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Dermatobia Redux

Raising two dipteran children was an interesting experience. It was embarrassing on a few occasions, when both of my arms started bleeding profusely in public; painful at times, to the point of waking me up in the middle of the night; and inconvenient during the last stages of the flies’ development, when I had to tape plastic containers to my arms to make sure that I will not lose the emerging larvae. But other than those minor discomforts it was really not a big deal. Perhaps my opinion would have been different had the bot flies decided to develop in my eyelids, but I actually grew to like my little guests, and watched their growth with the same mix of pleasure and apprehension as when I watch the development of any other interesting organism under my care.

Having two bot fly larvae embedded in my skin have also made me ponder once again the perplexing element of the human psyche that makes us abhor parasites but revere predators. Why is it that an animal that is actively trying to kill us, such as a lion, gets more respect than one that is only trying to nibble on us a little, without causing much harm? I strongly suspect that it has to do with our genetically encoded sense of “fairness” – we perceive parasites as sneaky and underhanded, whereas predators attack us head-on and thus expose themselves to our retaliation. They are brave, or so we think. This, of course, is a very naive and anthropomorphic interpretation of nature. A lion is no “braver” than a bot fly, who has to skillfully hunt mosquitos to assure the dispersal of her eggs and risk more dangers than a lion, a top predator with no natural enemies. Most importantly, to a bot fly we, humans, are a renewable resource – it is in the bot fly’s best interest that we live a very long life and thus can be “reused” – hence the minimum amount of suffering that this species causes. To a lion we are nothing more than a one-time meal. But we should not judge either species for their actions – there is no “good” or “bad” in nature – nature is amoral.

I am saying this to prepare you for a short video that I have made about my experience of raising a bot fly. I don’t want you to think that it is “creepy” or “weird”. It is simply a documentation of an interesting organism, who happens to develop in the skin of large mammals. But please be forewarned that this video includes a few sequences that some viewers may find disturbing. If you don’t want to have nightmares about things living inside you (which they already do, by the way), please don’t watch it. But if you are prepared to be open-minded and appreciate God’s wonderful creations in all their amazing glory, enjoy the show!

Puppy-killing scientist smuggles rainforest babies in body cavity

I am pretty sure that taking this very photo in Belize was the beginning of my adventure with the Human bot flies.

I am pretty sure that taking this very photo in Belize was the beginning of my adventure.

I almost got away with it – for five days I had covered my body and slathered insect repellant onto my skin with an almost religious zeal, but on the last day I faltered. I was in Belize, teaching macrophotography at the Bugshot workshop. The course was almost over, and so I relaxed and decided to shoot some red-eyed tree frogs in the rainforest around the lodge. I rolled up my sleeves but, because I had misplaced my insect repellant and was too lazy to look for it, I did not put any DEET on. Big mistake. As I photographed the frogs, clouds of mosquitos, perhaps sensing a new, unprotected warm body, went to town on my arms and face. But, this being my last day in Belize, I decided to ignore the little vampires and kept taking pictures. Later that day my arms were quite itchy, but it was nothing new or unusual.

That's a nice-looking butt – I knew that something was amiss when a strange tube started poking out of my skin. This turned out to be a bot fly's breathing tube.

That’s a nice-looking butt – I knew that something was amiss when a strange tube started poking out of my skin. This turned out to be a bot fly’s breathing tube.

Things started veering off course after I got home. Some of the mosquito bites kept itching and, rather than disappearing, started to get bigger. It didn’t take me long to realize that I had brought with me, embedded in tiny holes in my skin, larvae of the Human bot fly (Dermatobia hominis). This was not the first time for me to have this parasite. What was new was the number of these animals that had made my body their home – at least six of them were feeding on both my arms, with four spaced only a millimeter apart on my right forearm. In the end, only three of them survived the first week. One of the surviving larvae was on my elbow. It was a nasty little thing, very active and painful. It had to go. But I decided to keep the two remaining larvae. As strange as it sounds, I felt bad about killing them, but I also had never seen an adult bot fly, and this was my chance.

