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Genus Microdon

Fly with forked antennae - Microdon Syrphid - Microdon Larva in ant nest. - Microdon Microdon Fly - Microdon megalogaster Microdon Microdon ruficrus - male Microdontinae, I think - Microdon craigheadii
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Diptera (Flies)
No Taxon ("Aschiza")
Family Syrphidae (Hover Flies)
Subfamily Microdontinae
Genus Microdon
Synonyms and other taxonomic changes
Genus significantly depopulated by Reemer & Ståhls(1)
Explanation of Names
Microdon Meigen 1803
microdon = 'small tooth', refers to points or short spines at posterior corners of scutellum
~20 spp. in 3 subgenera in our area, ~130 spp. in 6 subgenera worldwide(1)
NB: the nearctic M. craigheadii Walton and over a dozen spp. worldwide are not placed to subgenera(1)
7-28 mm(1)
The larvae are round and dome-like, often with a reticulate patterning and a conspicuous single tubercle posteriorly (above the rim of the "dome") formed by the fused posterior spiracles.

Puparia look just like larvae but have two well-separated, nipple-like anterior horns (spiracles) which appear ~2 days after pupation.

Adults are medium-sized bee-mimics with distinctive long antennae.

Mating pair

Identification to species may be difficult. Having matched puparium + adult can be very helpful; knowing the host also helps. Puparia are typically found near the periphery of the nest and are relatively easy to rear. If you find an adult flying or perched, it's a good idea to scan the surrounding area for potential host nest, such as Formica mound, or a Camponotus nest in wood or soil.
Key to spp. (puparia & adults) in(2)
Worldwide, incl. much of North America; particularly diverse in the Neotropics(3)
Larvae live in ant nests; adults near ant colonies (do not visit flowers but may hover over ant mounds)(3); host associations listed in(4)
M. cothurnatus in WA: May-Jun and into Jul at higher elevations [Akre et. al.(1973)]; in NC, most spp. May-Jun, M. pachstylum Jul-Oct(5); in MN, M. tristis Jun, M. pseudoglobusus Aug; in OK, M. fulgens Jul(6)
2nd and 3rd instar larvae prey on host larvae (often Camponotus or Formica); 1st instar larvae have been observed probing host larvae, presumably prompting them to regurgitate pre-digested food fed to them by workers.(4)
Life Cycle
The newly hatched larvae are extremely mobile, and move rapidly into the ant nest(4). Larvae and puparia are accepted by ants and avoid attack by producing pseudopheromones (cuticular hydrocarbons) mimicking those of their ant host colony(7)(8). However adult Microdon lack the pseudopheromones and are not they will be attacked and dismembered. To avoid that fate, when nearing eclosion the puparia move close to nest exits and eclose in the early morning, when the ants have retreated deeper inside the nest and are more torpid due to lower temperatures. Upon eclosion, the fresh adult quickly exits the nest to avoid being attacked in its newly vulnerable form.
Microdon larvae were initially described as mollusks, and later as scale insects. Instars 1 and 2 mimic ant cocoons. If the nest is disturbed and workers start moving cocoons to safety, the Microdon larvae curl up to look like ant cocoons, and the ants will pick them up and relocate.(9)
Print References
Akre, R., Alpert, G., & Alpert, T. (1973). Life Cycle and Behavior of Microdon cothurnatus in Washington (Diptera: Syrphidae). J. Kans. Entomol. Soc., 46(3), 327-338. (1st page from JSTOR)
Akre, R., Garnett, W., & Zack, R. (1988). Biology and Behavior of Microdon piperi in the Pacific Northwest (Diptera: Syrphidae). J. Kans. Entomol. Soc., 61(4), 441-452. (1st page from JSTOR)
Akre, R., Garnett, W., & Zack, R. (1990). Ant Hosts of Microdon (Diptera: Syrphidae) in the Pacific Northwest. J. Kans. Entomol. Soc., 63(1), 175-178. (1st page from JSTOR)
Cole, F.R. (1923) Notes on the early stages of the syrphid genus Microdon (Diptera). Pomona Coll. J. Ent. Zool. 15: 18-19 (Full text)
Paulson, G., Akre R.D. (1994) A fly in ant's clothing. Nat. Hist. 103: 57-58 (Full text)
Works Cited
1.Generic revision and species classification of the Microdontinae (Diptera, Syrphidae)
Reemer M., Ståhls G. 2013. ZooKeys 288: 1–213.
2.Revisionary notes on nearctic Microdon flies (Diptera: Syrphidae)
Thompson F.C. 1981. Proc. Ent. Soc. Wash. 83: 725-758.
3.A generic conspectus of the Microdontinae (Diptera: Syrphidae) with the description of two new genera from Africa and China
Cheng X.-Y., Thompson F.C. 2008. Zootaxa 1879: 21–48.
4.Biology of Microdon fuscipennis (Diptera: Syrphidae) with interpretations of the reproductive strategies of Microdon species...
Duffield R.M. 1981. Proc. Entomol. Soc. Wash. 83: 716-724.
5.Insects of North Carolina
C.S. Brimley. 1938. North Carolina Department of Agriculture.
6.Syrphidae of Oklahoma (Diptera)
Shorter D.A., Drew W.A. 1976. Proc. Okla. Acad. Sci. 56: 75‒94.
7.Encyclopedia of Entomology, 2nd ed
Capinera J.L., ed. 2008. Springer, lxiii+4346 pp. (4 vols.).
8.Chemical Ecology of Insects 2
Carde R.T., Bell W.J. 1995. Springer. 435 pp.
9.Cocoon mimicry and predation by myrmecophilous Diptera (Diptera: Syrphidae)
Garnett W.B., Akre R.D., Sehlke G. 1985. Florida Entomologist 68: 615-621.