The Amazing Housefly Part 1: Stabilizers, Feet and Gender

You ever see some guy drive up into a parking lot, park his car, but then not get out for a bit? I see guys do that sometimes*, and I always assume it’s because they’re listening to something really good on the radio.**

*But never women. Let’s face it- guys are just weirder.

**Like one of those one-hit-wonder songs from groups that never put out another half-decent song- like Norman Greenbaum or Harvey Danger***- so you never bought the album, and you hang out till the end of the song because you don’t know when you’re going to hear it next. Or maybe they’re listening to a really funny comedy bit. Or maybe they’re just listening to one of those right-wing talk-radio jerks. Whatever.

***Confession: In the mid-90’s I actually bought a (the?) Harvey Danger album because I liked the song “Flagpole Sitta.” Terrible album.

But Friday morning after I pulled into my office parking lot, I sat in the parked car for a full 5 minutes, not to listen to “Spirit In The Sky”- the radio wasn’t even on- but to take close-up photos of the Housefly trapped inside the car, perched on the driver’s-side window. (Nice shot, eh?)

Tangent: I always imagine that for a fly- or any bug really- getting trapped in a car is like an alien abduction. 2 weeks ago we had a fly stuck in the car from Fruita, CO to Price, UT. What a weird trip it must’ve been for that fly.

As I’ve mentioned before, I have no idea how long I’m going to continue this project*. But before I end it- whenever that day comes- I have a list of things I really, really want to blog about, and I need to make sure I’ve covered all of the things on that list before I wrap it up. But here’s the thing: I haven’t actually written the list down, and if you asked me to recite the full list, I couldn’t. But every once in a while I’ll come across something, and just know it’s on the list. Houseflies are on the list.

*Though it is going to end one day, if only because I just hate things- 10 year-old sitcoms, “Rocky” movies, geriatric rock bands on yet another reunion tour- that never end. All things- especially good things- should have a beginning and an end.

All About Flies

Flies, true flies, of the order Diptera, are distinguished by their wing architecture. Dipteran have 2 wings, and 2 halteres behind those wings. Halteres are knobbed structures located behind the wings that are flapped rapidly during flight and act as flight stabilizers (maybe very roughly analogous to the tailfins of an aircraft, or the rear rotor of a helicopter.) (See diagram left from HowStuffWorks.) They almost certainly evolved from wings, as primitive insects had 2 pairs of wings, as many (most) species still do today. Dragonflies, damselflies, caddis flies, fireflies and butterflies are not true flies. Houseflies, horseflies, deerflies, botflies, gnats, midges and mosquitoes(!) are.

And, to start this series out with a bang, and show you just how awesome my little camera is, check this out: I got a photo of a Housefly haltere. Wow.

The fly in my car was a Housefly, Musca domestica. You can tell by the red eyes and the distinctive black and grey striped markings on the abdomen (pic below).

Those blue, metallic-looking flies people refer to as houseflies aren’t houseflies; they’re Bluebottle Flies, Calliphora vomitoria, which is a kind of blow-fly.

All About Houseflies

Houseflies occur all over the world, though they’re thought to have originated in Central Asia. They thrive in both tropical and temperate environments, and in both rural and urban settings. The Housefly lifecycle includes a complete metamorphosis from egg to larva (maggot) to pupa to adult. (pic below, not mine. BTW, adult at lower right = male, upper right = female, as we’ll see in just a bit.)

Females lay up to 500 eggs over a few days in batches of 75 to 150. Maggots hatch in a day or less, and immediately start feeding on whatever substrate they hatched in, ideally manure, though soils containing old manure also work. Horse manure is best for maggots, followed by human excrement, followed by cow manure.

After between 4 and 13 days the maggot pupates, transforming into an encased pupa while it develops into its adult phase. The pupal case is formed from the maggot’s last larval exoskeleton; maggots, like other invertebrates, need to regularly shed their old exoskeletons as they grow. The pupal stage lasts between 2 and 27 days, varying primarily due to temperature; in general the warmer it is, the faster the fly develops. The fly breaks out of the pupal case by hammering with a specialized organ on its head, the ptilinum.

Adult Houseflies can live as long as 2 months, but 2 to 4 weeks is more typical. They live longer at cooler temperatures, and lifespan appears to be extended by access to- get this- sugar. Once emerged from their pupal cases, their first order of business is eating; they need food before mating. The female requires protein (manure alone won’t cut it) to produce eggs, and so this is a time when she’s particularly likely to land on your hamburger. After mating, the female starts laying eggs in 4 to 20 days.

