And you thought you had trouble telling one butterfly species from another! Lepidopterists – scientists who study butterflies – have it even worse.
Take the case of the tiger swallowtail butterfly, a rather large, yellow butterfly with black tiger stripes. Each spring and early summer they flutter over the hills and valleys of eastern North America, sometimes in great numbers. But figuring out exactly what is, and what is not, a tiger swallowtail has baffled lepidopterists for more than three centuries.
The tiger swallowtail was the first butterfly to be painted by an artist from the New World. John White painted one on Roanoke Island, North Carolina, in 1587 while serving as the expedition leader of Sir Walter Raleigh’s third trip to America. Despite exaggerating the wing shape, his details were relatively accurate.
Linnaeus, the father of modern taxonomic naming, called the species Papilio glaucus in 1758. For over 150 years thereafter, it was commonly known as the tiger swallowtail. But Linnaeus had actually named the tiger swallowtail from a black-colored female – a rarer version of the species that looks just like the more common yellow females, except that the background color is darkly pigmented. These dark females generally occur from Massachusetts to Florida, the southern portion of the butterfly’s range.
Why are there dark-colored females? They are thought to have evolved to mimic the dark color of the foul-tasting and poisonous pipevine swallowtail butterfly. This is called Batesian mimicry, when one otherwise palatable species evolves to closely resemble an unpalatable cousin.
Linnaeus also observed the more common, yellow-colored female tiger swallowtail but decided it was another species altogether. A contemporary of Linnaeus documented a male tiger swallowtail, which is also yellow, but decided it, too, was its own species.
In the 1800s, biologists realized that the three species were only differentiated by color and lumped them together under the common name, “eastern tiger swallowtail.” Starting in at least 1906, however, lepidopterists noticed that the more northern populations were smaller and had slightly different markings. Some began recognizing this as a subspecies of the eastern tiger swallowtail and called it canadensis.
In 1991, biologists from Michigan State University announced that they had enough evidence to declare canadensis a separate species altogether – the Canadian tiger swallowtail. Their evidence included genetic differences, color and size differences, caterpillar food-plant use, lack of black-colored females in canadensis, and only a very narrow hybrid zone between the two species. It is now widely accepted that eastern tiger swallowtails are found southward and Canadian tiger swallowtails are found northward.
The range of the eastern tiger swallowtail just barely makes it into southern Vermont and New Hampshire. Most of the tiger swallowtails we see around here, therefore, are the Canadian tiger swallowtails.
Generally, Canadian tiger swallowtails are flying around from the beginning of May until the end of June, while eastern tiger swallowtails fly from the beginning of June into October. So if you see a tiger swallowtail sailing over a meadow from mid- to late summer, it just might be an eastern. But even up close, they look very similar. The Eastern is larger, with the underside marginal forewing band broken into yellow dots separated by black borders. On the underside of the Canadian hind wing, the black line nearest the body is very wide. Minute details for sure. Even worse is that, in the hybrid zone between species, there are many that appear intermediate. In Vermont and New Hampshire, we are in the thick of the hybrid zone.
But the story doesn’t end here. In 2002, another potential tiger was described by Harry Pavulaan and David Wright, two avocational lepidopterists. “When it became apparent that there were inconsistencies in the natural history of mountain populations versus lowland populations of tiger swallowtails, an intensive effort was made to study the field biology of the mountain populations,” said Pavulaan. They have named it the Appalachian tiger swallowtail and affectionally refer to it as “Appy”. So far, it is known only from the southern Appalachian Mountains.
Why do tiger swallowtails come in so many varieties that biologists have been trying for centuries to pin them down? Probably because tiger swallowtails have what are known as sibling species – two or more populations that are reproductively isolated from one another yet so similar in outward appearance as to be lumped together even by experts. Careful, intense study of details such as anatomy, biochemistry, and behavior can bring sibling species to light. But there can be many dead ends and tricks that confuse biologists.
If you want to wade into the debate and become an avocational lepidopterist yourself, now is the time. The lovely Canadian tiger swallowtails will be flitting over meadows near you all this month.