As explained at Henderson State University in Arkansas, the Monarch (Danaus plexippus) is the dangerous one, just poisonous enough to make birds ill but not kill them. The Viceroy benefits from mimicking the Monarch's color pattern, but only taste bad:
Another butterfly, smaller than the Monarch but mimicking its color pattern, can be found in areas inhabited by the Monarch. Compare the left photo of the Monarch to the right photo of its mimic. This mimic, called a Viceroy (Limenitis archippus), does not feed on milkweed and is not toxic. It would be acceptable food for a bird, but if the bird has already learned not to eat a Monarch, the bird will leave the Viceroy alone as well. If a bird ate a Viceroy first, it might not learn that the color pattern is meant to be a warning. That would make the mimicry less effective, so there are fewer Viceroys than Monarchs. Lower numbers of Viceroys increase the chances that the first contact a bird has would be with a Monarch.Nature. WOW! Double rainbow, man. Double rainbow!
Elizabeth SanFilippo explains at Gather how this butterfly issue relates to Lie to Me's "Double Blind" episode.
... (Tricia Helfer's character) Naomi (is) a Viceroy butterfly, mimicking the poisonous Monarch butterfly so predators stay away. Since Naomi is so skilled at deception and Lightman has difficulty reading her, he realizes it’s in his best interests to stay away, too. Although that doesn’t stop him from giving Naomi time to escape.I don't get to watch this show the way I used to. I caught this episode via cable's On Demand.