Speaking of eating, Houseflies- like Spiders- can eat only liquid food. They liquefy solid morsels by regurgitating digestive juices onto them (as of course you already know if you saw Jeff Goldblum in the remake of The Fly.)

This fast, productive lifestyle is extremely prolific. Flies in most of the US complete ~12 generations/ year, and closer to 20 down in the tropics. It’s been estimated that if an “Adam & Eve” pair of flies started in April, and every single one of their eggs survived and reproduced, and so on and so on, throughout the summer, by August they would have produced close to 200 quintillion (200 followed by 30 zeros) flies.

OK, but let’s get back to the pics I shot on my driver’s-side window, and the obvious question they raise: Why are my car windows so dirty*? Haha, no really, the question is: How do flies stick to glass? There are all kinds of answers floating around, from sticky feet to wet feet (capillary action) to apparently-smooth surfaces being actually pretty bumpy at a microscopic level to Van der Waal forces (which actually may well be partly at play as well) but the real answer is that a fly’s foot is pretty darn sophisticated.

*My car always needs washing. When people ask why my car is dirty, I tryy and spin it as a positive, attributing it to me go-getter-outdoorhead lifestyle. But really I’m just kind of lazy.

Fly legs- like most insect legs- are tipped with a pair of very small claws, called the tarsal claws. These claws can hang on to nearly any protuberance on a landing surface, but on glass or super-smooth plant surfaces there’s sometimes nothing to claw onto.

In between the tarsal claws, a Housefly has an organ called a pulvillus, which is a retractable floppy little sack, sort of like a deflated balloon. The pulvillus is coated with hundreds of teeny oily little hairs, the adhesion between which and the surface holds the fly in place.

Side Note: Hymenopterans- Bees, Ants, Wasps- have a similar but different structure, called the arolium.

So here’s the cool thing. In this photo you can just barely make out the extended, yellowish pulvillus holding the fly to the glass.

These pulvilli stick to all kinds of surfaces and things, including bacteria, and since Houseflies regularly land on manure, rotting flesh and human food, that’s how* they spread disease.

*One of the ways, anyway. The vomiting onto food-bits, and of course their feces “help” as well.

Back to names. As longtime readers go, I’ve thrown out about a thousand Latin names for different creatures in this blog, and most go in one ear and out the other. But I love the Housefly’s Latin name: Musca domestica. “Domestic” = “House” obviously, and “Musca” is practically the same as “Mosca”, which, as every Spanish-speaker or gringo who took high school Spanish knows, is the Spanish word for “Fly”. And that reminds me of a joke, which coincidentally, is a great setup for the 2 topics we’ll tackle about Houseflies: sex-determination and vision.

But here’s the thing: the joke only works in Spanish, because of the wordplay involved. But it’s easy Spanish, so surely if you live in pretty much anywhere in the US, you must know at least enough Spanish to follow along*.

El Chiste

Hay turista norteamericano en restaurante en Mexico.

Turista: ¡Oye camarero! ¡Hay un mosca en mi sopa!

El camarero mira la sopa…

Camarero: No señor, hay una mosca en su sopa.

Turista: ¡Ay caramba! ¡Que buena vista tienes!

*Really? You don’t speak any Spanish? OK, here’s the joke. An American tourist is in a restaurant in Mexico and he tells the waiter there’s a fly in his soup. But he uses the masculine version of the word “a” or “un” for the fly, or “mosca”, which is a feminine word in Spanish. The waiter corrects his Spanish with the feminine article, “una”, but the tourist misunderstands and thinks the waiter has identified the fly as female, as which point he exclaims, “Wow, what good vision you have!”

Tangent: If you don’t speak Spanish, you should. No, no, not because everyone in the US will speak Spanish in 10 years. That’s a bunch of xenophobic hokey. Recent immigrants speak Spanish, but their children and grandchildren overwhelmingly prefer English. No, the reasons are:

1- It’s a high ROI language to learn. You can travel to dozens of very different countries with some basic Spanish (as I highlighted in the Blue Piñon series.)

2- It’s a much better language than English. Seriously. It follows all the rules, each letter always makes one- and only one- sound only*, and a far lower proportion of the verbs are irregular than in English. Conjugation and syntax are simple and easy. At the same time though, it routinely conveys clearer meaning and specificity than English, through a complete subjunctive mode, a real 2^nd-person plural pronoun and conjugation (so you don’t have to say “y’all” or “you guys” all the time), a clear distinction between 2^nd person formal and informal conjugation (so you don’t talk to a Nobel prize-winner like you do to your toddler) and many, many examples of greater verb specificity, as highlighted in “ser” vs. “estar”, and “saber” vs. “concocer.”

