# Instar
An instar is a developmental stage of an immature insect between two successive molts (ecdyses); the first instar is the period between hatching from the egg and the first molt[1](https://ipm.ucanr.edu/PMG/glossary.html). The word applies to both larval and nymphal stages and is numbered by order of appearance after egg hatching, as in first larval instar or third instar[2](https://www.fabinet.up.ac.za/publication/pdfs/4548-978-3-031-11553-0.pdf).
| Key fact | Detail |
|---|---|
| Definition | The form of an immature insect between two molts; the first instar runs from hatching to the first molt[1](https://ipm.ucanr.edu/PMG/glossary.html) |
| Numbering | Instars are numbered in order after egg hatching (1st instar, 2nd instar, and so on)[2](https://www.fabinet.up.ac.za/publication/pdfs/4548-978-3-031-11553-0.pdf) |
| Typical counts | In some species the number of instars is constant, typically from 3 to 15; in others it varies with temperature, food availability, or other environmental factors[3](https://genent.cals.ncsu.edu/bug-bytes/morphogenesis/) |
| Related term | The stadium is strictly the duration of an instar, beginning at ecdysis[2](https://www.fabinet.up.ac.za/publication/pdfs/4548-978-3-031-11553-0.pdf) • [4](https://academic.oup.com/aesa/article-abstract/71/4/491/86048) |
| Hormonal control | 20-hydroxyecdysone initiates each molt; juvenile hormone present during a molt produces another juvenile instar rather than a pupa or adult[5](https://www.ncbi.nlm.nih.gov/books/NBK9986/) |
| Record counts | The fire brat *Thermobia domestica* undergoes 60 molts if instars are counted between any two molts; the mayfly *Stenacron interpunctatum canadense* has 45 larval molts[6](https://entnemdept.ufl.edu/walker/ufbir/files/pdf/UFBIR_Chapter13.pdf) |
## Definition and terminology
Insects grow in discrete steps because their rigid exoskeleton cannot expand much; it must be shed and replaced with a larger one as the insect grows, a process called molting[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/). The life stage between each molt is the instar[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/).
Entomologists distinguish three related terms. The instar is the form of the animal between molts; the stadium is strictly the duration of an instar; and the stage refers to the major ontogenetic divisions of the life cycle, such as larval, pupal, and adult[2](https://www.fabinet.up.ac.za/publication/pdfs/4548-978-3-031-11553-0.pdf). A stadium begins at the moment of ecdysis, when the instar emerges from an egg shell, an old larval cuticle, or a pupal case[4](https://academic.oup.com/aesa/article-abstract/71/4/491/86048).
The term has been defined in two ways in the literature. One definition treats an instar as the stage of the insect between successive molts, without restricting it to immatures; the other restricts it to the stage between molts of the immature insect[6](https://entnemdept.ufl.edu/walker/ufbir/files/pdf/UFBIR_Chapter13.pdf). Which definition is used changes how counts are reported, since adult insects of some primitive groups continue to molt.
## Number of instars
In some species the number of instars is constant, typically from 3 to 15, while in others it varies in response to temperature, food availability, or other environmental factors[3](https://genent.cals.ncsu.edu/bug-bytes/morphogenesis/). Larvae and nymphs typically go through a characteristic number of instars during growth and enter metamorphosis when they reach a species-specific threshold size[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC6711285/).
**Plasticity in instar number.** The endocrine arrangement that governs molting provides plasticity to deal with food deprivation in the later instars: some insects can sustain juvenile hormone production and intercalate an additional instar to accommodate the required growth[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC6711285/).
Counts can be far higher in primitive wingless insects. In a survey of records, the fire brat *Thermobia domestica* holds the record with 60 molts if instar refers to any stage between molts; if only larval instars count, the mayfly *Stenacron interpunctatum canadense* leads with 45 molts[6](https://entnemdept.ufl.edu/walker/ufbir/files/pdf/UFBIR_Chapter13.pdf).
## Hormonal control
Molting is governed by hormones. Cuticle secretion and the molt cycle are controlled by ecdysone, a steroid hormone secreted by a gland in the thorax, which is in turn controlled by a hormone from the brain; when the brain receives the appropriate stimulus, the insect molts[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/).
Insect molting and metamorphosis are controlled by two principal effector hormones: the steroid 20-hydroxyecdysone and the lipid juvenile hormone. 20-hydroxyecdysone initiates and coordinates each molt, while the presence of juvenile hormone during a molt ensures that the result is another instar rather than a pupa or an adult[5](https://www.ncbi.nlm.nih.gov/books/NBK9986/). As juvenile hormone concentrations decline across successive molts, more adult characters appear until the adult stage is produced[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/).
## Instars in different developmental types
The immature stages between molts are called nymphs in insects with simple (incomplete) metamorphosis and larvae in insects with complete metamorphosis[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/). Nymphs usually share the habitat and host of the adults and lack wings and reproductive organs; larvae of moths, flies, and beetles, the caterpillars, maggots, and grubs, often look and live quite differently from the adults and transform through a pupa before becoming adult[7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/).
Wing development proceeds across instars in insects with gradual metamorphosis. In the first instar, wings can be entirely missing; they then emerge as small wing buds in either the second or third instar, enlarging at every molt until the adult emerges with fully formed, functional wings.[3](https://genent.cals.ncsu.edu/bug-bytes/morphogenesis/)
## Use in practice
Because each instar is a discrete, countable period, instar identification is used in describing insect development in agricultural and pest-management contexts, where the developmental stage of a pest population determines its vulnerability and the timing of controls[1](https://ipm.ucanr.edu/PMG/glossary.html) • [7](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/).
## References
1. [Glossary, UC Statewide Integrated Pest Management Program, University of California](https://ipm.ucanr.edu/PMG/glossary.html)
2. [Forest Entomology and Pathology (Springer, chapter PDF), FABI, University of Pretoria](https://www.fabinet.up.ac.za/publication/pdfs/4548-978-3-031-11553-0.pdf)
3. [Morphogenesis, ENT 425 General Entomology, North Carolina State University](https://genent.cals.ncsu.edu/bug-bytes/morphogenesis/)
4. [Note on the Use of the Terms Instar and Stage, Annals of the Entomological Society of America 71(4)](https://academic.oup.com/aesa/article-abstract/71/4/491/86048)
5. [Metamorphosis: The Hormonal Reactivation of Development, Developmental Biology, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK9986/)
6. [Chapter 13: Most Instars, University of Florida Book of Insect Records](https://entnemdept.ufl.edu/walker/ufbir/files/pdf/UFBIR_Chapter13.pdf)
7. [Insect Growth and Development, WSU Tree Fruit, Washington State University](https://treefruit.wsu.edu/crop-protection/opm/insect-growth-and-development/)
8. [The evolution of insect metamorphosis: a developmental and endocrine view, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6711285/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Development and comparative physiology › Clade-specific and postembryonic development › Metamorphosis and larval development › Insect larval forms and instars*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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