Human bot flies are well known to entomologists and people living in warm, tropical parts of Central and South America. I cannot think of any of my biologist friends working there who didn’t have a torsalo living in their skin at some point or another, often in such very inconvenient places as the eyelid, the upper lip, or the top of the head. These get extracted through a variety of methods that often involve suffocating the larva with glycerine jelly, raw steak, or duct tape, and then pulling or squeezing the larva out of the skin. These methods usually work, but there is always a risk of leaving a part of the bot’s body in the wound, which may lead to infection. On those occasions where I needed to remove a larva, I preferred to use a suction venom extractor, which enlarges the opening of the wound (warble) and pulls the larva out, still alive and in one piece. I only discovered this method, first described 13 years ago (Boggild et al. 2002. Clin. Infect. Dis. 35: 336-8), after a visit to my doctor. Her solution was to perform a surgery by cutting my arm open. I said “thanks, but no thanks” and did my own research on furuncular myasis.

A mature larva of the Human bot fly (Dermatobia hominis) is an impressively armored animal. And yet it caused relatively little discomfort when feeding, deeply embedded in the skin of its host, me.

A mature larva of the Human bot fly (Dermatobia hominis) is an impressively armored animal. And yet it caused relatively little discomfort when feeding, deeply embedded in the skin of its host, me.

Human bot flies (D. hominis), despite their name, are not interested in our species only. They will gladly feed on other primates, as well as ungulates and other large mammals. Similarly, other members of the bot fly family (Oesteridae), who preferentially target small mammals, will occasionally find themselves on humans. But we get infected with D. hominis more often than with other bot flies because of this species’ unusual strategy of dispersing its eggs. Rather than laying them on the ground in the vicinity of mammalian burrows, the way other bot flies do, the D. hominis female catches and lays her eggs on other exoparasites: mosquitos, ticks, and deer flies. The eggs hatch while on the intermediate host and drop onto the skin of the ultimate host, often a human, when they sense its body heat. Frequently they will use the hole made by the mosquito to enter the skin but they can also use a hair follicle to get inside. Even the newly hatched larvae are covered with spines that point up, which makes pulling them out from the warble very difficult.

The puparium of the Human bot fly. The tufts on the front of the body are anterior spiracles that allow the animal to breathe when it matures in this stage underground. As the puparium ages it changes color from light brown to black. Remarkably, the spiracles stay the same, orange color.

The puparium of the Human bot fly. The tufts on the front of the body are the anterior spiracles that allow the animal to breathe as it matures  underground. As the puparium ages it changes color from light brown to black. Remarkably, the spiracles stay the same, orange color.

Once in the skin, the larva undergoes three molts and in 7-10 weeks grows from the size of a grain of sugar to that of a peanut. Throughout this time the warble enlarges and occasionally bleeds, but otherwise it is relatively painless, unless the larva decides to munch on nerve endings. These wounds rarely get infected as the larva very likely produces antibiotic secretions. Once fully grown, the larva crawls out of the warble and falls to the ground, where it quickly buries itself and turns into a puparium. The wound usually heals completely within a couple of days. All in all, not a big deal. But some people, for whatever reason, don’t like to have a squishy, almost harmless animal living in their skin.

Although we don’t think about them as such, Human bot flies are beautiful rainforest animals, as much a part of that ecosystem as howler monkeys and Morpho butterflies.

A mature Human bot fly (Dermatobia hominis). Although we don’t think about them as such, these flies are beautiful rainforest animals, as much a part of that ecosystem as howler monkeys and Morpho butterflies.

A mounting body of research indicates that many parasites have evolved a way of manipulating the behavior of their hosts. A parasitic horsehair worm will make its otherwise terrestrial grasshopper jump into the water, where it then ruptures the grasshopper’s body and swims away. Parasitoid wasps who have just left the emaciated body of a caterpillar will be actively protected by their brain-washed host. Humans also fall victim to parasitic manipulation – there is evidence that toxoplasmosis, a disease caused by protozoan Toxoplasma gondii, makes men less intelligent and prone to take greater risks (it has to do with increasing the likelihood of ending up as food for large cats, Toxoplasma’s ultimate host; inexplicably, the effect on women is a statistically significant increase in their intelligence.)

After I had decided to keep two of my botflies and let them reach maturity, I began to wonder – have the generations of entomologists, who let these flies live in their skin as a kind of geeky right of passage, inadvertently selected for a strain of bot flies that manipulate human behavior towards letting the flies live? Or do I just have toxoplasmosis?

A newly eclosed Human bot fly, with traces of the ptilinum on its head, a reversible pouch that gets inflated with hemolymph to help the young fly break free from the puparium.

