Every year I have these caterpillars on my citrus trees and i enjoy watching them grow. They start off as tiny things, less than a quarter of an inch, and resemble bird droppings.  Later in their development, they resemble a snake complete with a faux tongue.  After reading up on them, the bird dropping resemblance is camouflage to help them hide from predators and the snake resemblance is disguise to keep it form being eaten.  If you look carefully at the one in the middle of the picture you will notice what looks like red horns or a snake's tongue.  It is normally hidden (I touched this one for the photo), but when the caterpillar feels threatened, it will deploy the tongue and also emit a foul smell to try to repel predators.  The tongue-like organ is called an osmeterium and all species of this particular caterpillar have one, what is unique is that each species has a slightly different color.  Wiki has this to say about it:

"The first three instars of the caterpillar are bird dropping mimics, coloration that helps protect it from predators. In later instars, the eyespots on the thorax serve to deter birds. Like all members of the family Papilionidae, the caterpillar of P. glaucus possesses an osmeterium, an orange, fleshy organ that emits foul-smelling terpenes to repel predators. Normally hidden, the osmeterium is located on the first segment of the thorax, and can be everted when the caterpillar feels threatened. The combination of eyespots and osmeterium makes the caterpillar resemble a snake."

For those who would like to know, the snipit from Wiki mentions "instars".  An instar is the phase of the larvae (caterpillar) between molts.  This particular caterpillar has five instars.

In case you are wondering, here is the butterfly that this caterpillar will eventually become:
This is the Giant Swallowtail, papilio cresphonte, the largest butterfly in North America with a wingspan of four to six inches.  It preferrs citrus for depositing its eggs.

I like watching these develop and the damage to the plant is minimal so I do not remove or kill them.  Often, I call a friend who raises butterfles and she comes and retrieves them, raises them, and releases the butterfles.