There was a week last month when I was very into collecting dead horse flies. This is not a hobby that I had pictured for myself, but here we are. It happened like this: I was working outside, sitting on the Visitor Center patio at Gilsland Farm, when I heard a tiny ‘smack’ on the window next to me. I looked down and saw a horse fly on its back. Upon further investigation, I noticed that this fly’s eyeballs were gorgeous. Anyone who’s been told that they have pretty eyes has nothing on this guy. For the rest of the week, I was stalking the patio looking for additional victims, putting them under a microscope, and showing my coworkers pictures of dead flies in meetings. Since this passion project was on the clock, I figured I ought to make something out of it, so here are some fun facts about insects in the order Diptera (the true flies) to flaunt at your next dinner party or first date.
EYES
Tell me these aren’t some of the prettiest eyes you’ve ever seen. Not all flies have such colorful eyes, but horse and deer flies, species in the family Tabanidae, are known for them. Like most insects, flies have compound eyes made of many tightly-packed light-sensitive columns called ommatidia. Each has a lens, and the lens acts like a tiny prism, refracting certain wavelengths of light (aka colors) based on the thickness of the lens cuticle. Various species have their own special eyeball pattern, which means that there’s likely some benefit to having stripes, polka dots, and even zigzags of varied lens cuticle thickness. Perhaps it enhances contrast or filters out certain colors, making others more pronounced. Sadly, this is not a very researched subject. Also, despite having many lenses, fly brains aggregate the images from each lens into a single image, so they only see one of you.
STAYING CLEAN
I don’t blame anyone for thinking that flies are gross. Things like scat and decomposing organic material provide moisture, food, and warmth for eggs and growing larva. This doesn’t mean that flies enjoy being dirty, though. Flies actually clean themselves incessantly and thoroughly. Watch a perched fly, and you’ll notice it grooming itself like a mammal, and probably a lot like you in the shower! Flies groom in a predictable pattern, first using their front legs to clean their eyes and antennae (the important sensory organs), then switching to their back legs to clean their abdomen, wings, and lastly their thorax. Periodically they brush accumulated debris from their legs. Researchers at Howard Hughes Medical Institute’s Janelia Research Campus looked into what was controlling this sequence, and they hypothesize that higher-priority cleaning movements, like wiping the eyes, suppress lower priority movements, like cleaning the thorax. This type of research could be the foundation to other fun mysteries, like what’s behind the complex sequential courtship dances of some bird species.
FLY DIVERSITY
Flies come in a wide variety of colors, patterns, and lifestyles, and most want nothing to do with humans. These are just a handful of the one hundred and fifty plus families of flies worldwide, which together represent over 110,000 described species.
Robber flies (family Asilidae) are ambush predators. Their beefy thorax helps them catch and hold on to prey. Watch for them darting from vegetation to catch insects in flight.
Many flower flies, also called hoverflies (family Syrphidae), are bee mimics. Like many groups of flies, they’re important pollinators. There’s currently a Maine Flower Fly Survey collecting data on iNaturalist. Anyone can join the project and contribute! Find them hovering over flowers and foliage.
Soldier flies (family Stratiomyidae) love some garbage, but they’re harmless to people and are usually quite sluggish. Certain species are wasp mimics.
Long-legged flies (family Dolichopodidae) are often found on foliage, not so much near garbage. These tiny metallic gems hunt many insects that are considered pests, like aphids, termites, and mosquito larva. They’re very common; I often see them on the leaves of outdoor ornamental plants.
I won’t go into detail on Flesh Flies (family Sarcophagidae), because the name speaks for itself, but one fun fact is that they give birth to live young, rather than eggs. That’s kinda cool, right? Right??
Tachinid flies (family Tachinidae) are parasitic. Eggs are laid on or near host insects, which become food for the hatched tachinid larva. This makes them handy as biological control agents targeting invasive insects. The fly pictured below has its eye on that (non-invasive) Brown-hooded Owlet Moth caterpillar!
There are actually two families of “fruit flies,” Tephritidae and Drosophilidae, encompassing over 9,000 species. The species that those fly researchers covered in dust and watched take a bath is in the family Drosophilidae. I’m personally Team Tephritidae because of their neat eye and wing patterns.
Bee flies (family Bombydiilae) are also called bomber flies, because many species launch eggs coated in sand (to weigh them down and mask their scent), into the burrows of solitary bees and wasps. Some, like this Tiger Bee Fly, are quite large (and harmless to humans).
Did I do it? Did I make you like flies? It’s okay if not, but I hope to at least leave you with a vague sense of wonder about the diversity of life whizzing past our heads every day!
By the way, if you’re wondering how to tell if a flying insect is a card-carrying dipteran, there are a couple of things to look for first. One is two wings, rather than four. The ancestors of modern flies had forewings and hindwings, like bees and butterflies. The hindwings evolved into tiny club-shaped structures called halteres on modern flies. Halteres are used for balance in flight. The eyes are another clue: true flies have huge eyes that take up most of their face. Bee and wasp eyes are much smaller.
*Most of these photos were taken with a macro lens attachment on an iPhone 13. These lenses are a lot of fun, especially with newer phone cameras. The model I use is the SANDMARC 25mm macro lens.
