PROC. ENTOMOL. SOC. WASH. 83(4), 1981, pp. 725-758 REVISIONARY NOTES ON NEARCTIC MICRODON FLIES (DIPTERA: SYRPHIDAE) F. Christian Thompson Systematic Entomology Laboratory, IIBIH, Agric. Res., Sci. and Educ. Admin., USDA, % National Museum of Natural History, Washington, D.C. 20560. Abstract. — A key to the adults and puparia of the species of Microdon Meigen found in America north of Mexico is given. For each species a short synonymy and summary of distribution and ant host records (new records for 11 species) are given. Lectotypes are designated for M. aundentus Fa-bricius, M. globosus Fabricius, and M. luanitohensis Curran. New syn-onyms are given for M. globosus (=cilbipilis Curran, =conflictus Curran, ^pseudoglobosus Curran, and =hutchingsi Curran), M. lanceolatus Adams {=nu)destus Knab, ^senilis Knab, and =similis Jones), IVf. ruficms Willis-ton i=basicornis Curran and =champhuiu Curran), and M. tristis Loew {=robusta Telford). Three new species are described: M. abditus Thompson (New Hampshire, type-locality; eastern North America), M. abstrusus Thompson (Maryland, type-locality; Pennsylvania and West Virginia), and M. adventitius Thompson (Georgia). Microdons have maggots that look like molluscan slugs and live with ants. The adults are typical flies. Interest in microdon biology has increased re-cently (Greene, 1955; van Pelt and van Peh, 1972; Akre et al., 1973; Duffield, 1981), prompting a review of their taxonomy. My work has been done in conjunction with that of Duffield (1981), whose paper serves as a general introduction to mine. Most of the north temperate species of Microdon are similar to one another, and their taxonomy has been considered difficult. Williston (1887), Johnson (1916), Knab (1917), and Curran (1925) have struggled with the taxonomy of Nearctic microdons, each successively contributing more to its resolution. Their works have been restricted to adults and have tended to divide intraspecific variation more finely and thus to recognize more species. I depart from this trend and recognize fewer species. Immatures and adults have been studied together, with the result that adults have been discovered to display more intraspecific variation than previously assumed possible. Akre et al. (1973) show that two distinct color morphs of cothur-
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