Every time I show someone a real, live, brand new baby Monarch caterpillar they say the same thing: "I never realized they were so small!"
When they first hatch out of the egg they look nothing like they will in even a few hours from when they start munching away. The photo at left was taken using a 10x diopter magnifier on my macro lens. They are about 1.5mm long and have no coloring.
When they hatch they will most often eat their egg shell first and then start nibbling at the underside of the leaf.
5th (largest), 4th, and 3rd instar caterpillars
What Is An Instar?
An instar is simply the term given to each stage of growth that a caterpillar goes through. Monarch caterpillars go through 5 stages, denoted by molting, or shedding their outer skin, called the cuticle, in order to grow larger. You will know your caterpillar is going to molt when it suddenly stops eating and finds a place to rest in a straight line. Do not disturb your caterpillars when they do this. They are not sick or hurt; they are going to get much bigger! Sometimes they will sit still like this for a very long time--for hours!