One Species, Two Morphs

Sometimes members of the same species can look quite different, usually due to sexual dimorphism. For the female Eastern Tiger Swallowtail though, butterflies of the same sex can also look very different.

Here is the dark morph of the Eastern Tiger Swallowtail. All dark morphs are females, and this morph is usually seen later in the season, August and onward. It actually has the underlying pattern that the yellow morph Eastern Tiger Swallowtails (see below) has, but instead of a black-on-yellow pattern the dark morph is more like a dark-black-on-light-black pattern. The dark morph is considered to be participating in Batesian mimicry, since they resemble the poisonous Pipevine Swallowtail.

July 28, 2021 at Sourland Mountain Preserve
Photo 148654982, (c) jpviolette, some rights reserved (CC BY-NC)

I didn't see a yellow morph female Eastern Tiger Swallowtail on this trip, but the yellow morph female Eastern Tiger Swallowtail looks fairly similar to the males. They're yellow-and-black, though the females additionally have some of the blue along the edge of the back wings, similar to what you see in the dark morph photo above.

Morph-talk: 
  • The yellow morphs of the Eastern Tiger Swallowtail exhibit some sexual dimorphism between the males and the females due to the bluish spots on the females. 
  • The males (all yellow morphs) and the female dark morphs exhibit fairly extreme sexual dimorphism.
  • And the yellow morph females and the dark morph females exhibit dimorphism, which is just polymorphism with only 2 morphs.

July 28, 2021 at Sourland Mountain Preserve
Photo 148655145, (c) jpviolette, some rights reserved (CC BY-NC)


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