Thursday, August 5, 2010

A few butterflies

Eastern Tiger Swallowtail, Papilio glaucus, male. Females have much more blue coloration on the hindwing. In our neck of the woods, male "tigers" are absolutely unmistakable and a very common sight. Swift and powerful of flight, they are often seen coursing high in the forest canopy, and the females often lay their eggs well aloft in Tulip Trees and ash.

While male Eastern Tiger Swallowtails are straightforward enough in terms of identification, not so with some of the females. This is a female, but it's one of the dark morphs. They can be real foolers at first, which is precisely why they look the way that they do. Where I took this shot, in southern West Virginia, it seemed that nearly all of the female tigers were of the dark form.

As one moves northward and out of the range of a certain other butterfly - I'll showcase that one in a bit - yellow or typical forms of the female increasingly dominate. Why? Batesian mimicry. This is the term for non-toxic butterflies that have evolved coloration that resembles highly toxic butterflies. The black-form tigers resemble the Pipevine Swallowtail, Battus philenor, which is full of harmful alkaloids derived from its host plants in the Aristolochiaceae, or Pipevine Family. As predators have learned to avoid Pipevine Swallowtails, the Batesian Mimicry theory states that they will also avoid look-alikes. So, it doesn't do the tigers any good to mimic the poisonous pipevines in areas where the latter is scarce or nonexistent.

After some experience, the dark morph female tigers can be readily told be their large size and distinctive fast flight, and rather acutely angled wingtips. But, a good look such as seen above, and you can make out the telltale tiger stripes bleeding through the dark coloration.

A pair of Pipevine Swallowtails, caught in flagrante delicto. This is the poisonous beast that the female tigers are mimicking. Ohio is near the northern limits of the range of pipevine swallowtails, and probably their only option for a host plant here is the inconspicuous woodland herb Virginia Snakeroot, Aristolochia serpentaria. This plant is rather uncommon and scattered, and if you know it, you can better appreciate the plant-finding abilities of the butterfly.

In West Virginia, where the above photo was taken, the large high-climbing Pipevine, Aristolochia macrophylla, becomes abundant and so does the swallowtail. And the incidence of Batesian Mimicry in female Eastern Tiger Swallowtails spikes sharply.

Finally, without a doubt one of North America's flashiest butterflies, the Red-spotted Purple, Limenitis arthemis. Believe it or not, this one is closely related to the Viceroy, L. archippus, which is the Monarch look-alike. The two species will even hybridize, and the result is something I hope to see firsthand someday.

Red-spotted Purples are rather catholic in their tastes, hosting on cherries (Prunus), and a variety of species in the Salicaceae Family, which includes willows, aspens, and cottonwoods. Thus, and luckily for us, the purples are common.


Ricky the Sheltie said...

We walked the Cedar Falls trail in Hocking Hills on Monday and saw more tiger swallowtails and red spotted purples than ever before. They were everywhere! Is it a big year for them?

Randy Emmitt said...

Enjoyed this posted and it was well written and beautifully illustrated to boot. I've seen thousands of Pipevined but can not recall ever seeing a mated pair.

Ricky here in NC were are booming with Eastern Tiger Swallowtails also, just did a post about it.

Russell Reynolds said...

I never knew that the tiger swallowtail female was dark like that. I see those i think they are black swallowtails. I will be lookin' these guys over better now.