Common Name: Eastern Tiger Swallowtail – The black markings on a yellow background are similar to a tiger’s stripes and the protuberances that extend from the back of each wing are like the forked tail feathers of several of the birds commonly called swallows. There are eastern and western variants of the butterfly (that are almost identical in appearance).
Scientific Name: Papilio glaucus – The generic name is the Latin word papilio which means ‘butterfly.’ The species name is from Greek Mythology; Glaucus was a fisherman who ate a divine herb planted by the Titan Cronus and was transformed into an ocean deity. Glaucus is also a Latin word meaning ‘bluish-grey’ which may be a double-entendre for the subtle markings at the inner rear wing.
Potpourri: The inimitable palette that evolution has bestowed on butterflies has captured the imagination of humanity in art and poesy throughout history. Swallowtail butterflies of the Superfamily Papilionoidea are considered to be true butterflies and are therefore in a sense the epitome of the butterflies and moths of the Order Lepidoptera; there are about 225 species worldwide and 40 in North America. Recent genetic research has confirmed that the Papilionoidea are monophyletic – they have a single common ancestor and are therefore a taxonomically well-defined grouping. Carolinus Linnaeus, the originator of the taxonomic nomenclature system, chose Papilio, the Latin word for butterfly as the type, or characteristic genus of the order, selecting individual species names from Greek Mythology (the type species is P. machaon, the European Swallowtail; Machaon is mentioned in Homer’s The Iliad). The Tiger Swallowtail is in turn the most iconic butterfly in North America; it appears in a 1587 drawing made by John White, the commander of Sir Walter Raleigh’s ill-fated colony on Roanoke Island. Fittingly, the Eastern Tiger Swallowtail is the state insect of Virginia, a name given to the area including Roanoke by Raleigh for his patron, England’s Queen Elizabeth I, and the state butterfly of North Carolina, where the “Lost Colony” is now situated.
Butterflies instantiate beauty, their delicate and diaphanous wings a veritable tableau of attractive hues that would seemingly broadcast a signal beacon to any meal-minded carnivore for a quick and relatively easy snack. That they are ubiquitous, flitting in mass processions called flutters to alight in large conclaves, belies the verisimilitude to hapless prey – there must be something else going on – and there is. No less an authority than the venerable Britannica Encyclopedia offers a clue: “It is chiefly against the sight-hunting predators that the Lepidoptera have evolved a multiplicity of defense mechanisms that are unequalled by those of any other group of animals.” The erratic, punch-drunk flight of the adult butterfly is physically deceptive, posing a directional challenge to the vectored attack of an intended predator. The targeting is further exacerbated by the contrasting striped markings that make it very difficult to determine the direction of flight, the image being bifurcated into two or more separate entities.
The butterfly larva, more commonly known as a caterpillar (a word with a curious etymology that derives from an Old North French word catepelose which literally means ‘hairy cat’) is likewise a seemingly defenseless and delectable treat of soft body pulp. However, the apatetic coloration is in this case the opposite of the flamboyance of the adult, the green serving to hide the larva among the foliage on which it forages. In the event that it is discovered, chemical warfare is employed. Swallowtail caterpillars are endowed with a retractable, forked organ called an osmeterium that is extended to exude an aromatic hydrocarbon called a terpene which has a mephitic odor to deter predators. The combination of crypsis to hide and olfaction to repel is a successful adaptation to survival – the larva must pupate to become an adult to propagate.
However, the unique and signature evolutionary adaptation that distinguishes the butterflies and moths from all other animals is the miniscule, imbricated scales that cover nearly the entire surface of the adult’s body, most notably the wings; the name Lepidoptera is from the Greek lepid meaning scale and ptera meaning wing, or, taken together, scale-winged. Butterfly and moth scales are formed from a single cell and are held at an angle of about 45 degrees to the wing membrane. Each scale is a single color determined by particular chemical pigments including melanins, uric acid and flavones and by the texture of the scale in its reflection of light, producing iridescence, metallic and pearly effects. The macroscopic impression of the butterfly wing is the result of the merging of many colors, much like the pointillist technique introduced by the French impressionist Paul Seurat in the 19th Century; a color plexus. According to research in evolutionary developmental biology (Evo-Devo to aficionados), scales are based on the same genes that flies use to make sensory bristles, evolving to become flat, wide and insensate. Butterflies and moths emerged with scales in the Permian Period about 250 million years ago probably as a means of defense, the easily detached and slippery scales affording a means of escape from spider snares and inept predators. The meretricious wings of the butterfly are its final line of defense; they are not only hard to catch, they are hard to grasp. The aerodynamic overdesign of the wings affords another layer of protection; the flimsy wing tips readily tear off when subject to shear forces to allow the butterfly to escape – a fact attested to by the many adults encountered with missing wing segments.
