instar

{{short description|Developmental stage of arthropods between moults}}

Image:Larval development- Imperial moth.JPG) development from egg to pupa, showing all the different instars]]

An instar ({{IPAc-en|audio=En-us-instar.ogg|ˈ|ɪ|n|s|t|ɑr|}}, from the Latin īnstar 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (ecdysis) until sexual maturity is reached.{{cite book |last1=Allaby |first1=Michael |title=A dictionary of ecology |date=2005 |publisher=Oxford University Press |location=New York |isbn=9780198609056 |page=234 |edition=3rd |url=https://books.google.com/books?id=9780198609056}} Arthropods must shed the 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 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.{{cite journal |last1=Knopf |first1=K. W. |last2=Habeck |first2=D. H. |title=Life History and Biology of Samea multiplicalis |journal=Environmental Entomology |date=1 June 1976 |volume=5 |issue=3 |pages=539–542 |doi=10.1093/ee/5.3.539 }} 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 larval forms of holometabolous (complete metamorphism) or nymphal forms of hemimetabolous (incomplete metamorphism) insects, but an instar can be any developmental stage including pupa or imago (the adult, which does not moult in insects).

Image:Common_mormon_(Papilio_Polyetes)_catapillars.jpg]]

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 {{by who?|date=February 2023}} that the number of instars can be physiologically constant per species in some insect orders, as for example Diptera and Hymenoptera. The number of larval instars is not directly related to the 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 and that may have an effect on how many molts an insect will undergo{{snd}} an example of this is seen in the lepidopteran tobacco budworm.{{Cite web|url=http://entnemdept.ufl.edu/creatures/field/tobacco_budworm.htm|title=tobacco budworm - Heliothis virescens (Fabricius)|website=entnemdept.ufl.edu|access-date=2017-11-09}} 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{{cite journal |last1=Fox |first1=Eduardo Gonçalves Paterson |last2=Solis |first2=Daniel Russ |last3=Rossi |first3=Mônica Lanzoni |last4=Eizemberg |first4=Roberto |last5=Taveira |first5=Luiz Pilize |last6=Bressan-Nascimento |first6=Suzete |title=The preimaginal stages of the ensign wasp Evania appendigaster (Hymenoptera, Evaniidae), a cockroach egg predator |journal=Invertebrate Biology |date=June 2012 |volume=131 |issue=2 |pages=133–143 |doi=10.1111/j.1744-7410.2012.00261.x |doi-access=free |bibcode=2012InvBi.131..133F }}{{cite journal |last1=Bressan-Nascimento |first1=S. |last2=Fox |first2=E.G.P. |last3=Pilizi |first3=L.G.T. |title=Effects of different temperatures on the life history of Evania appendigaster L. (Hymenoptera: Evaniidae), a solitary oothecal parasitoid of Periplaneta americana L. (Dictyoptera: Blattidae) |journal=Biological Control |date=February 2010 |volume=52 |issue=2 |pages=104–109 |doi=10.1016/j.biocontrol.2009.10.005 |bibcode=2010BiolC..52..104B }} and in the red imported fire ant.{{cite journal |last1=Porter |first1=Sanford D. |title=Impact of temperature on colony growth and developmental rates of the ant, Solenopsis invicta |journal=Journal of Insect Physiology |date=1988 |volume=34 |issue=12 |pages=1127–1133 |doi=10.1016/0022-1910(88)90215-6 |bibcode=1988JInsP..34.1127P }}{{cite journal |last1=Fox |first1=Eduardo Gonçalves Paterson |last2=Solis |first2=Daniel Russ |last3=Rossi |first3=Mônica Lanzoni |last4=Delabie |first4=Jacques Hubert Charles |last5=de Souza |first5=Rodrigo Fernando |last6=Bueno |first6=Odair Correa |title=Comparative Immature Morphology of Brazilian Fire Ants (Hymenoptera: Formicidae: Solenopsis) |journal=Psyche: A Journal of Entomology |date=2012 |volume=2012 |pages=1–10 |doi=10.1155/2012/183284 |doi-access=free |hdl=11449/73193 |hdl-access=free }} The number of larval instars in ants has been the subject of a number of recent investigations,{{cite journal |last1=Fox |first1=Eduardo G. P. |last2=Smith |first2=Adrian A. |last3=Gibson |first3=Joshua C. |last4=Solis |first4=Daniel R. [UNESP |title=Larvae of trap jaw ants, Odontomachus LATREILLE, 1804 (Hymenoptera: Formicidae): morphology and biological notes |journal=Myrmecological News |date=1 October 2017 |pages=17–28 |hdl=11449/163472 }} and no instance of temperature-related variation in numbers of instars has yet been recorded.{{cite journal |last1=Russ Solis |first1=Daniel |last2=Gonçalves Paterson Fox |first2=Eduardo |last3=Mayumi Kato |first3=Luciane |last4=Massuretti de jesus |first4=Carlos |last5=Teruyoshi Yabuki |first5=Antonio |last6=Eugênia de Carvalho Campos |first6=Ana |last7=Correa Bueno |first7=Odair |title=Morphological Description of the Immatures of the Ant |journal=Journal of Insect Science |date=March 2010 |volume=10 |issue=15 |pages=15 |doi=10.1673/031.010.1501 |pmc=3388976 |pmid=20575746 }}