabolous insects
An alternative hypothesis proposes that the pupa represents all of the nymphal stages collapsed into one, and that holometabolous larvae are essentially free-living embryos whose development was paused at an early point. The hormonal evidence currently favors the first explanation, but the debate is not fully settled. Either way, the invention of a pupal stage was a pivotal event in insect evolution, allowing larval and adult forms to specialize for completely different ecological roles, one for feeding and growth, the other for reproduction and dispersal.
The Subimago of Mayflies
Mayflies are the only living insects that molt again after developing functional wings. They emerge from their aquatic nymph stage as a winged form called the subimago, which then sheds its skin one final time to become the true adult (imago). This makes the subimago something like a partial parallel to the pupa, although it is fully mobile and capable of flight. Research on the mayfly Cloeon dipterum found that the foreleg dramatically grows between the last nymphal instar, the subimago, and the adult, suggesting that the subimago stage may persist because the insect needs one additional growth step that cannot be accomplished in a single molt.22PubMed Central. Regulation of metamorphosis in neopteran insects is conserved in the paleopteran Cloeon dipterum (Ephemeroptera) The mayfly subimago offers a window into how transitional stages between juvenile and adult forms can persist when there is a developmental need for them.
Metamorphosis Without a Pupa in Frogs
Frogs undergo their own dramatic metamorphosis, transforming from aquatic, gill-breathing, tail-bearing tadpoles into air-breathing, four-legged adults. There is no pupal stage; the tadpole remodels itself while still swimming and feeding (though feeding slows or stops in some species near the climax of metamorphosis). The master hormone is not ecdysone but thyroid hormone. Metamorphosis simply will not happen without it, and adding thyroid hormone to the water can trigger premature transformation.23PubMed Central. Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids
The parallel to insect metamorphosis is real but limited. Both systems use a hormone-driven switch to remodel the body from a juvenile feeding form to a reproductive adult form, and both involve programmed cell death to remove structures the adult does not need (the tadpole’s tail, for instance, is reabsorbed). But frog metamorphosis happens in the open, with the animal remaining active, rather than inside a sealed pupal case. This difference highlights what makes the insect pupa distinctive: it is a protected, quiescent environment where remodeling can proceed without the animal needing to move, eat, or escape predators. The trade-off is vulnerability, which is why pupal casings and cocoons are often so impressively engineered.24PubMed. A developmental switch induced by thyroid hormone: Xenopus laevis metamorphosis