A small percentage of adult dark-morph female eastern tiger swallowtails, Papilio glaucus L. (Papilionidae), display wing coloration patterns intermediate between those of normal yellow-morph and normal dark-morph females. One of these patterns (which, for the sake of convenience, I refer to in this paper as the “trigon” pattern) involves normal melanization of the scales in the basal portions of the dorsal wing surfaces but incomplete melanization of the scales in the central portions, resulting in the appearance of an inverted triangle (trigon) centered on the insect's body when the wings are outstretched. Between mid-July and mid- August 2010, I observed trigon-type female P. glaucus in the Terre Haute (Vigo Co.), Indiana region for the first time since I had begun systematically observing the P. glaucus in the area in 2002. Five trigon-type females out of 2,388 females were observed, suggesting that the trigon type comprised about 0.2% of the local female P. glaucus population. An examination of photographs of P. glaucus posted on an insect/spider identification website by users from throughout P. glaucus's range revealed a notable increase in the percentage of trigon-type females in 2010, suggesting that the Vigo Co. increase was not a localized phenomenon. I concluded that the unusually high temperatures that prevailed in the summer of 2010 affected wing pigment production during pupal development, thereby serving to make the trigon phenotype more prevalent that year. Other evidence further suggests that in general the prevalence of the trigon phenotype is associated with elevated temperatures (e.g. severe heat events). The findings of the present study suggest that if record high temperatures continue to prevail throughout P. glaucus's range during pupal development times, dark female P. glaucus expressing the trigon (and other intermediate) color patterns could become more prevalent, potentially altering the dynamics of dark female P. glaucus's mimetic relationship with Battus philenor in the most impacted populations.
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