Nineteenth-century American entomologist and botanist Thaddeus W. Harris called the red spotted purple “the most beautiful of our butterflies that I have seen,” when he first observed one in Milton.

The red spotted purple is one of two varieties of brush-footed butterflies that have the scientific name Limenitis arthemis. The other is the white admiral. Collectively, they have been called red spotted admirals, an appropriate amalgamation of their names. The adage that “compromise makes nations great and marriages happy” must also apply to lepidopterists, scientists that study butterflies.

These two types of butterflies are distinguished at the subspecies level, with the tertiary names astyanax and arthemis, respectively, added to their scientific name. When several variant forms exist within the same species, as in this case, the species is called polytypic.

Although these two varieties are the same species, they look distinctly different and have their own natural ranges. 

The red spotted purple is much more likely to be seen on this Island (though less common on Nantucket) and in states to our south. It is a medium-sized (three-inch wingspan) butterfly that resembles a swallowtail without the tail. It has dark wings with blue, red and white dots on the upper wing’s edges, and is quite plentiful on the Island. These butterflies fly until mid to late September and overwinter as larvae.

By contrast, it is unusual to see white admiral butterflies on the Island, as they are more commonly found in states to our north. In fact, the late birder and naturalist Vern Laux reported the only recent Martha’s Vineyard sighting back in 2000. 

White admirals are slightly smaller and have a distinctive thick white band — perhaps like an admiral’s epaulets or shoulder-board — on the top side of their wings. Off-Island, especially out in western Massachusetts, this subspecies is more common. 

Massachusetts is a ‘blend zone’ between the two forms — states to our north are home to white admirals and southern states shelter red spotted purples. 

The interesting part of being in a blend zone is the opportunity to see firsthand the result of the two subspecies coming together, mingling and reproducing. One such ‘intergrade’ (as they are called) has been documented on Martha’s Vineyard, in 2001. These intergrades have partial white banding, or at least traces of white banding, on the wings of red spotted purples, or red spots on the dorsal hind wings of the white admiral.

Where the rubber hits the road and butterfly enthusiasts linger are where ‘intergrades’ or hybrids exists. For a naturalist in between the butterfly zones, the opportunity to see something rare or special is always is a possibility. Thus, when a dead red spotted purple was brought in the Felix Neck Sanctuary last week, it was carefully examined for traces of the bands of a white admiral. No such luck this time.

Perhaps the lessons of this butterfly are to look closely, appreciate our similarities and differences, and celebrate the specialness in between.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown, and author of Martha’s Vineyard: A Field Guide to Island Nature.