A newly eclosed Human bot fly, with traces of the ptilinum on its head, a reversible pouch that gets inflated with hemolymph to help the young fly break free from the puparium.

In any case, the flies survived in my skin for nearly 10 weeks, successfully emerged, pupated, and are now enjoying a brief life as adults. Brief, because adult bot flies have no functional mouthparts and cannot feed, which means that they only live for a few days. They are quite pretty – I would go as far as to say that, among insects, they undergo one of the most dramatic transitions from ugly to cute during their development.

It was an interesting experience and I am glad that I managed to bring these insects to maturity. But rest assured that the next time I am in Belize my bottle of DEET will never leave my pocket.

Stay tuned for a video with some awesome sequences showing the development of my bot fly!

Update: The video is now available.

A composite photo showing the stages of the Human bot fly’s development. The size difference between the first and the third larval instal is particularly striking.

A composite photo showing the stages of the Human bot fly’s development. The size difference between the first and the third larval instars is particularly striking.

Postscript
I let my bot flies live and I went to great pains to make sure that they survived their inadvertent exodus from their native land of Belize. Will this endear me to people who wanted to crucify me for killing a puppy-sized spider a few months ago? I am guessing, no. Do I give a crap? Take a guess. Incidentally, now that the dust has mostly settled, I can repeat that I did not kill the puppy-sized spider – another scientist collected and preserved it – although this bit of information somehow didn’t register with the online media. There was no point in clarifying this because it is completely irrelevant to the issue of scientific collecting – I have killed and preserved my share of specimens, and I will always defend biologists who have the unpleasant duty to do so.

Postscript 2
Gil Wizen has written about his experience of raising a dipteran child on his blog.

Mozambique Diary: Not all flies fly

Tsetse fly (Glossina sp.) from Gorongosa feeding on my blood. Luckily, tsetses in this area do not carry the dreaded sleeping sickness (but it does not make it any less painful).

Tsetse fly (Glossina sp.) from Gorongosa feeding on my blood. Luckily, tsetses in Gorongosa do not carry the dreaded sleeping sickness.

After a long hike in the scorching heat of the African savanna the cool, shady patch of tall miombo forest looked like heaven to us. I was in the southern part of Gorongosa, looking with a few friends for some elusive species of arthropods. But we were having little luck finding any and after several hours of strenuous walking the morale was low. As we stepped under the dark, inviting canopy of the forest, the drop in the temperature was palpable and we all relaxed, slowed down the pace, and the mood in the group immediately improved. But then, suddenly, somebody yelped “Ouch!” and at the same moment I felt a painful pin-prick at the back of my neck. Crap, tsetse flies! We looked around – they were everywhere. Clouds of them. We could see groups of dozens clumping on vegetation, taking into the air the instant they noticed the movement of our bodies. We ran.

A painting (undoubtedly the first and only) of a bat fly (Penicillidia sp.) burrowing in the fur of a Long-winged bat (Miniopterus).

A painting (undoubtedly the first and only) of a bat fly (Penicillidia sp.) burrowing in the fur of a Long-winged bat (Miniopterus).

Tsetse flies have long had a reputation for being one of the scourges of Africa, alongside malaria, crocodiles, and the plague of locusts. And deservedly so – some species of tsetses, all members of the genus Glossina, are vectors of nasty protozoans, including Trypanosoma brucei, the cause of the deadly sleeping sickness. Luckily for us, Gorongosa tsetses carry a different Trypanosoma species, T. congolense. This protozoan does not affect humans but unfortunately causes the chronic Nagana disease in cattle and horses, which explains the nearly complete absence of these animals around the park and in almost the entire region of central Mozambique. But knowing that tsetse bites are not going to kill us did not make them any more pleasant. Tsetses are large flies, about the size of a bee, and their skin-piercing mouthparts are much thicker than those of a mosquito. In other words, it hurts like hell when one jabs you with its proboscis, and you flail your arms like a madman to shoo it away while the fly escapes unharmed.

Members of the family Streblidae, such as this Raymondia sp., collected from the Hildebrandt's horseshoe bat (Rhinolophus hildebrandtii), often exhibit interesting adaptations in their wing morphology, such as the ability to fold them longitudinally along the back. This presumably helps them move swiftly in the pelage of their hosts.

