Genus Microdon

Genus significantly depopulated by Reemer & Ståhls(1)

Microdon Meigen 1803

~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.

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)

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 protected...so 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)