Ten facts about stilt-legged Flies (Micropezidae)
I'm truly thrilled that flies (Diptera) are making their debut on the ten facts series here on Expiscor. I'm equally thrilled that Morgan Jackson wrote these ten facts. He's an extraordinary entomologist, PhD candidate, and wonderful guy (as an aside, Morgan is partly to blame for getting me involved with blogging). You can follow his blog, or catch him on twitter.
So, here are ten amazing facts about diverse and beautiful flies in the family Micropezidae:
1. There are more than 700 species of stilt-legged flies, and they’re found on every continent, including Antarctica (sort of, see #9). While 700 species is a relatively low diversity compared to other families of flies and insects, they’ve still got most vertebrate groups beat.
2. One of the early people to work on micropezid taxonomy & systematics was Dr. Willi Hennig. While he’s better known for his work on phylogenetic systematics and cladistics (the process of classifying species by their shared, evolutionarily-derived characters rather than overall similarity), Hennig cut his scientific teeth on stilt-legged fly taxonomy. In fact, by the time he was 23, he had already published 7 papers spanning nearly 300 pages describing 9 new genera and 88 new species within the Micropezidae! He’d go on to describe another 4 new micropezid genera and an additional dozen new micropezid species by the end of his career, in addition to his many other contributions to Dipterology and evolutionary biology.
3. The greatest diversity of stilt-legged flies can be found in the Neotropics (Central & South America), particularly within the incredibly diverse subfamily Taeniapterinae. These flies are generally large, colourfully patterned, and often easily found in the tropics while they stand on foliage pretending they’re not actually flies! The majority of taeniapterine stilt-legged flies are convincing mimics of Hymenoptera, like ants (Formicidae), ichneumon wasps (Icneumonoidea) and spider wasps (Pompilidae).
4. What makes these flies so convincing is not only how they look, but also how they act. Most taeniapterine stilt-legged flies have dual-coloured front legs which they hold out in front of their head and wave up and down and all around, while running or flitting around the forest understory. If you’re not paying close attention, it’s easy to mistake these fly legs for ant or wasp antennae, which is exactly what the flies want you to think. This type of behavioural mimicry is also known in some flower flies (Syrphidae), but seems to be much more common (and I’ll say convincing) in stilt-legged flies.
5. This arm waving behaviour isn’t only just for mimicry purposes however. It’s been reported that at least one species of stilt-legged fly will do it while “sleeping”, potentially as an early predator detection system! In her natural history documentary “Mimicry, Sleep and Sex”, Patricia Ortiz shows how Ptilosphen viriolatus form nightly aggregations on certain plants, taking up positions on the tips of leaves and remaining there with their legs slowly waving up and down, back and forth, making sure nothing in search of a midnight snack catches them sleeping. Imagine dozens of flies all standing on the same plant and striking the quintessential sleep-walking pose and you’ve got the right idea!
6. Stilt-legged flies also use their long front legs as a flagging system for wooing a lady friend (Ortiz has excellent video of this in the documentary linked above). Many species have been observed undergoing complex mating rituals including arm waving, eye-stroking, and even “kissing” (or at least the dipteran equivalent; no tongue, but plenty wet).
7. Sometimes being a mimic and having a complex mating ritual doesn’t work out as you’d hope. In 1999, Dr. William Eberhard, who’s an insect sex specialist (I bet your high school guidance counselor didn’t present that as a potential career for you back in the day), observed a male stilt-legged fly in the genus Plocoscelus begin courting an Ectatomma ant. Everything was going smoothly until the ant responded by tackling the fly off the leaf, grabbing it in its mandibles, and carrying it back to its nest for a meal. And you thought your first date was awkward…
8. One of the more bizarre species of stilt-legged fly is found only in southwestern Australia, and literally serves as the punchline to the old joke “What do you call a wingless fly? A walk!”. Badisis ambulans is a wingless species whose generic and species names both translate to “walking” (Badisis is Greek, ambulans is Latin). Who ever said taxonomists don’t have a sense of humour? Badisis ambulans is unique because it lives in a species of Pitcher Plant (Cephalotus follicularis) as a larva, presumably feeding off the the decaying insects who fall in and die. While the plant is listed as Vulnerable by the IUCN, it is quite commonly cultivated, but unfortunately without it’s little walking buddy.
9. Badisis ambulans isn’t the only wingless stilt-legged fly. Out in the middle of the southern Indian Ocean on the subantarctic Heard and Kerguelen Islands (some of the most remote islands on Earth) lives a species of stilt-legged fly that currently makes up it’s very own subfamily, Calycopteryx moseleyi (subfamily Calycopteriginae). This species has been little studied, largely because it takes significant effort to get to the islands where it’s found, but it is suspected to be in decline on Kerguelen Island as invasive, feral rabbits are feasting on a plant that it is thought to be closely associated with, Kerguelen cabbage (Pringlea antiscobutica).
10. Despite all of the facts and incredible behaviour I’ve shared here, we know exceedingly little about the taxonomy, biology and natural history of these flies, and as can be seen with Badisis and Calycopteryx, we may be running out of time. While we know how or where to find many species as adults, we only know the larval habitats of a handful of species, and there are likely hundreds of species awaiting discovery and description. As with most of flies, there are enough unanswered questions to keep entomologists and evolutionary biologists busy for decades to come!