Viceroy: top, Monarch: bottom.

They look alike, but in fact they both taste awful to a red-wing blackbird. For almost a century, the "mimicry" explanation was so convincing that nobody tested it.

Viceroy copyright Paul A. Opler, Courtesy USGS Northern Prairie Wildlife Research Center. Monarch copyright 1996-1997, courtesy Bill & Marty Welch.

Monarch among the misconceptions
It's no accident that viceroy butterflies look like monarchs. According to a scientific fable that survived almost 100 years, monarchs taste lousy because they eat toxin-rich milkweed, so birds learned to avoid them. Viceroys evolved to have a similar appearance because birds avoided eating viceroys that looked like monarchs.

The fable was considered a classic example of evolution at work -- of how organisms change to suit their environment.

This so-called "Batesian" mimicry involves a model, the monarch, and a mimic, the viceroy. It stands to reason that a vulnerable butterfly would evolve to look like something that was immune to a major predator.

Eventually, the pretty theory was skewered by some ugly facts. Standing to reason is not the same thing as truth, and when entomologists ran the experiment, they found Batesian mimicry was not involved. In fact, the viceroy was as unpalatable as the monarch, at least to red-wing blackbirds. After eating viceroy abdomens, the birds often shook their heads, guzzled water, or acted agitated -- all indications that the viceroy tasted, well, like a big bad bug.

In all, the birds totally ate only 41 percent of viceroys, compared with about 45 percent of monarchs and 98 percent of controls.

But the mimicry hypothesis was not totally misplaced. In the "what's-in-it-for-me?" calculus that is ecology, the situation may actually represent Mullerian, not Batesian, mimicry. Both viceroys and monarchs benefit from the similarity in appearance -- it's a more efficient way to "teach" predators that both species taste gross (see "The Viceroy Butterfly..." in the bibliography).

While we're on the subject of bogus biology, you do know Charles Darwin devised his theory of evolution while studying finches at the Galapagos Islands... Wrong again!

You can count on science textbooks for accuracy.