southern armyworm (Spodoptera edidania).
I've actually seen a lot of small caterpillars skeletonizing the leaves of my tomato. Apparently, they're the younger larval instars of this guy. Most caterpillars go through several larval instar stages, which can make it difficult for folks like me to definitively identify them. This guy's darker coloration suggests that he (or she) is a later instar stage.
Speaking of identifying caterpillars, it looks like I misidentified this caterpillar. It's actually a tobacco hornworm (Manduca sexta) and not a tomato hornworm (Manduca quinquemaculata). Tomato hornworms have eight white V-shaped markings on their side while tobacco hornworms have seven diagonal white lines.