This week we consider the official State butterfly emblem of Tennessee, the Zebra Swallowtail. Unlike the South American Kites, so many of which are so little known, a great deal has been learned about the Zebra Swallowtail. Much of the information available has been learned in my lifetime. In fact one reason why the Zebra remains my favorite butterfly, to the extent that I have one, is that it serves as a reminder to scientists to be cautious about thinking they know more than they do. Basically everything I learned about this butterfly in college has turned out to be wrong.
Photo from carolinanature.com, where the author mentions that this individual was found in Virginia. Bodies are mostly black above, mostly white below, with stripes along the sides. Antennae are brown to amber. Wings are vividly striped, making the Zebra hard to ignore, but the stripes vary, as does the size of the butterfly. Hind wings always have tails, often very long tails, when the butterflies first eclose, though one or both tails may be lost later. The proboscis, the long hair-thin hollow tube that is a butterfly's tongue, is long in proportion to the butterfly's head but still much shorter than some Swallowtail butterflies' probosces are.
The Zebra was my favorite species because, among the early spring butterflies I watched, it seemed to have the "just right" attitude toward humans, neither pushing itself forward nor hiding but calmly letting itself be watched. If more information about the butterfly had been available to me, I might not have liked it so much. Zebra Swallowtails behave nicely toward humans but they are asocial and do not behave very nicely toward one another.
Other Swallowtails have gone through a few name changes as scientists decided the genus Papilio was too full and needed splitting up. The Zebra Swallowtail holds a record. It was first named Papilio marcellus n 1779. Since then, people have given it the genus names Boreographium, Eurytides, Graphium, Neographium, Protographium, Iphiclides, Iphidicles, Cosmodesmus, and Protesilaus, and the species and subspecies names ajax, abbottii, annonae, broweri, carolinianus, cubensis, floridensis, lecontei, nigrosuffusa (or nigrosuffusus), pricei, telamonides, tockhorni, and walshi.
Recent changes in the genus name amount to quibbling. The Kites resemble an Asian family of small long-tailed swallowtails, the genus Graphium. Some scientists thought the Kites belonged in the genus Graphium. More scientists disagreed, and debated whether the Kites might have evolved earlier than the Graphiums overseas (Protographium) or later (Neographium). The position of this web site is that living things have evolved and are still evolving, within the range of what is possible for their genotype--what is called microevolution--and Zebra Swallowtail populations give an especially clear and pretty example of microevolution, every year, but if mutant individuals in one species have ever evolved into anything more different than a subspecies or race--which would be called macroevolution--nobody has documented it. Trying to guess how one species might have evolved out of another species is unscientific and not a very useful way to pass time. With the species name Boreographium, however, we come to a name change proposed for valid scientific reasons. Boreo means North, and the Zebra Swallowtail is the northernmost of the Kite species.
Eurytides is a nice descriptive name when read as coming from the Greek words eury eidos, "broad shape," comparing the Zebra Swallowtail's wings with the Zebra Longwing's. It can also be read as "son of Eurytus." Ancient Greek literature records at least three characters called Eurytus. One was a war hero, one a king renowned for his skill in archery, and one was remembered mainly for having a son called Clonus.
Several characters in Greek literature had names that include the word eury, "broad, wide." It seems to have been understood in the philosophical sense, used in combinations like Euryale, "wide sea," Eurybe, "grand strength," Eurydice , "wide judgment, universal law," Eurymachus, "wide battle," and Eurynome, "broad realm."
Iphiclides means "son of Iphicles" in Greek. It's not used as the name of a character, though Iphicles was Heracles' little brother and people might have been proud to claim him as an ancestor. Iphiclides is the genus name of the Eurasian Scarce Swallowtail, which has white wings with black stripes. Iphidicles at least started out as a misprint.
Protesilaus was a hero of Greek literature. According to the Iliad, the troops he commanded had been warned that the first man who disembarked at Troy would be killed in the battle. Protesilaus, whose name comes from protos, "first," and Laos, "the people," deliberately chose to go first and be killed as a sacrifice for his men. His father's name was Iphiclus, possibly a descendant of Iphicles, and his name has been preserved as the species name of another Kite Swallowtail.
Cosmodesmus seems to be the Greek words kosmos, "the universe, creation," and desmos, "band, connection," but Google doesn't find it as a character name in ancient literature. It's the sort of name that might have been invented for a character in eighteenth or nineteenth century fiction, but Google doesn't find the reference.
