Monday, July 17, 2006

Back in Black

An interesting find we had on our field trip on Saturday was a black form female Eastern Tiger Swallowtails. Most of us are probably familiar with the Tiger Swallowtail. I consider it the other poster child of the butterfly world (the first being the Monarch). A typical male or female is a stunning yellow and black as seen in the picture below. Some females are black though. In some areas of the northern US, most females will be black. These black forms tend to be more prominent in areas with more Pipevine Swallowtails which they are thought to mimic.

Distinguishing the black form from the other black swallowtails of the east can be a bit of a challenge. The key difference to me is that the Tiger Swallowtail retains a "shadow" of the yellow and black pattern on the underside of the forewing as evident in the picture below. The other "black" swallowtails are more "black".

The hind wings of the black form are powdery blue above with a black band dividing the blue areas. This band is absent in the sometimes confused female Spicebush Swallowtail. Pipevine Swallowtails have a single row of orange spots below like the Tiger Swallowtail, but also have a large patch of irridescent blue-green. Spicebush Swallowtail and Black Swallowtail have two rows of orange spots below.

Above the Eastern Tiger Swallowtail black form female has a very thin row of white spots on the upper forewing margin that is easily spotted in the field. No other "black" swallowtail in the east has these markings.

Female Tiger Swallowtail - Notice the tiger pattern shadow and the row of white spots on the forewing

Pipevine Swallowtail - Notice the irridescent patch

Black Swallowtail - Notice two rows of spots on hindwing below

Female Spicebush Swallowtail - No black line dividing the blue patch on the hindwing above

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