Butterflies are nature’s mimes. The observed preponderance of their alternative wing patterns known as Batesian mimicry became one of the most convincing demonstrations of Darwin’s revolutionary theory. Henry Bates spent over ten years (the first four years with Alfred Russel Wallace who is credited by some with having first posited the concept of natural selection) in the Amazon jungle collecting 14, 712 animal species before his return to England in 1859 just before the publication of Darwin’s On the Origin of Species. In his study of butterflies, he observed an anatomical resemblance of one butterfly – (known as the mimic) for another (known as the model) where the modelled butterfly was unpalatable to predators. Darwin’s admiration for Bates and his insights into the evolution of butterfly coloration as an anti-predator adaptation is well documented, he was the reviewer and editor of the seminal Naturalist on the River Amazon published in 1863, In it, Bates notes that “It may be said, therefore, that on these expanded membranes nature writes, as on a tablet, the story of the modification of species.” Darwin could hardly have asked for a more convincing example of his theory, perhaps even more so than the finches of the Galapagos Islands whose beak variations inspired his insight.
The Tiger Swallowtail offers one of the more interesting examples of Batesian mimicry. Whereas all males of the species are yellow, the females can be either yellow or black. Melanism, the presence of dark pigmentation in the skin, feathers, hair or scales, is one of most prevalent color anomalies in the animal kingdom. There is no such thing as a panther; all of the large cats (leopards, jaguars, lions and tigers) are in the genus Panthera; the name panther is colloquially applied to any that have black fur – Black Panther is redundant. Female Tiger Swallowtail Butterfly evolution has resulted in a melanic variant because of Batesian mimicry. The model in this case is another black-winged butterfly commonly called the Pipevine or Green Swallowtail (Battus philenor), which is highly unpalatable to most butterfly predators. B. philenor larvae subsist on plants from the genus Aristolochia that is comprised of vines and herbaceous shrubs that are commonly called Pipevine, Dutchman’s pipe or Birthwort due to the shape of the flowers. The plants produce aristolochic acid, a lethal toxin if consumed in adequate quantities; Pipevine butterfly larvae evolved with a tolerance to the toxin that accumulates as a chemical constituent of their bodies that imparts unpalatability. Butterfly predators have over time adapted to the unpleasant taste of the Pipevine Swallowtail by learning to avoid black butterflies. Since butterflies are diurnal, black is a color that starkly stands out in bright daylight. The use of colors that stand out so as to call attention to an animal is called aposematism; in this case black wings promote avoidance. The black female Tiger
Swallowtail takes advantage by becoming the mimic of its toxic cousin. Recent genomic research of this phenomenon has revealed that it is a very simple process; Melanism is controlled by a single gene that converts the yellow background to black. Once the mutation occurred (random genetic variation), the increased survival rate of the black females (natural selection) resulted in black female offspring. Batesian mimicry is amply demonstrated by the fact that black female Tiger Swallowtail butterflies are only found in the South – the range of the Pipevine butterfly; there are none in the north.
The facility with which butterflies change their wing patterns to become new species raises the question: How do males of a species identify a consanguineous female? In the case of the male Tiger Swallowtail the difficulty is exacerbated not only by the yellow and black variants of the female but by the presence of a number of other black butterflies, including a Black Swallowtail butterfly. While there are some visual cues and possibly some pheromone cues, evolution engendered an incontrovertible solution: specialized genitalia. The male reproductive organ, called an aedeagus in insects, is a complex structure with a variety of projecting spines, scales tufts and teeth that will only fit into the genital cavity of a female butterfly of the same species (as shown at left). Vive la difference!