Some of the species names reflect the tradition of naming Swallowtail species after characters in literature. Marcellus was a popular name in ancient Rome; among the historical characters called Marcellus were some early Christians.
Ajax was a warrior in the Trojan War story. His father's name was Telamon, so he was sometimes called Ajax Telamonides in the Iliad. Linnaeus called the Zebra Swallowtail Papilio ajax, and some people have used "Ajax" as this butterfly's English name.
Of the other names given to this butterfly, abbottii, broweri, lecontei, pricei, tockhorni, and walshi all commemorate people; carolinianus, cubensis, and floridensis obviously refer to places where the butterflies were found (though cubensis may have been recorded in error); annonae describes the plant family that includes its food plant, and nigrosuffusa ("suffused with black") describes the autumn brood as distinct from the spring or summer brood.
The plethora of proposed species names for this species reflects its variability. We shall, if we live so long, meet another North American butterfly whose reaction to temperature produces even greater variation in color patterns, but a majority of individual butterflies conform to either one color pattern or the other. Zebras seem almost as variable as the larger animals for which they're named. Today scientists prefer to remember that, as early as 1973, some scientists were expressing doubts that what are now recognized as the three general ranges of temperature-determined looks ever were separate species.
The butterfly that left egg on the scientists' faces made news around the world. "Protographium Marcellus" is a band name. Zebra Swallowtails occasionally flutter through pop culture, too; photos and paintings of them are sold as posters, and they are sometimes printed onto various objects at Zazzle (there are about a half-dozen Zebra images at my Zazzle store, PriscillaKnits).
Basically, individuals that have lived through some freezing weather are smaller and lighter in color, likely to be described as white butterflies with black or brown stripes. Those who have never been cold in their lives are larger and darker, likely to be described as black butterflies with pale green stripes. In between these extremes is a full range of intermediate sizes and colorations. The length of the "swallow tails" also varies according to temperature; longer tails tend to accompany bigger, darker wings. Body hair also varies; bigger individuals can have hair rather than scales all over their hind wings as well as bodies, while smaller ones look sleek and smooth. Males have scent folds on the inside edges of the hind wings, so especially hairy wings may be typical of male butterflies, though my sources didn't discuss gender differences in the hairiness of this species.
Photo from keysmoths.com. This furry animal was found on an island south of Florida.
Photo from carolinanature.com. This smoother specimen is typical of Zebra Swallowtails found in the Carolinas, Virginia, Tennessee, and further north.
In Florida, the seasons, to the extent that the Florida peninsula has seasons, produce a white spring brood, a greenish summer brood, and a black autumn brood every year. Further north, the bigger and darker forms are seldom seen. All Zebra Swallowtails in Virginia look closer to the spring brood in Florida than to the other two broods; nevertheless, their size and color vary enough that they have been believed to be distinct subspecies rather than temperature-related variants.
Into the 1980s, the three broods observed in Florida were generally thought to be three distinct species, despite a 1973 study that opened questions about this. They are three different generations in the same family lines, but they look like three different species.
Individual variations are visible, not only between our spring, summer, and autumn broods but between siblings whose host trees grew at different altitudes. You can take eggs laid by a big, dark, southern autumn butterfly, put them in the refrigerator or freezer for varying amounts of time before they hatch, and get a range from the smallest/palest to the biggest/darkest butterflies. This clearly shows that the look of Zebra Swallowtails does not qualify as a subspecies difference.
Photo from keysmoths.com, which notes that Zebra Swallowtails don't live and breed on the Florida Keys, but stray onto those islands often.
Males and females look pretty much alike, with enough individual variation that the only real way to tell the sex of a Zebra Swallowtail is to observe its behavior. (They recognize each other by scent; humans don't notice their scents.) The hairy patches on the hind wings usually look more conspicuous in males, but are also influenced by temperature.. In some, not all, photos of couples, one butterfly is just noticeably larger and less vividly colored than the other; I found one photo where a pair had been examined under a microscope and the slightly larger and less colorful butterfly, conforming to the usual rule for Swallowtails, was female. Both sexes sip clean water and flower nectar. Zebras who show an interest in polluted water are usually but not always male. Neither is particularly shy; neither is particularly interested in licking sweat from human skin, as composter species often are. They don't seem to mind being watched, but don't fly up in watchers' faces in the threat display some other large butterflies make. Males are more likely to seem "hyperactive" as they flit about in the sun, hoping to meet females. Couples do not always make it obvious which is male and which is female but, after mating, females spend most of their time looking for suitable places to lay eggs. All butterflies who lay eggs are females but occasionally one finds a female butterfly doing something other than laying eggs..
