An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

, such as

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...

, between each

moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...

(''ecdysis''), until

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definitio ...

is reached. Arthropods must shed the

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After shedding their exoskeleton (moulting), the juvenile arthropods continue in their life cycle until they either pupate or moult again. The instar period of growth is fixed; however, in some insects, like the salvinia stem-borer moth, the number of instars depends on early larval nutrition. Some arthropods can continue to moult after sexual maturity, but the stages between these subsequent moults are generally not called instars. For most insect species, an ''instar'' is the developmental stage of the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

l forms of

holometabolous Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the superorder Endopterygot ...

(complete metamorphism) or

nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are ty ...

al forms of

hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode of development of certa ...

(incomplete metamorphism) insects, but an instar can be any developmental stage including

pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...

imago In biology, the imago (Latin for "image") is the last stage an insect attains during its metamorphosis, its process of growth and development; it is also called the imaginal stage, the stage in which the insect attains maturity. It follows the f ...

(the adult, which does not moult in insects). The number of instars an insect undergoes often depends on the species and the environmental conditions, as described for a number of species of Lepidoptera. However, it is believed that the number of instars can be physiologically constant per species in some insect orders, as for example

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

and

Hymenoptera Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...

. Yet, the number of larval instars is not directly related to speed of development. For instance, environmental conditions may dramatically affect the developmental rates of species and still have no impact on the number of larval instars. As examples, lower temperatures and lower humidity often slow the rate of development - an example is seen in the lepidopteran

tobacco budworm ''Helicoverpa armigera'' is a species of Lepidoptera in the family Noctuidae. It is known as the cotton bollworm, corn earworm, Old World (African) bollworm, or scarce bordered straw (the lattermost in the UK, where it is a migrant). The larvae f ...

and that may have an effect on how many molts caterpillars will undergo. On the other hand, temperature affects the development rates of a number of hymenopterans without affecting numbers of instars or larval morphology, as observed in the ensign wasp and in the red imported fire ant. The number of larval instars in ants has been the subject of a number of recent investigations, and no instance of temperature-related variation in numbers of instars has yet been recorded.

References

* {{reflist Insect developmental biology Arthropod morphology