Friday, April 26, 2013

Instars

3rd instar Imperial Moth (TJ Couch)
Merriam Webster defines instar as:  a stage in the life of an arthropod (as an insect) between two successive molts; also : an individual in a specified instar

What does that mean for caterpillars? Caterpillar skin can only stretch so far and caterpillars grow in size at a remarkable pace. Once their skin can no longer stretch to accommodate them they must shed their skin in a process called molting. Underneath their skin, a new layer of skin has been forming that is larger than needed, giving the caterpillar a little room to grow. The caterpillar's skin will split and the larva will squeeze and walk out of his old skin. 


A whole new look
The shiny new skin of the caterpillar may look just like their old skin, or it can look totally different  Some caterpillars change dramatically from instar to instar.

Spicebush Swallowtail 4th and 5th instars

For instance, the Spicebush Swallowtail caterpillars pictured here are only a few days of difference in age. This species starts it's time as a caterpillar as a dark brown, tiny larva. After their third molting their skin is green and at their fifth molting their new skin is a brilliant pinkish orange. 

Each color change provides a benefit for this particular caterpillar. while small they are brown and look a bit like a dark spot on a leaf. While green they are extremely hard to see when wrapped up in bright green leaves. In their last stage their bright orange serves as a warning color and helps to ward away predators.

1st instar Imperial Moth larvae (TJ Couch)
A gauge of growth
Lepidopterists use instars to track the growth of caterpillars and to better understand their development. 

Here at MOSI Outside instars help us to track the growth of caterpillars and project when we will need to collect more eggs or caterpillars as previous larvae grow and ready themselves for chrysalis. It's also a lot easier to say "Please pull down the fifth instar Spacebush" that to request "Could you hand me those crazy huge neon orange caterpillars?".

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