Photo from lazynaturalist.com. This couple are mating face to face; the female is almost entirely hidden behind the male.
Zebras can be a challenge to photographers. Like most Swallowtails they tend to flutter their wings even while eating, making it hard to snap a good clear shot. The secret is to persist. Swallowtails are most likely to spread out their wings in cool weather when they are warming themselves in the sun. Clear shots of the underwings are most easily obtained when a butterfly is drinking deep, focussed on sucking up nectar; Swallowtails fan their wings when they start to eat or drink, but may slow down or stop the wing movement when they find something good to the last drop. Because it's hard to get clear photos of them, yet the butterflies are locally common and pretty and popular, every wildlife photographer seems to want to publish a photo of a Zebra Swallowtail. Wikipedia had to thin their collection. Wikimedia Commons has thirty beautiful photos in the category "adult butterfly sipping nectar from a flower" alone.
Observing these behavior patterns, Michael Q. Powell was able to snap a rare, clear portrait of a Zebra Swallowtail's face. Furrier individuals can have long eyelash hairs all the way around the large compound eyes. Lots of photographs of eastern Virginia butterflies can be seen at https://michaelqpowell.com .
Another way to get clear photos of butterflies is to find them near the end of their shot lifespans. They fly for a few weeks, and then some seem to feel tired and sit down on the ground and wait for thir hearts to stop, while others drop suddenly to the ground and lie dead. An unusually vivid photo essay with close-up shots of small, commonplace things, including some Swallowtail butterflies, is at https://reinventingclaire.com/tag/eurytides-marcellus/ .
At all stages of their lives Zebra Swallowtails exist in a symbiotic relationship with their host plant, the pawpaw tree, Asimina triloba. (Georgianature.com mentions that science now recognizes a few different species of pawpaw in the Southern States, and Zebra Swallowtails can live on at least four of those.) If you want to see one or two Zebras three times a year, plant one of these native shrubs. The trees and butterflies find it easiest to thrive in the Southeastern States, fairly close to streams. They are known to live as far north as Pennsylvania, Ohio, and Nebraska, but they're rare in the northern part of their range. The butterflies sometimes stray into Michigan and Ontario but are not believed to breed there.
Photo from MarylandBiodiversity.com. This summer Zebra shares the Monarch's taste for milkweed blossoms with many other insects. Zebras have shorter proboscs than most Swallowtails, which limits them to feeding on relatively smaller or shallower flowers. Many people mention their taste for white clover. I often see them pollinating violets. Their favorite flowers also include blueberry and blackberry blossoms. The spring generation sometimes fly in time to pollinate redbud trees.
But they especially pollinate the pawpaw flower, which has little appeal to other butterflies. They rarely stray very far from a pawpaw tree.
Asimina triloba flower photo from elizabethswildflowerblog. Along the Gulf Coast a few other species of Asimina grow. Some of them have more colorful flowers. Triloba flowers usually look black but in a strong light, as here, they show dull red.
Generally Zebra Swallowtails like to be the only one of their sex and species in the neighborhood. An adult butterfly's life revolves around producing the next generation of its kind. Zebras like to make sure their next generation will have adequate food--a healthy pawpaw tree, or main limb of one, for every caterpillar. In most places you never see more than two of them together. In places where pawpaw trees are very abundant, however, people have photographed small groups of males sipping water from the same puddles.
Photo from Butterflies Of America.
Photo from MarylandBiodiversity. Since Zebras don't compete with other butterflies for food, we might expect that they'd be more comfortable with drinking buddies of other species than with other Zebras, and so they seem to be. Though their habits are generally cleaner than Tiger Swallowtails', both sexes in both species sip clear water, sometimes at the same puddles. Zebra Swallowtails seem to have very little social instinct, but do participate in the group flapping behavior Tigers do when the crowd at a puddle is disturbed. (However, when a mob of butterflies rise from their puddle and flap around a larger animal who might be a predator, if a butterfly is bold enough to fly at the intruder's face, it will be a Tiger or a Silver-Spotted Skipper.)
