An 1840s British visitor to Illinois noted that “among the novel discomforts of the West, that of insects is one of no trifling character. The whole earth and air seems teeming with them….”  A big bunch of them, including mayflies, teemed at Nauvoo.
Last week Theric Jepson rhapsodized:
Remember the mayfly? Small, transparent and fragile. Yet infinitely complex. Purple and green at the same time. The endlessly intertwining veins in its wings a lesson in simple beauty-the complexity of the universe captured in microcosm-in a flitting life that lasts one day. 
Remembering the mayfly in history also reveals some of the “endless[s] intertwining” of human language and experience. Since the 1840s, some Midwesterners have lumped mayflies Hexagenia limbata and H. bilineata and a few other taxons under the name “Mormon fly.” 
Figure 1: H. limbata (© Lynette Schimming, Cirrus Digital Imaging)
Juvenile mayflies live under water before molting into airborne adults that fly about for a day, maybe two, and then die. The thing is, they do it all at once. Millions and millions flit around in a great, frenetic singles’ conference in the sky-big enough to register on radar. Their mass expiration dumps mounds of bug bits on everything and reeks like rotting-fish. 
“These damned Nauvoo flies” (1845) seem to have started out as a type of troponym, a name derived from a place. Several weeks before the Battle of Nauvoo, a canoeing out-of-towner “noticed…a certain fly or miller…in immense numbers…called the Mormon fly” and said to be “found on these rapids alone.” 
In 1868, an entomologist noted that the troponym applied to two different geneses and that “why either of the two should be called the ‘Mormon fly,’ [was] an insolvable mystery” since the Mormons only arrived in 1840.”  The doctor has a point. Local lore notwithstanding, mayflies did not confine themselves, temporally or spatially, to Mormon Nauvoo. Why, then, did people make big truck about them?
For one thing, the Mississippi hatches were overwhelmingly enormous. More categorically, though, people construct identities, even for insects, which sometimes manifest in charactonyms, or names based on (perceived) attributes. A scientific report (1890) explained that H. bilineata often gathers “in such multitudes as to have acquired the name ‘mormon fly.'”  An 1889 joke attributed the name to the flies being “so prolific.”  More recently, David Howlett explained that Nauvoo locals associate Mormon tourists with the flies “since both cover the landscape during the summer….”  Thus, “Mormon” is constructed to indicate behavior (gathering) and attributes (numerous) instead of the place. 
L: Mayflies cover vehicle, Ontario (© Cory Flynn); R: Hatch on radar, 2003 Jul 25, La Crosse, MN (black dot), 100 km (60 mi) top to bottom (NOAA.gov 1, 2, 3)
Nibbling up the food chain, we leave proximity and similarity for a relationship: “Legend has it that when we kicked out Joseph Smith and followers from Illinois, they put a curse on the river and Mormon flies were the result” (2008).  Not only were the flies like Mormons, they also served them.  The tale has some basis. Mormons claimed that when they left Illinois they “cursed it in the name of their God.”  1861 and 1888 statements by (non-Mormon) Nauvoo residents asserted a curse because of the expulsion.  The earliest linkage—and that only partial—I’ve found, however, comes from 1925: “there were millions and millions of flies—“Mormon” flies, they call them…. I came to the conclusion that a curse had indeed rested upon the place.” 
Identifying local pestilence with “unclean” or “evil” groups has a long, sin-crusted history.  Equally ancient is the converse ascription of divine intent in one’s enemies’ misfortunes. This form of othering can manifest traumatically when specific allegations ignite over-pressured social conformations into ethno-racial violence against a particular group. More commonly, a “maintenance” form of othering-frequently only half-serious, usually without conscious animosity-helps keeps that group closest to the release-valve and danger. Thus: bawdy tales of Black sexual prowess, White claim of rape, lynching. In however small a way, the curse of the “Mormon fly” probably contributed to violence against Mormons in their first century. Mystic Mormon curse, successful (allegedly hypnotic) missionary, mobbing.
