I almost didn't see this caterpillar because at first glance it looked very much like the brown, curled-up edge of a beat-up, tattered oak leaf about to fall to the ground. In other words, it's camouflaged to be hard to see on a tree leaf in fall! Its head is at the bottom so notice that right behind the head there's a green area. Old, ready-to-fall oak leaves often bear both brown and green splotches, so this caterpillar even takes that into account!
What's amazing here is that this caterpillar, the Wavy-lined Emerald Caterpillar, Synchlora aerata, is camouflaged with pieces of the Zinnia blossom it is eating. The grainy, orange-yellow items sticking to the caterpillar's back are anthers removed from the Zinnia's disk flower. You can see some of them in the larger picture. Actually, Wavy-lined Emerald Caterpillars are not so uncommon in North America. They metamorphose into a greenish moth you sometimes see. The caterpillars eat on many kinds of flowers, not just Zinnias, so their camouflage covering may consist of many things other than Zinnia anthers, giving the caterpillar many possible colors and shapes, but always looking amazingly like the flower it is eating!
What do you see at the right? No, not a messy bird-dropping, but a caterpillar that looks like a bird dropping, even so far as having white patches on it simulating the white paste coating many bird droppings, which is mostly uric acid, as explained on our famous Bird Excretion Page. Well you can see why a caterpillar might benefit from looking like bird poop: Birds don't want to eat other birds' excrement.
Of course caterpillars don't think out any of this camouflage business. Millions of years of evolution simply caused caterpillars to have these particular shapes, colorations and behaviors. For, the easier the caterpillars were for birds and other predators to see them, the more they got eaten, and the eaten caterpillars didn't survive to produce moths that would produce the next generation of caterpillars that were easy to see...
You may be interested in reviewing books about caterpillars available at Amazon.com in the US, Canada and the UK by clicking here.