Swallowtail variants (2015.04.01 [Wed.])


  I think something strange may be happening to the insect fauna of Port Island. I wrote previously about our cicadas, and now I'd like to touch on the island's swallowtail population. Let's begin at the beginning...

  A few days ago, Mr. O, the proprietor of the Rokko Insectarium, visited my home and showed interest in one case of specimens — "What do we have here?" he said, eyeing an Asian swallowtail (Papilio xuthus) I collected the previous summer. In August of 2013, I was still living on Port Island, and there was a mikan tree on my way to work that I knew attracted many swallowtails, so I used to keep a folded up butterfly net in my bag as I walked to the CDB. I was fortunate to catch a number of lovely specimens, one of which was the summer-morph male Mr. O was so drawn to.

  As you can see from Figs. 1 and 2, it is strikingly yellow, which made it almost impossible to miss when it flew, and at first had me thinking it was an Old World swallowtail (Papilio machaon). But P. machaon don't typically congregate around mikan trees, and when I captured it I could see its patterning was that of P. xuthus, which I later confirmed in a field guide had the characteristic black hindwing markings of a summer male. The summer morph itself was not extraordinary, but its black and yellowish-tan coloration was. The black regions were larger than ordinary with markedly thicker black stripes; the overall effect suggested incomplete melanization. But within the black pattern local white spaces lent the whole an effect uncannily akin to that of a human gaze.

  The psychological effect was like a simulacrum of human vision, and the more I stared at those 'eyes,' the more they seemed to stare back at me. What's more, some black patterns on the lower forewing resemble eyebrows and the insect's abdomen could be taken for the bridge of a nose, giving the whole the look of a sharp-featured man gazing keenly ahead. What a find! I was so taken with Mr. O's exclamations of wonder that I ordered a special case and am keeping it as a prized specimen, as you can see in the figures.

  For the sake of comparison, I also show a normal Asian swallowtail in Fig. 3. I captured this one in 2012, and its blue and red highlights have since faded, leaving it perfectly monochromatic. This is the typical phenotype for the species (which I also now note includes white-tipped antennae). The wing patterns are essentially similar to those of the yellow morph, and if you look for it, you can see the resemblance to a human face, although not with the same vividness. I think that it must have been those staring eyes that attracted Mr. O's fascination.

  Soon after I caught the yellow swallowtail, I spotted another of its kind flitting about the garden outside the CDB, but when I raced back from the parking lot after retrieving my net, it had already flitted away. I kept the net close at hand thereafter, hoping for a second chance, but in vain. If that turned out to be the same yellowish type, then I would say that a new swallowtail variant may have been established in this area.

  Speaking of which, I recall another interesting swallowtail here on the island. In late March of 2014, while it was still occasionally chilly, I spotted a tiny Asian swallowtail in the same garden spot. That day happened to be warm with clear skies, and I believe it was around 2:00 in the afternoon. In any case, this butterfly was minuscule for a swallowtail, more like an Vanessa indica in size. 

  So what the heck is going on here on Port Island? Mr. O thinks it may be due to the limited space. Recently, mating between lesser emperor (Anax parthenope) and blue-spotted emperor (A. nigrofasciatus nigrofasciatus) dragonflies has been confirmed, suggesting we are in the midst of a 'hybrid zone,' in which intermediate individuals frequently occur. But the butterfly I introduced here is clearly an Asian swallowtail. Perhaps instead it represents the consequence of a bottleneck, as we say in population genetics. Post Island is manmade, and while many species may live here, as might be expected it is too small to be a true ecosystem. I wonder how quickly alleles would spread given this spatial scale and number of individuals. It might take only the slightest 'push' to establish a low-frequency allele in a small population. Perhaps the unusual insects I have encountered on this island are the products of a kind of experiment in evolution.