A harsher form of maintenance othering moves the label from pest to person: “independent study and creative thinking are essential to encounters with those pesky Mormon-flies” (2008).  An 1880 Mormon editorial used the phrase “‘Mormon’ flies” sarcastically, suggesting it had currency as a slur. 
Mayfly from Italy (species not specified), Wikipedia
When I describe my research to associates, they usually trip over the idea that “Whiteness” does not always refer to pigmentation. In the same way that settlers could look at the same bug in New York and Illinois and label one of them “Mormon,” we “see” features in others that independent observation might not confirm. I think the following statement cuts to the green-blooded heart of the matter: “we call them mormon flies…. I’m really not for sure why. I think it’s just a little nickname they were given to be mean….” 
 William Oliver, Eight Months in Illinois: With Information to Immigrants (Newcastel-upon-Tyne: William Andrew Mitchell, 1843), 148.
 “Mayfly” applies to the thousands of species in the order Ephemeroptera. Caddisflies are also sometimes gathered under the “Mormon” label. Extra trivia at no extra charge: In fly-fishing, the popular “Mormon Girl Stone” imitates a gravid, female Isoperla mormona (little yellow stonefly), which is rather like (to a commoner like me) the caddisfly grouped as Mormon fly. Ernest G. Schwiebert, Nymphs Volume II: Stoneflies, Caddisflies, and Other Important Insects: Including the Lesser Mayflies, expanded edition (Guilford, CT: Lyons Press, 2007) p76-77.
 “Burlington, Iowa has an authentic record of a pile eight feet deep forming around an electric light pole one night when a heavy flight was in progress.” Harry Edwin Jaques, How to Know the Insects; an Illustrated Key to the More Common Families of Insects, with Suggestions for Collecting, Mounting and Studying Them (Mt. Pleasant, IA: self-published, 1941) p42.
“The 100-degree heat in Burlington, Iowa…spawned swarms of smelly Morman [sic] flies, police said. …The fly swarms slickened streets…and caused a ‘black snowstorm’ that posed a traffic threat. ‘When they get on you, they can really be awful,’ a police dispatcher said. ‘They smell like old, dead fish, and they stick to you like flypaper.'” United Press International, “Midwest expects only limited relief from heat,” The Daily Record, Ellensburg, WA, 1987 Jun 15, p13.
 The “Nauvoo flies” reference came from an 1845 Aug-Sep trip. Nathaniel F. Moore, Diary of a Trip from New York to the Falls of St. Anthony in 1845, edited by Stanley Pargellis and Ruth Lapham Butler (Chicago: University of Chicago Press, 1946) p46, as quoted in George W Givens, In Old Nauvoo: Everyday Life in the City of Joseph (Salt Lake City: Shadow Mountain, 1990) p35; Charles Lanman, A Summer in the Wilderness: Embracing a Canoe Voyage Up the Mississippi and Around Lake Superior (New York: D Appleton, 1847) entry for Prairie Du Chun, 1846 Jul, p34. The Battle of Nauvoo was in 1846 Sep. The usage in both cases suggests an established name, but I failed to locate earlier citations. The next citation I found comes from 1849 when the mouth-less wonders appeared in an epic (in length and style, not necessarily execution) poem about Native American Peoples and the West. (The adult form only lives one day; it does not eat; it has no mouth.) “And here the upper rapids we approached; / Here we beheld in numbers, flying about, / At hour of eve, found here, and nowhere else, / A certain insect, called the Mormon fly. / Where they alight until they die, remain, / Unless disturbed or from their hold unloosed.” Elbert H. Smith, Ma-ka-tai-me-she-kia-kiak, or, Black Hawk, and Scenes in the West. A National Poem: In Six Cantos (New York, 1849) p65: Canto I.65 lines 1-6.
 The quote continues: “There was somewhat more plausible ground for calling the Chinch bug the ‘Mormon louse;’ for that little pest really did swarm for the first time in Illinois about the same year that the Mormons settled there.” I’ll write more on lice later. Walsh identifies the insects as Macronema zebratum Hagen (Caddis-worm fly) = Macrostemum zebratum (Zebra Caddisfly) and Palingenia bilineata = H. bilineata. Benjamin D Walsh, “The Bughunter in Egypt: A Journal of an Entomological Tour into South Illinois by the Senior Editor,” American Entomologist 1 no 1 (1868 Sep):6-7. For similar troponymic use, see OS Westcott, “Nature Study,” Journal of Proceedings and Addresses of the Thirty-fifth Annual Meeting of the National Educational Association (Chicago: University of Chicago, 1896) p 139-41 .
