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 https://www.youtube.com/watch?v=EbHyq3RwtxI.
(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin. Also to Ba Rea ((www.basrelief.org) for her instar confirmation.)