*Well, except “h”, which makes no sound.

3- If there really is any kind of justice in the universe and an afterlife of any sort, it is an absolute certain given that in that next life, English-only speakers will be mowing the lawns, washing the dishes and cleaning the bathrooms.

Nested Tangent: I should confess that my own Spanish, though sufficient for travel, is pretty lousy. From time to time I endeavor to improve it, but I just don’t travel often enough. Lately I’ve been watching Univision in the morning while riding the trainer, but after following along a bit, I get lost, zone out and end up just spinning and staring at the weather-ladies*. Then I realize with a start that I’m essentially just watching soft-porn and switch the channel before Awesome Wife catches me.

*Seriously. Why are the meteorologists on Spanish-language TV so unbelievably-smoking hot?

But the joke brings up the whole fascinating topic of gender, and specifically gender-determination, in Houseflies.

All About Sex-Determination (In Flies)

With humans and other mammals sex is determined by sex-specific chromosomes: XX = female, XY male.

But insects are more complicated. Ants and Bees for example use a haploid-diploid system: females have 2 sets of chromosomes (like us) while males have just 1 (and arise from unfertilized eggs.) Flies are all over the place. Fruitflies for instance, which diverged from Houseflies ~120 million years ago, use an “X0” system, meaning that 2 “X”s make a female, and 1 “X” makes a male, and there is no “Y”.

Houseflies are a bit more like us. They have 12 chromosomes arranged in 6 pairs. One of those pairs is either “XY” or ”XX” and determines whether the fly is male or female, with the “Y” chromosome being passed through the male-line only, just like with us.

And that’s how it works. Sometimes.

Other Housefly populations have been discovered in which there is no “Y.” All of them- female and male- are XX. In these flies, another gene, called “M”, has been located that sits on one of the other non-sex-specific chromosomes, or autosome, (i.e. not the “X”) which creates a male Housefly when present.

Tangent: There’s a fascinating “Y”-less sex-determination system in the mammalian world. In recent years it’s been discovered that all Mole Voles, Ellobius lutescens, native to the Caucasus Mountains, are X-only- both females and males. There is no Y chromosome, nor does SRY, the gene on the mammalian Y chromosome responsible for “male-ness”, appear to exist on any other chromosome. The analogy isn’t perfect; Mole Voles are all (male and female) X0, rather than XX, with just a single X chromosome, and a diploid chromosome count of 17. So far, the sex-determining gene and its location are unknown.

The Mole Vole is of particular interest to biologists, because the mammalian Y chromosome is small, seems to be shrinking over time, and some researchers suspect that it may eventually become “extinct”, leading to the head-scratcher of how humans in the distant future might reproduce.

OK, so that’s interesting, but it gets even weirder. It turns out there are other Houseflies in which all the flies- male and female- are both XX and have M. In these flies, another gene- called FD, which is present in, and passed down by, female flies- deactivates M, and so sex is determined by the female, not the male. FD is a special variant of a gene, F, which is present in all Houseflies, and expresses “female-ness”, but is usually switched off by M. But the FD variant is dominant even when M is present, and flies carrying this (originally) mutant gene are always female.

Confused yet? There are even some other variants, but the point is that Houseflies show a wide range of- or a high degree of “plasticity” with respect to- methods of sex-determination, making them fascinating subjects for study.

Let’s return though to our waiter. Could the tourist have been right? Could the waiter have actually determined the sex of the fly by looking at it? It’s unlikely. But we can, and we can do it with my awesome driver’s-side window photos.

Male houseflies have larger eyes than females, set closer together. The bigger the separation between the eyes, the likelier that Housefly is female.

In this case the distance between the eyes is about as great- or greater than the diameter of the eye at its widest point, so we know it’s female.

Why are the male Housefly’s eyes so much bigger? To see better, one might reasonably (and correctly) assume, but that just raises another question: Why do males Houseflies need to see better than female Houseflies? To which the answer is: to find female Houseflies. Males that find females quickly and more often leave behind more descendants.

But this in turn raises a 3^rd question, which is how does a larger eye help the male Housefly see better? And this leads to the whole super-amazing architecture of Housefly eyes, which turns out to be way more complex, fascinating and all-around way cool than you ever knew.

Next Up: The Eye of the Fly