Monarch or Viceroy? How to Tell the Orange Butterflies Apart

By late summer the orange butterflies are everywhere — nectaring on goldenrod, gliding over meadows, gathering strength for the long journeys ahead. Most people call every big orange-and-black butterfly a monarch, but a good share of them are actually viceroys, a separate species that has evolved to look almost exactly like the monarch. The resemblance is so close that even careful observers second-guess themselves. The good news is that one field mark tells them apart instantly, from either side, whether the wings are open or shut.

The One Mark That Settles It: the Hindwing Line

Forget size and color for a second and look at the lower wings (the hindwings). A viceroy has a bold black line that arcs across the veins of each hindwing. A monarch does not have this line. That single black crossbar is the giveaway, and once you know to look for it you will never be stuck again:

• Viceroy: a distinct black line curving across each hindwing, cutting across the orange veins
• Monarch: hindwing veins are outlined in black, but there is no extra line crossing them

This works on a resting butterfly with its wings folded up and on one basking with wings spread, which makes it far more reliable than trying to judge size on a butterfly that will not hold still.

Other Ways to Tell Them Apart

If you get a longer look, several other clues line up with the hindwing test:

• Size: monarchs are noticeably larger, with a wingspan around 3.5 to 4 inches; viceroys are smaller at roughly 2.5 to 3 inches
• Wing edges: the viceroy’s wings look a little more scalloped and angular along the margins
• Flight style: monarchs sail and glide with the wings held up in a shallow V; viceroys tend to flap quickly and then glide flat
• Behavior: in fall, big loose groups of orange butterflies streaming or clustering together are almost always migrating monarchs, since viceroys do not make that mass migration

Why the Viceroy Copies the Monarch

The mimicry is not an accident. Monarchs are toxic to birds because their caterpillars eat milkweed and store its bitter compounds, so a predator that gets sick on one monarch learns to avoid the orange-and-black pattern. For a long time the viceroy was considered a harmless copycat cashing in on that reputation. Researchers now think both butterflies are genuinely distasteful to predators, so they reinforce each other’s warning coloring — a partnership rather than a bluff. Either way, the shared costume is why they look so alike.

Their life histories differ, though. Viceroy caterpillars feed on willow, poplar, and cottonwood rather than milkweed, and viceroys stay put for the winter as partly grown caterpillars instead of flying south. So the butterflies you see funneling through in September and October, headed for Mexico, are the monarchs.

A Note on the Fall Monarch Migration

Late summer into fall is the best time of year to practice this ID, because it is exactly when monarchs are on the move. The generation that emerges now is the one that migrates thousands of miles to overwintering grounds, so a meadow full of goldenrod and asters can be a genuine spectacle. Learning to pick the viceroys out of the crowd makes the show even better.

Not sure whether the butterfly on the goldenrod is a monarch, a viceroy, or one of their other look-alikes like the queen or soldier? Snap a photo and let Spotter identify it, and add another wingbeat of the meadow to what you can name on sight.