A tale of four swallowtails

Black Swallowtail butterflies have frequented the backyard garden every summer, laying their eggs on the dill.  They usually arrive early and stay well into late summer, so there might be as many as two crops of Black Swallowtail caterpillars in the backyard (see an earlier post on this subject).

A Black Swallowtail butterfly lapped up nectar from early spring blooming Dutchman’s breeches.  Note double row of yellow spots on the top (dorsal) side of the wing and double row of orange spots on the under (ventral) side of the wing.

I wasn’t surprised to see what I thought was a Black Swallowtail butterfly zooming around the wildflower garden, nectaring on the coneflowers the other day, and went out to get some photos.

But the markings are not quite right for a Black Swallowtail butterfly.  The large whitish-orange splotches on the outer margins of the wing are only very small dashes of color.  

A large blue area showing on the hind wing and two rows of yellow dashes means this is not a Black Swallowtail, but the black form of the Eastern Tiger Swallowtail instead.

Black Swallowtail, Spicebush Swallowtail and Pipevine Swallowtail butterflies co-occur along with the black form of the Eastern Tiger Swallowtail throughout most of their ranges in the midwestern and eastern parts of the U.S.  The four species are part of a large Batesian mimicry complex, with the Black, the black form of the Eastern Tiger, and Spicebush butterflies copying the color pattern of the unpalatable Pipevine Swallowtail to protect them from predation. At least three other species, as well as the males of several of the six species mimic the female Pipevine’s color patterns.

The model — the unpalatable Pipevine Swallowtail.  Photo by Bob Moul.

It is thought that the more palatable female butterflies, in particular, profit from mimicking the unpalatable species because they spend more time with their wings extended while laying eggs, and thus increase their vulnerability to predation during that time.  Mimicry works well when predators have experienced the real thing — i.e., the unpalatable Pipevine Swallowtail butterfly.  However, since its range does not include Minnesota (yet), the color patterns of the mimics won’t work to their advantage here.

A different look — for a good reason

I was excited to find what I thought was a different species of swallowtail caterpillar on the dill plants in the back yard the other day — black with yellow spots.

But then a closer look revealed there were just two color morphs of the same Black Swallowtail caterpillar munching away on the dill plants:  a few of the familiar green and white with yellow spots and black bands, along with more of the mostly black with yellow spots variety.

Green (summer version) and Black (fall version) of the Black Swallowtail caterpillar.

Why would a black form suddenly be more common than the green and white form of the caterpillar?  As you might recall from an earlier post on this species (Munching the Dill), Black Swallowtail caterpillars vary quite a bit during their development, changing from black with yellow-spotted 1st and 2nd instars (molts) to the familiar green and white morph in later development.

The first two instars are black, gradually developing spots. The later instars are highly variable in color, with a white, green, brown, or black background.

It seems that caterpillars developing in the early fall weather with its decreasing day length and cooler temperatures have somehow turned off the color change that normally happens between 2nd and 3rd instar development in order to retain their black color.  Searching for an answer to how and why this might be the case, I stumbled on a paper by Wade Hazel that addressed these very questions.

The how of the color difference between the two caterpillar morphs is explained by their exposure to short days during their first two to three instars of development:  if the daylength is 12 hours or less, black swallowtail caterpillars stay black; if daylength is greater than 12 hours, the caterpillars adopt their typical green, white, and yellow coloration.

The answer to why this color change to the black morph occurs in the fall is explained by their ability to absorb more solar radiation, raising their body temperature a degree or two higher than the green morph in bright sunlight, and thus accelerating their development during the unpredictable fall weather.

Dark-colored caterpillars heat up faster and are warmer than paler-bodied ones, when exposed to bright sun.  Data from Wade Hazel’s 2002 paper published in Evolution 56: 342-348.

A black caterpillar can reach the pupal stage 3 days faster than a green one based on these body temperature differences. Reaching the pupal stage is vitally important at this time in the fall because the pupa is the freeze resistant stage that can overwinter until the following spring.

