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A Monarch’s First Instar

Monarchs, like other butterflies and moths, undergo complete metamorphosis — they have an egg, larva (caterpillar), pupa (chrysalis), and adult stage. During a monarch’s larval stage it eats almost constantly, pausing only to shed its skin. The period between each shedding of the skin, or molt, is called an instar.  Monarchs have five larval instars and during their larval stage grow to almost 2,000 times their original mass.

The first meal a monarch larva has is its eggshell and it quickly moves on to milkweed leaf hairs.  Soon thereafter it begins to eat the leaf in earnest, often making an arc-shaped cutting. 

During this first instar, which typically lasts one to three days, the larva’s appearance changes considerably. When it hatches, the monarch larva is pale green or grayish-white, shiny, and almost translucent. It has no stripes or other markings. It’s about 2 mm long, with front tentacles appearing as tiny little bumps. Its back tentacles are barely visible.  By the end of the first instar it begins to have a pattern of black (or dark brown), yellow and white bands, and the 6mm-long body no longer looks transparent and shiny.

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Caterpillars Eating & Molting

The larval stage of a butterfly or moth is spent doing little but eating.  Only as a caterpillar will these insects have chewing mouthparts, and they waste no time in using them. As they eat, caterpillars increase in size and their skin (exoskeleton) becomes tighter and tighter, as it doesn’t grow larger.  The caterpillar grows a new, larger exoskeleton underneath the outer skin and then sheds, or molts, the old one. Most caterpillars molt five times.  At first, the new exoskeleton is very soft and not very protective, but it soon hardens. The shed exoskeleton is often eaten before the caterpillar ingests more plant food.

There are names for the caterpillar’s stage of development in between each molt, called “instars.”  When the caterpillar hatches from its egg, it is referred to as a “first instar” caterpillar.  After its first molt, the caterpillar is referred to as a “second instar,” and so on up until the exoskeleton is shed for the final time, revealing the chrysalis (if it’s a butterfly).

The Monarch in the photograph is a very new 4th instar instar caterpillar (see antennae which haven’t hardened).  It has shed three times.  Its third exoskeleton (which it has just shed) is on the milkweed leaf behind the caterpillar. To see a real-time video of a Monarch molting go to

(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin.  Also to Ba Rea (( for her instar confirmation.)

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