In eastern Virginia, Thisbiolife documented quite a large lek. Were these butterflies all reared in cages and released together? In many species immature males hang out in groups called leks, where they compete and cooperate, evade or fend off predators, and wait to grow up and find mates, together. It's not at all unusual to see Zebra Swallowtails in large leks, but usually one finds either one or two Zebras in a large lek of a more gregarious species such as Tiger Swallowtails or Spring Azures.
Male Zebras are not as positively attracted to nastiness as butterflies of composter species often are, but I have seen them choose, among otherwise equally desirable puddles, a puddle downhill from the dung and carrion that attract the composter species. Females, like other Swallowtails, can usually get their minerals from the males, but occasionally visit polluted water themselves. Both sexes drink fresh water and flower nectar.
Photo by Rogue Taylor, donated to Inaturalist. Zebras aren't usually attracted to human sweat, but possibly this one was, or perhaps a sweet drink had been spilled, a flower brushed against...?
Zebra Swallowtails generally have three generations each year, which can be called the spring, summer, and autumn broods. When they breed, as they occasionally do, in the Northern States there may be time for only two broods. In all regions spring butterflies tend to be smaller and lighter than autumn butterflies. Spring butterflies in Pennsylvania have wingspans just over two inches, typically two and a half, while autumn butterflies in Florida have wingspans up to four inches. I think of Zebras as our smallest Swallowtails--in Virginia--but, further south, they can be bigger than our Tiger Swallowtails/ Sources that give a narrower size range are likely to be talking about their local populations.. Wingspans over three and a half inches are normally seen in southern Florida.
Spring butterflies have pupated through the winter, which means they are likely to have survived being frozen solid. They probably don't remember being frozen and don't seem emotionally affected by it, but the experience has affected their growth. They are delicate sprites who don't look as if they'd be able to crossbreed with the big hairy individuals who were their parents. Nevertheless, their offspring will be bigger and darker than they were, and the autumn generation will be bigger and darker than the summer generation. The species microevolves around the range of individual variation for its species within each year.
According to the photo evidence at Inaturalist, Zebra Swallowtails can mate back to back, side to side, or facing each other around a twig. The back to back position allows one butterfly to enfold the other's wing tips between its own; the side to side position allows them to look to predators like one oversized butterfly. They spend some time together; one photographer was able to snap four clearly focussed photos of one pair from different angles. If a third butterfly is in the neighborhood, it may approach the couple, who seem resolutely to ignore the gawker.
Mother butterflies look for fresh, tender leaves for their young to eat. They fly around a tree (or a sprout that is trying to become a tree) and check the size and shape before testing leaves with their feet, which seem to have a sense of taste. Meena Haribal and Paul Feeny have identified a specific chemical scent that tells the butterfly which leaves will be ideal for her eggs to hatch on. The larger leaves on mature pawpaw trees can be thick, dry, and tough and contain more of the mildly toxic acetogenins (biochemicals) than the caterpillars need to eat. Zebra Swallowtail caterpillars are well camouflaged and able to evade being seen, but may be least difficult to find on the new leaves at the tips of branches on sprouts hardly taller than humans.
The infant butterfly hatches from an egg laid on a pawpaw leaf. Eggs look like little round beads, pale green at first, ripening to amber as the caterpillar prepares to emerge. Eggs, caterpillars, and chrysalides are almost always found on the underside of a pawpaw leaf.
Photo from Butterflies of America.
The caterpillar's first meal is the shell from which it emerged. Mother butterflies are usually very careful to lay each egg on a separate tree, or at least a separate limb of a mature tree. (Despite triloba being sometimes called "the tall pawpaw," it's hard for me to describe them as tall or large trees. Beside the buckeye, maple, sycamore, willow, and poplar trees that grow nearby, even the taller triloba are small trees.)
Photo from mdc.mo.gov.
Newly hatched caterpillars have mostly black skins with some pale brown stripes and some bristles, and have a humpbacked look that becomes more pronounced as they grow up.
