I believed I'd seen these before but couldn't remember exactly. After consulting with my entomologist friend, I learned that they are the larvae of a Giant Swallowtail butterfly (Heraclides cresphontes). Then I recalled briefly seeing an adult swallowtail of some type flitting about the lime tree about two weeks ago. It was an impressive butterfly but I couldn't identify it, so I grabbed the camera and shot a few pictures while it was flying about. They aren't great pictures because it wouldn't stop moving, but they do document what is was and the egg laying activity (see photo further down).
Back to the larvae, they are called Orange Dogs. I believe the orange reference is to the fruit because they feed on citrus plants and they clearly aren't orange in color. As far as the dog reference, look at the face on this one. The larva's resemblance to bird poop is probably pretty effective camouflage. They are also distasteful to birds and can emit a foul odor if threatened.
This species was previously quite rare in California. They have spread from the southeast through Texas and the southwest and are now becoming more established here. They reportedly don't do significant damage to plants. I'm relieved at that because it means the big citrus growers won't be targeting it for eradication. Still, commercial citrus groves are probably no good for this species because they are sprayed with so many pesticides. Organically grown citrus in home gardens is probably their best place for egg laying.
The adult is said to be the largest butterfly in North America, with a wing span of up to 6 inches. I can believe it based on the size of the larvae and the one I saw laying eggs in October.The chrysalis will resemble a dried, curled up leaf which may make it difficult for me to find. As an adult it will only live about 2 weeks, but that should be long enough for it to find a mate and lay more eggs.
A funny thing about this photo is that there is already a larva on a leaf in the upper right corner of the photo. I never saw it when I was taking the photo because I was focused on the butterfly, and I never saw it when reviewing the photos. I only saw it after cropping the photo to include it here. It makes me think about how many interesting thing I probably miss every day.