 H Garman, “A Preliminary Report on the Animals of the Mississippi Bottoms Near Quincy, Illinois in August, 1888. Part I,” Bulletin of the Illinois State Laboratory of Natural History 3 (1890):123-184 .
 “All Sorts of Items,” Daily Evening Bulletin, San Francisco, 1889 Aug 05, p4 col E. Another joked that “Dubuque, Iowa, is complaining of the Mormon fly. The country at large grumbles because the Mormon doesn’t fly.” New York World, as quoted in “All Sorts of Items,” Daily Evening Bulletin, San Francisco, 1889 Jul 30, p4 col B.
 David Howlett, comment at blog The Juvenile Instructor, 2009 Jul 15. Another behavioral attribution is: “They call them Mormon Flies cause they are fawking everywhere you turn and they don’t leave you alone.” <Chemical442>, response in discussion, “Nastiest Bug in Your Area?” 2003 Apr 08.
 Illinoisans in the 1840s certainly found Mormon gathering troublesome, but I have yet to find evidence that they connect the name with the behaviors-other than the observation that they were, in many cases, looking at exactly the same species that they knew from elsewhere, so there must have been something prompting the new name.
“> The writer continues: “The real reason…at least for the area I’m from…is that we are upstream of the Keokuk dam and the eggs tend to settle.” <Up peterscope>, comment in thread “Who needs a plague of locusts when we have enough mayflies to show up on weather radar and create drifts,” Fark, 2006 Jun 07. “The story is that when the Mormons were chased out, they placed a curse on the river.” <bobbie d>, “The Mayfly,” blog: bobbies world, 2008 Jul 17.
 Some observers collated Mormons and Indians in similar fashion: Mormons were suspect because they lived near Native American Peoples, because they acted liked Indians, because they merged with Indians and directed marauding parties (all supposedly, of course). Proximity, similarity, unity. I take this idea of different levels of comparison from Paul Reeve’s 2009 MHA presentation on construction Mormon racial identities. ***
 “The Saints have gone out of that country [Illinois], and we are glad. When they left, they shook off the dust of their garments against the state, and cursed it in the name of their God, and this curse is sealed upon it by the tears of the afflicted and distressed, who have groaned and cried unto God under the weight and enormity of its injustice and oppression. [¶] They will not return to the State from which they have been exiled to inherit the curse which they have placed upon it.” From editorials comment following an extract from the New York Sun on the plight of Mormons on the plains. “Reaction in Public Feeling (Extracted from the New York Sun)” The Latter-day Saints’ Millennial Star 9 no 3 (1847 Feb 01):37-8. In 1843, Joseph Smith said, “[Lucian Woodworth] has prophesied that if these buildings [Nauvoo Temple, House] go down, it will curse the place. I verily know it is true.” Joseph Smith, HC 5:284 (1843 Feb 21). Printed in “History of Joseph Smith,” Millennial Star 20 no 37 (1858 Sep 11):581-4 . The Nauvoo curses were not alone. Nineteenth-century Mormons prayed for vengeance on their enemies, prophesied that areas would suffer, and formally cursed land. For examples: “we prayed the Lord to kill the mob” (George A Smith, 1857 Aug 02, Journal of Discourses, 5.107); “I pray that they may go to hell” (John Taylor, 1858 Jan 10, JD 7.122); “I prophesy in the name of the Lord of Israel…that in a few years the government will be utterly overthrown and wasted …” (Joseph Smith, HC 5:394***); Heber C Kimball’s 1868 “yellow dog” prophecy about Missouri (as quoted by J Golden Kimball, Conference Report, 1930 Oct, p58), and Wilford Woodruffs cursing of “the Benton County mob” and Paris, AK (Kenney, Scott G, ed., Wilford Woodruff’s Journal:1833-1898 Typescript. 9 vols. (Midvale, UT: Signature Books, 1983), vol 1, 1836 Oct 12). See also D&C 84:92, Mormon 1:18, and Helaman 13:31-36.
