When Dave and I saw a couple of orange butterflies flitting near us on the access driveway while we were checking the bluebird nestboxes on our prairie trail on Sunday, we were excited, thinking they were monarch butterflies. Monarchs, of course, have seen precipitous declines over the last few years due to a variety of factors including reduced availability of their essential food, milkweed.
Being the careful reporter I try to be, and knowing that there is another butterfly that looks very like the monarch, I did a little research before posting these photos. And it was a good thing I did, because this is a viceroy butterfly, not a monarch.
Here (below) is a photo of an actual monarch that was visiting the purple coneflowers in our front yard in 2010:
The monarch has more large light spots on the leading edges of its forewings and more small spots along the trailing edges of its wings, and the viceroy has a narrow line across its hindwings, which the monarch does not have and which is probably the easiest field mark to go by. The monarch also has pale lines across its body, while in the top photo of the viceroy, above, you barely even notice the uniformly dark body. The monarch is also slightly larger than the viceroy, but the difference is hard to tell without a side-by-side comparison in the field.
Viceroys feed on poplar, cottonwood and willow trees, and like the monarchs have (relatively recently) been found to be distasteful to bird predators, due at least in part to their bodies' retention of the salicylic acid found in their food. Their similarity to the monarch is now being argued to be a likely example of Müllerian mimicry, with each unpalatable species benefiting from its similarity to the other, rather than Batesian mimicry, where a palatable or nondangerous species benefits from its similarity to a unpalatable or dangerous species. (For more on the mimicry issue, see http://www.britannica.com/blogs/2011/05/mutual-mimicry-viceroy-monarch.)
In my research I came across a wonderful Minnesota nature blog I hadn't discovered before: Backyard Biology, authored by a mother-daughter pair (one a recently retired biology professor and the other a former biology major, nurse, and at-home mom). Their viceroy-monarch comparison appears here. They've recently been writing about the importance of prairies, as I have, with beautiful photos. Check it out.