Don’t eat me!

There were a total of 15 Black Swallowtail caterpillars munching on my dill patch this past couple of weeks — not much dill is left.  Now, the mature larvae are about 1 1/2 inches long, and their little round bodies look like the perfect food for the insectivorous birds around here.  But the birds leave them alone.  Why?

Don’t I look tasty?  No, as any bird brain can tell you, yellow and black means “stay back!”

True to that old saying, “you are what you eat”, swallowtail and other caterpillars sequester their host plant’s noxious chemicals in their own bodies, which makes them unpalatable.  (It’s not just the Monarch Butterfly that can do this!)  And they have an added defense just under the skin of the back of their head where two fleshy projections emerge, when the head is touched lightly, to spray bad-tasting terpenes in the face of the would-be predator.   I discovered this interesting feature by accident and managed to photograph the 1 second deployment of the caterpillar’s “horns”.

Bright orange warning flags go up when the head is touched lightly. Unfortunately the display only lasts a couple of seconds.

Unfortunately (for the photographer), the wind was blowing and it was rather cold for caterpillars, so these warning displays were not as active as they might have been.

The first caterpillar got tired of me poking it in the head, so I tried a different individual, who was really too cold to give the flag raising much effort.

The younger larvae of this caterpillar apparently don’t use this chemical defense, and are not as unpalatable because they haven’t eaten enough of the host plant. So they mimic a bird dropping instead, hoping to avoid detection.  In addition, spines projecting from their skin are irritating to tender bird mouths.

Photos by Bob Moul

However, by far the most aggressive defense by caterpillars I have read about is that of Lonomia obliqua, the exceptionally hairy larva of the Giant Silkworm Moth.  if the hairs break off and become embedded in human skin, they cause a chain reaction that basically causes the individual to bleed to death internally.

Caterpillar of the Giant Silkworm moth, Lonomia obliqua. Its hairs contain potent anti-clotting agents.  Stay away from this one!!  Photo by Terra Vermelha.

As more tourists visit the Amazon region where this species lives, there are more reports of envenomation by stepping or brushing against the caterpillar, with subsequent hemorrhagic shock and death just days later.  You can read more about this beast here.

Munching the dill

I haven’t seen many butterflies this summer, but they have obviously visited the backyard garden.  Yesterday when I went out to collect some dill for pickle-making, I found these Black Swallowtail caterpillars munching my dill plants.

A female Black Swallowtail must have visited here a couple of weeks ago, because these are almost fully grown larvae that will soon crawl away to pupate.

I watched while these two devoured stem after stem of the dill, eating their way from the ends of the delicate leaves down to the woody part of the stem.  Every now and then, they stopped to eject a little square packet of excreta.

This one is starting off at the tip of one branch that still has a few leaves.

The mouth parts don’t look all that massive, but they make short work of this stem.

The first three pairs of appendages (thoracic area) will develop into the true legs of the adult. The fleshy pads in the abdominal region are stumpy projections that have suction cups to assist locomotion.

Black Swallowtail caterpillars exhibit a variety of color phases during development, as shown in these wonderful photos by Bob Moul.

The first two instars are black with a white band around the middle, gradually developing spots. The last instar is black and yellow, gradually becoming greener as it gets larger. Unlike the Monarch chrysalis, the Black Swallowtail pupa is supported by a silken thread wound around its middle, as well as at the tip of the former abdomen. The male and female are distinctly different in the markings on their hind wings.

Black Swallowtails prefer to lay their eggs on plants in the carrot family, especially dill, Queen Anne’s lace, fennel, parsley.  This first generation of butterflies will mate and lay eggs, but then those larvae (second generation) will pupate over the winter, finally emerging when it becomes warm enough again the following summer to complete their metamorphosis and start the life cycle all over again.  This means that the two generations of caterpillars have different programming for development, some completing their metamorphosis quickly within two weeks, but others entering a dormant phase that can last for months.  Interesting control mechanisms at work here.