This second instar has just molted. Once they have filled with fluid (which takes about an hour), they will often turn around and eat the skin they have just shed. The head capsule from the molt can be seen to the right of the caterpillar's head.
Second Instar Trenching
This second instar is stemming the flow or "trenching" in order to feed on the milkweed leaf. The dark spot clinging to the edge of the leaf is frass, or caterpillar droppings.
Second Instar Feeding Pattern
This second instar caterpillar has a similar feeding pattern the a first instar. They will eat layers of the leaf only occasionally eating a hole through the surface of the leaf.
Second Instar ~7 Millimeters
This early second instar caterpillar is approximately 7 millimeters. The black, yellow and white striped pattern is now obvious.
Second Instar Late Stage with Dropped Head Capsule
This second instar will soon molt and become a third instar. Look for a dropped head capsule as well as little movement for a day or two prior to a molt. Note the empty egg casing in front of the caterpillar. First instars will often only eat part of the egg when they hatch.