I watched the caterpillar chew an outline into the leaf (you can see it in the above photo) and when I came back the section of leaf had been entirely removed. With a voracious appetite, the little caterpillar is about 4x as big as it was last Wednesday. It's also taken on the typical coloration of monarch caterpillars. I wonder if in their first instar (each molt of the exoskeleton is called an instar) they don't have any warning colors because they haven't ingested any toxin from the milkweed yet. What's interesting is that two other species that depend on milkweed in their life history also have bright colors - milkweed or oleander aphids (Aphis nerii) and small and large milkweed bugs (Oncopeltus fasciatus and Lygaeus kalmii, respectively). Neither of these have appeared on the milkweed yet this year, but were present last year - maybe they'll show up later in the summer.
Etymology notes: Danaus comes from the Greek mythological figure who had 50 daughters (monarchs have multiple generations in a year). He was the twin brother of the more well known Aegyptus (they were descendents of Io, the woman the ever-womanizing Zeus turned into a heifer to hide from the ever-jealous and vengeful Hera). Aegyptus went on to become the king of Egypt and sired a convenient 50 sons, all of whom - save one - were killed by the Danaids (the collective name for Danaus's 50 daughters, like a flock of crows is a murder, buzzards a wake, coyotes a band, and house cats a clowder - more here).
While there are several Plexippuses found in Greek mythology, it's not difficult to narrow down the namesake to the Plexippus that was one of Aegyptus's sons. The sons drew straws (well, sort of) to marry one of the daughters. Danaus had no interest in letting his daughters marry his twin brothers sons so they 51 of them fled to Argos. The Aegyptus and his 50 jilted sons tracked them down. Danaus readily submitted, not wanting harm to fall upon Argos, but told his daughters to kill their husbands on the first night of their marriage. Plexippus married - and was murdered by -Amphicomone
One daughter, Hypermnestra, spared her husband, Lynceus, as he alone respected his wife's desire to remain a virgin. As you can imagine, he was pretty irate at the fate of his brothers and killed Danaus. He and Hypermnestra remained wedded and started the lineage of rulers of Argos (which includes such notables as Perseus). The other daughters chose new husbands by holding footraces.
Other milkweeds or tigers (the common name for members of the Danaus genus) have specific epithets (the second part of the latin name) that derive from other sons of Aegyptus (like chrysippus).