The Golden-legged Mydas (Mydas tibialis)

This past August while visiting the Weldon Spring Site Interpretive Center in St. Charles, County MO, I stumbled upon one of my favorites that I have not seen since taking entomology at the University close to 20 years ago. When first encountering this insect you immediately think it must be one of the spider wasps or perhaps the great black wasp (Sphex pensylvanicus). For those who don’t immediately flee the area and instead look a little closer, you will see this is actually a very special species of fly.

Mydas tibialis, the golden-legged mydas nectaring from one of its favorite food sources, the blossom of Eryngium yuccifolium (rattlesnake master).

Mydas tibialis (golden-legged mydas) are Batesian mimics, meaning they are harmless mimics of a potentially harmful species, such as wasps. The adult form of mydas flies are purportedly short-lived. They spend the most of their lives underground where they feed on grubs in the soil.

In order to truly appreciate the size of a mydas fly, one must see them in person. If you are familiar with the size of a typical rattlesnake master inflorescence, then you might be able to appreciate this from these photos.

After doing a short bit of research, there doesn’t seem to be nearly enough known about the life history of our mydas flies. This is a shame. Not only are they fascinating animals with much waiting to be discovered but it also looks like they can be good biocontrol agents. Hopefully it won’t be another 20 years before I find one again.

Mydas tibialis, the golden giant of the Dipterans.

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