Photo from Butterflies Of America. This hatchling is peering about in its shortsighted way. The glossy black surface of the head, visible here, is the top and back part analogous to a helmet. The small group of working eyes, found close to the mouth on the same side of the animal with its feet, don't work very well for long-range vision; Vincent Dethier observed, speaking mainly of moth caterpillars, that all the caterpillars whose vision had been tested seemed to qualify as "legally blind," apparently not seeing further than about a yard ahead. Caterpillars approach and avoid us with such disregard for us as persons because they are not able to see us as persons, and species more curious than Zebra Swallowtails--which tend to stay in one place and concentrate on eating--probably have to crawl about on us in order to satisfy their curiosity about what kind of trees we are.
Most caterpillars' skins, after the very first one, are green to match the leaves they live on. Some have a mottled grey color produced by black and white crosswise pinstripes. A few are brown. At close range all of these colors can be seen as patterns of fine horizontal stripes. All Zebra Swallowtail caterpillars have yellow-orange osmeteria, very large and colorful in proportion to the size of the caterpillars. They don't merely put out their "stink horns" and smell unappetizing; they may actively rub the osmeterium and the mouth, alternately, against a perceived enemy.
This photo, also from Butterflies Of America, shows a caterpillar in a state of righteous indignation, probably because a researcher keeps prodding it with straws. To a hungry spider the odor those"stink horns" release may be toxic as well as disgusting. The caterpillars try to touch an attacker with the osmeterium, smearing the rancid odor the osmeterium secretes all over their enemy. During this display they also vomit, smearing undigested plant material full of acetogenins over the enemy. A kind person will not subject a caterpillar to this kind of stress merely for display. It gets rid of most of the ants and spiders that occasionally attack the caterpillars. It seems to be less effective on the wasps and flies that lay their eggs on caterpillars' back ends. Damman found that the osmeterium is more effective in spring than in late summer.
Acetogenins keep vertebrate animals, like deer or humans, from eating pawpaw leaves; they have emetic properties. Their effect even on contact with smaller animals seems to be even more undesirable. Like most Swallowtail caterpillars, Zebras are harmless to humans because we feel no interest in eating them.
Zebra Swallowtail caterpillars continue to eat their own shed skins throughout their caterpillar lives, and lack the instinct to avoid eating the skins of their own species if the skins are not their own or are still occupied by a sibling. In nature they almost never see their siblings. If you rear these caterpillars you must prevent their meeting one another. Bigger siblings often pursue and eat smaller siblings when these caterpillars are not isolated. Their host plant could not afford to allow any overpopulation of this species.
Photo by Sara Bright.
Pawpaw trees bear a soft, bland fruit, "Nebraska Bananas," which most people find delicious, but some people have allergy-type reactions to it. Very few animals try to eat any part of the pawpaw tree except the ripe fruit. As with bananas, the fruits can be eaten while their peels have that nice clean look, but are easier to digest when the peels turn black.
In some places, however, a third party may join the symbiotic relationship Zebras thrive on fresh new pawpaw leaves; late summer caterpillars may be at a disadvantage since the leaves available to them are older and tougher. Sometimes a pyralid (borer) moth caterpillar attacks a tree branch. Its ravages stimulate the tree to put out its reserve leaves, just in time for the Zebra caterpillars to eat the new leaves. This treatment seems as if it would be hard on the trees, but trees can spare a few leaves in late summer, so even this arrangement seems to work for the benefit of all concerned.
Photo by Donald J. Hall. This unsightly mess attracts Zebra Swallowtail caterpillars but is not caused by them. Zebra Swallowtail caterpillars have none of their parents' appeal--they're sluggish, ugly little stinkers who don't even like one another--but the one who made the mess is the smaller, duller caterpillar lurking inside the little "house" of sand, bark, dirt, and silk.
As it matures, the caterpillar's humpbacked shape increases to the point where it looks more like its future chrysalis than butterfly caterpillars usually do.
Photo from Butterflies of America. This is the most common type of caterpillar.
Photo from Butterflies of America. This is the most common type of chrysalis.
The pupal skin has a dead-leaf look, light green or light tan, The caterpillars pupate on the undersides of pawpaw leaves, where few predators care to look for them. Pupae tend to match the colors of the leaves to which they attach themselves. Autumn chrysalides are often light brown like dead leaves; summer and autumn chrysalides are sometimes green and dark brown like damaged leaves; spring and summer caterpillars usually stay green while pupating.
Close to the time for eclosion, this butterfly, photographed at Davesgarden.com, can be seen through its chrysalis. Wings and legs will stretch out after eclosion but the tongue is close to its full size and forms a black line down the middle of the body.
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