Instar

Instar is a term in entomology which refers to the developmental stage of arthropods, such as insects, between each molt until sexual maturity is reached. 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 moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again. This period of growth between moulting is called an instar.

Etymology

The term "instar" comes from the Latin verb instare, meaning "to stand upon or near", and was coined by Louis Agassiz in 1846. It is a calque of the German word Stadium, which refers to a stage or period of development.

Instar Stages

Insects, being Ecdysozoa, must shed their exoskeleton in order to increase in size. This process is called ecdysis, and each stage between periods of moulting for insects in the taxon endopterygota is called an instar, or stadium, and each stage between periods of moulting in the rest of the insects is called a nymph. The number of instars an insect undergoes often depends on the species and the environmental conditions, as some insects are known to add more instars when food is abundant.

Instar Determination

The determination of instars is made based on the time at which a moult occurs. The instar period of growth is fixed and does not change, regardless of the immediate physical condition of the insect. However, in some insects, a process called supernumerary instar can occur, where an extra instar will develop if the insect has been severely damaged, or if it has not reached a certain size by the time it reaches its last scheduled moult.

See Also

• Arthropod
• Ecdysis
• Louis Agassiz
• Supernumerary instar

References

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