 In 1861, “many of the citizens of Nauvoo and near vicinity expressed their opinion that the ‘driving out of the Mormons had left a curse upon the county that would not be removed until they should be permitted to return.'” Joseph Smith, III, “Biography of Joseph Smith” in Edward W Tullidge, Life of Joseph the Prophet, 2nd edition (Plano, IL: Board of Publication of the Reorganized Church of Jesus Christ of Latter Day Saints, 1880) p781. The first edition did not include the Smith III biography; it seems Smith was still working on the biography at the beginning of 1880 (p801). The biography was included in The History of the Reorganized Church of Jesus Christ of Latter Day Saints (Independence, MO: Herald Publishing House, 1897) 3.270. “We believe you people must have left a curse upon this place, for nothing prospers here now.” “Visit to Nauvoo,” Deseret Weekly News, Saturday, 1889 Jan 05, p3, quoting AW Beach, who was summarizing the statements of “quite a number of people in Nauvoo” with whom he conversed in 1888 Dec. See also some of Nate Oman’s ruminations on Mormon cursing.
 Andrew Jensen, speech in General Conference of the Church of Jesus Christ of Latter-day Saints, Salt Lake City, UT, Conference Report (1925 Oct).
 For examples: Gypsies, Jews, the Mountain Meadows Massacre, and on and on and on. I assume the history of Gypsies and Jews require no explanation; one of the pre- and post- justifications for the Mountain Meadows Massacre was that the travelers had poisoned a well.
 <Jerusha>, comment in thread “Answering Jeff Lindsay,” Catholic Answers, 2008 May 12. See also: “I’m not going to waste my time swatting Mormon flies,” <falcon>, comment (2009 May 06) at blog Mormon Coffee, “If You Only Had a Few Minutes,” by Aaron Shafovaloff (2009 Apr 27); (Nauvoo resident): “it is very strange visiting with all the wacko’s walking around with their shirts and ties on. What happened to the days when you could have a beer or glass of wine and not worry about the mormon flies[?]”<cljchw>, comment at blog Nauvoo & the Mormon Invasion, “In the Beginning…” by <LaVelle>, 2005 Aug 01; see also Steven DJ Sills, Tokyo to Tijuana: Gabriele Departing America (Project Gutenberg, 2004) p12: Ch26 ¶2, 21.
 “But the ‘women who feel themselves wronged’ and desire the patronizing pity and advertized charity of the females who run the establishment, do not seem to be conspicuously numerous. The ‘Mormon’ flies do not flock into the parlors of the ‘Christian’ Industrial Home. What is the matter? Why, the pious people who have made a failure of the concern attempt to cover their utter and complete defeat by ‘Christian’ defamation and bearing false witness against their neighbors.” No author listed, “‘Christian’ Industrial Home Falsehoods,” The Deseret Weekly 39 no 16 (Salt Lake City, 1880 Oct 12) p501. As before, the quote finds itself in thin company. I identified very few written instances of “Mormon fly” in the nineteenth century, and almost all of them were casual references to the creature itself. Thus, the sobriquet might not have been common, might not have been the sort of thing one wrote down, or maybe the little buggers are hiding from my search algorithms.
 <beckyluvs4>, comment on board, “DIE, CICADAS!! DIE!!!!,” 2008 Jun 09. Also: kudos to Bob Saar for his article “Their names are legion” (TheHawkeye.com, Burlington, Iowa, 2008 Jul 13). “We can’t call them mayflies anymore. [¶] We can’t call them that other name, either. They are not “Mormon flies” even though some of you still use that term. That’s not cool. It’s an insult to a lot of good people for no reason other than that some people hated some other people over a century ago. …You might not like it if they were called Popepests or Christ critters or Buddha bugs. No one likes to have their religion or icons smeared like riverflies under the wheels of a car.”