Red-spotted Purple Butterfly

The Red-spotted Purple is mostly purple and has red spots at the wing tips

Common Name: Red-spotted Purple and White Admiral – Butterfly names are in most cases descriptive, using color and patterns as leitmotif. The mostly dark blue wings tinged with enough red to produce purple culminating in red wing spots provides one of the more mnemonic names. The alternative name White Admiral is a result of one of the more tantalizing tales of the lepidopterans as they change colors and patterns in mimicry detailed below.

Scientific Name: Limenitis arthemis –  The genus name literally means harbor goddess in Greek. The nautical association is apparently related to or is a result of  the fact that they are called the admiral butterflies, as in White Admiral. The species name is from Artemis (Diana in Roman mythology), the Greek goddess of the hunt and hence the woods. A butterfly as metaphor for a goddess captures the graceful beauty of both.    

The White Admiral has a single broad white stripe – like a US Navy admiral

Potpourri: The  Red-spotted Purple and White Admiral are the same species, Limenitis arthemis. Mimicry, the term for an animal mimicking another object in shape and/or color, is an evolutionary and genetic  response to  the inexorable tug of survival. Although it may seem especially notable in this case because of the striking result afforded by the difference between white striped and stripe-less wings, mimicry in its broadest sense is widespread. Some prey animals change colors according to age and season to provide better camouflage. The spotted fawn turns light tan as a doe or buck in summer and darker in winter to match the scenery. Predators must do the same in order to hide from their quarry long enough effect the coup-de-grace at the last moment. There are no black panthers, only melanoid leopards and jaguars becoming night stalkers (both are in the genus Panthera). Aposematism is similar to mimicry in that coloration is used to ward off predators. But rather than being cryptic, the colors stand out  against the background in sharp contrast, alerting the wary predator that poisons there lurk. The juvenile red eft of the red-spotted newt is a good example of aposematism … a defenseless amphibian that protects itself with vivid orange hues similar to those  used by hunters to accentuate visibility.

White Admirals, the northern version of the Red-spotted Purple, are named for their prominent white stripes. It is perhaps only coincidence that the progression of officer ranks in the U.S. Navy ranges from an ensign’s single narrow to a single broad stripe for an admiral. The Red Admiral (Vanessa atalanta) which has a similarly placed red stripe is otherwise unrelated. Boldly contrasting prominent stripes on two species suggests purpose and coevolution. While striping may be related to species or mate recognition, it is more likely a matter of predator avoidance, the moving flashes of colored streaks creating a disorienting stroboscopic effect. [1]  On progressing geographically southward, the White Admiral’s broad stripe disappears and the red spots move forward to the edge of the wing tip to become both red-spotted and purple. This rather extraordinary transformation is a combination of the aping of mimicry and the warning of aposematism, a hybrid scheme called apatetic in general or Batesian in particular.  The wing is now more uniformly dark in color, resembling that of a butterfly of a different genus and species ― a poisonous doppelgänger.

The Red Admiral has a single red stripe

It is widely known that the Monarch butterfly is unpalatable to birds because its caterpillars eat milkweed (Asclepias syriaca) that produces cardiac glycosides that are toxic to most animals. It is mimicked by the Viceroy (Limenitis archippus) butterfly, a generic cousin of the Red-spotted Purple, as a matter of enhanced survival. [2] The Green Swallowtail butterfly (Battus philenor) is better known as the Pipevine Swallowtail because its larvae feed on Dutchman’s Pipe (Aristolochia durior), a vine that produces a toxin called aristolochic acid. Since the range of the toxic Pipevine Swallowtail butterfly extends only as far as its namesake food, it is a southern butterfly because that is where the vines are. [3]  The change is not cognitive choice, but rather choice by chance. The White Admirals that ventured south with less prominent stripes survived more frequently since they were more likely to be avoided by predators. Over time and subsequent mating of diminished stripe White Admirals, the stripes disappeared altogether and the Red-spotted Purple became the southern variant, sometimes listed as a separate subspecies Limenitis arthemis astyanax. The name extends the mythological association to include Astyanax, the son of Trojan hero Hector who was defenestrated by the Greek Achilles so he could not avenge the death of his father.

The Pipevine Swallowtail is copied by the Red-spotted Purple to escape predation by birds. It is called Batesian mimicry.

The kaleidoscopic patterns of butterfly wings are among the most artistic creations of nature. Their evolution that began during the Cretaceous Period 150 million years ago was marked by three random mutation “inventions” that radiated in time and space along the way to produce the 18,000 plus extant named species. [4]  The first and defining mutation was wing scales from which the name of the order Lepidoptera was derived, lepis meaning scale and ptera wing in Greek.  Scales are genetically modified sensory bristles, that became flat. senseless, and slippery, probably to avoid capture … the survivors passed the scale genes along. The second invention was changing the scale colors, possible because each scale is from a single cell with control of hue and texture, the combination producing different shadings and sometimes even iridescence. Lastly there was pattern, the genetics of placing colors in ordered arrangement. Spots in general and eyespots in particular start in the caterpillar stage, where an organizer puts them in the right position on the wing, a disc at this point. Colors are added in the chrysalis phase so that the adult butterfly wing emerges after metamorphosis with spots. These are usually at the margins of the wing so that a predator would first strike there, removing only a small portion of the wing as the butterfly flitted to safety. The efficacy of this is demonstrable, as many lepidopterans are found with a bite out of one wing. [5]

