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An instar (, from the Latin ''
īnstar
īnstar
'', "form", "likeness") is a developmental stage of
arthropods An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart fr ...

arthropods
, such as
insects Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, Thorax (ins ...

insects
, between each
moult In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
(''ecdysis''), until
sexual maturity Sexual maturity is the capability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxono ...
is reached. Arthropods must shed the
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
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 moulting, i.e. shedding their exoskeleton, 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 animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Ove ...
l forms of
holometabolous Holometabolism, also called complete metamorphosis Metamorphosis is a biological process by which an animal physically develops after birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical c ...
(complete metamorphism) or
nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Gree ...
al forms of
hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete Metamorphosis (biology), metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode ...
(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 insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropo ...

pupa
or
imago In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

imago
(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 (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings ...

Diptera
and
Hymenoptera Hymenoptera is a large order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or wa ...

Hymenoptera
. 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
tobacco budworm
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