Butterflies are among the most studied of all animals, surely more a matter of beauty and ease of net capture than for their scientific import as just another type of insect. Henry Walter Bates spent eleven years in the Amazon rainforest in the mid-nineteenth century, identifying 8,000 species that were then new to science, many of them butterflies. His studies led to the observation that some butterflies had patterns that were quite similar in appearance to unrelated species that were unpalatable to birds. He hypothesized that birds would learn to avoid them after only a few experiences and that this would then perpetuate the verisimilitude. When he returned to England in 1859 to recover from his epic jungle ordeal, he presented a paper on his discovery of butterfly mutations and to what he considered to be one of the best examples of  the “origin of all species and all adaptations.” [6] The phenomenon, known ever after as Batesian mimicry, became one of Darwin’s favorite examples of his epochal Origin of Species which had just been published. The two developed an enduring friendship, corresponding periodically on the new ideas of evolution. Bates became one of the primary adherents to the nascent theory, writing on one occasion that “I think I have got a glimpse into the laboratory where Nature manufactures her new species.” [7] The headwinds of religious dogma required decades to overcome, but gradually and fitfully the theory has gained near universal acceptance excepting those that adhere to biblical literalism.

With the advent of DNA as a roadmap of evolutionary change, Darwin’s insight only remains a theory insofar as it cannot be proven according to the scientific method of testing, which would require going back in time to reset the biological clock. The White Admiral conversion to Red-spotted Purple is one of the most documented of butterfly DNA subjects because of the infraspecific Mason-Dixon line that separates them. Proceeding north, the White Admiral prevails, while the far south is dominated by the mimetic Red-spotted Purple. The validity of the Batesian mimicry has been well established. A thirty year data set of Fourth of July Butterfly Counts confirmed that mimicry occurs even when the population of the unpalatable Pipevine Swallowtail species is low and that a sharp phenotypic geographical transition marks the boundary.[8] Between the two extremes, there is range over which hybridization occurs, affording a singular opportunity to study the interaction between the two variants according to DNA changes. Scientific research has established that the White Admiral variant is monophyletic (single ancestor) and that the hybridization of mimicry occurred just once. The hybrids that exist in the transition zone are thought to be due to mating between the two, producing on occasion a Red-spotted Purple with faint or partial white stripes. [9] More recently the location of the mutation responsible for Batesian mimicry on the genes of two different types of butterflies (Limenitis and Heliconius) that diverged 65 million years ago demonstrates the coevolution of this important survival trait. [10] Genetic confirmation provides the scientific “how” corresponding to the Batesian “why,” proof  for all practical purposes of Darwin’s “theory.”

The employment of Batesian mimicry of Limenitis arthemis in scientific research of butterfly sex  must surely have been considered for the Ig Nobel Prize in biology. One of the more compelling examples of female reproductive choice is sperm retention and storage after mating for fertilization at a later, more auspicious time. In that this would enhance the survival of subsequent generations, it has coevolved across the animal kingdom to include some insects, butterflies among them. It is also the case that many animals mate more than once; males with genetically driven propensity to sire as many offspring as possible and females to ensure successful insemination with the best possible mate characteristics. It is hard to say for sure, but it may also be that both enjoy it. Among the more profound questions facing biology is whether the sperm from a second mating male displaces that of the first or whether the two mix together to produce hybrids. Using the wing patterns that resulted as the biological metric, 17 females were mated with 34 males to conduct the experiment (it was not reported if this was consensual). The results were used to determine “insect mate-seeking strategies and individual fitness.” In that it was the first male’s sperm that prevailed, the conclusion was that it was not in the best interests of either the female or the male to mate multiple times. This then led to the conclusion that “virgin females apparently are sought by males and probably are more receptive to courtship and successful mating than are ones which have mated previously.” [11] This, at least, is the same theory espoused by some college fraternities and numerous religious denominations.

References

1. https://entnemdept.ufl.edu/creatures/bfly/red-spotted_purple.htm    

2. Marshall, S. Insects, Their Natural History and Diversity, Firefly Books, Buffalo, New York, 2006, pp 161-167.

3. Milne, L. and M. National Audubon Society Field Guide to Insects and Spiders, Alfred A. Knopf, New York, 1980, pp718-719.

4. Heikkilä, M. et al. “Cretaceous origin and repeated tertiary diversification of the redefined butterflies”. Proceedings. Biological Sciences. 22 March 2012 Volume 279  Number 1731 pp 1093–1099.

5. Brunetti, C. et al. “The generation and diversification of butterfly eyespot color patterns”. Current Biology. 16 October 2001 Volume 11 (20) pp 1578–1585

6. Bates, H. “Contributions to an insect fauna of the Amazon valley. Lepidoptera: Heliconidae”. Transactions of the Linnean Society. 21 November 1861 Volume 23 Number 3. pp 495–566.

7. Carrol, S. Endless Forms Most Beautiful, W.W. Norton, New York, 2005,  pp 197-219.

8. Ries, L. and  Mullen, S.   “A Rare Model Limits the Distribution of Its More Common Mimic: A Twist on Frequency-Dependent Batesian Mimicry” Evolution. 4 July 2008, Volume 62 (7) pp 1798–1803.

9. Savage, W.; Mullen, S. “A single origin of Batesian mimicry among hybridizing populations of admiral butterflies (Limenitis arthemis) rejects an evolutionary reversion to the ancestral phenotype”. Proceedings of the Royal Society B: Biological Sciences. 15 April 2009 Volume 276  Number 1667  pp 2557–2565 at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2686656/  

10. Gallant, J. et al “Ancient homology underlies adaptive mimetic diversity across butterflies” Nature Communications, 8 September 2014 Volume 5, p 4817.

11. Platt, A. and Allen, J. “Sperm Precedence and Competition in Doubly-Mated Limenitis arthemis-astyanax Butterflies (Rhopalocera: Nymphalidae)”. Annals of the Entomological Society of America. 1 September 2001 Volume 94 (5) pp 654–663.

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