Members of the family Streblidae, such as this Raymondia sp., collected from the Hildebrandt’s horseshoe bat (Rhinolophus hildebrandtii), often exhibit interesting adaptations in their wing morphology, such as the ability to fold them longitudinally along the back. This presumably helps them move swiftly in the pelage of their hosts.

But count yourself lucky. Imagine instead that you cannot shoo them away. You try to smack one but it runs, hides in your hair or some place where you are not able to reach, and it continues to bite. It only leaves your body to give birth somewhere in your house but then immediately runs back, guided by your scent and body heat. Oh, and imagine that this fly is the size of your fist (or a small puppy). Welcome to the world that bats are forced to live in.

Tsetses are members of a large group of flies, the superfamily Hippoboscoidea, all of which are exclusively hematophagous – blood is the only food that they are interested in. The tsetse family (Glossinidae) is the most basal (unsophisticated, one might say) member of this lineage of insects – they are always looking for a blood meal but never evolved the ability to stay with their tasty host. Bats are unlucky to have been colonized by two much savvier families of flies, the Nycteribiidae and Streblidae. These insects know the value of a good host and, once they landed on the furry back of a bat, they never leave it again. Over millions of years of coevolution with their mammalian hosts the bat flies have undergone a remarkable transition. From a free-flying ancestor, most likely very similar to today’s tsetse flies, emerged several lineages of highly modified, often completely wingless, spider-like creatures. Their body became flattened and very hard, making it almost impossible to squash them against the skin. In the family Nyctiberiidae the head turned into a small appendage that can be safely tucked away in a protective groove on the back and all traces of wings completely disappeared. These flies cannot survive for long outside of their host’s body and only feel at home when scurrying at an alarming speed in its dense fur. Their feet are armed with large claws that make it almost impossible to dislodge them from the hair of their host. They really don’t look like flies and when a friend spotted one on the body of a bat she called me to collect the bat’s “pet spider.”

In the closely related family Streblidae the wings may or may not be present, but even in the winged species the body is modified for squeezing through the fur, and members of the subfamily Ascodipterinae go even further in their commitment to the host. Much further. Once a female lands on a bat she sheds her wings and legs (yes, legs) and burrows head-first into the skin. Once there, her head and thorax sink into her own abdomen, and the skin of the bat overgrows her body. She becomes one with her host.

Penicillidia bat flies (Nycteribiidae) are some of the most unusual members of the order Diptera and hardly resemble their winged relatives. This individual was collected from a Long-winged bat (Miniopterus natalensis) in Gorongosa, Mozambique.

Penicillidia bat flies (Nycteribiidae) are some of the most unusual members of the order Diptera and hardly resemble their winged relatives. This individual was collected from a Long-winged bat (Miniopterus natalensis) in Gorongosa, Mozambique.

Female bat flies, like their relatives tsetse flies, are remarkably good mothers. The great majority of insects relies on what ecologist call “r-selection” in their reproduction – they lay hundreds or thousands of eggs, betting on one or two of them making it to adulthood. Bat flies, on the other hand, rely on “K-selection” – like humans, they prefer to invest a lot in a much smaller number of offspring, hoping that they will all make it to the reproductive age. They are larviparous – instead of laying eggs the female gives birth to a single, fully developed larva, which immediately turns into a pupa. While in her mother’s body, the larva feeds on “milk glands”, analogous to the mammalian mammary glands (if they were located in the uterus), and develops safely protected from the elements and predators. When the time comes for the mother to give birth she walks off the bat’s body and attaches the larva to the wall of the bat’s roosting place, usually a cave (which explains why bats that roost in rolled-up leaves and other less permanent places have fewer ectoparasites). Then she turns back and runs towards her host, guided by the smell and the heat of its body.

The recent Ebola crisis brought back the attention of the medical community to bats as potential reservoirs of the virus. Although there is no evidence that bats are in fact harboring the virus, there seems to be some correlation between instances of the outbreak and the presence of large numbers of bats in the affected areas. While reading the literature on both Ebola and bat flies I found it rather curious that nobody has tested bat flies for the presence of the virus – these are relatively very long lived (195 days on average) insects, who always stay (as pupae) at the roosting sites of bats, even when the hosts leave to forage elsewhere. They often move from one host species to another and, this point makes me really wonder why nobody has seriously looked at these flies as potential vectors, occasionally drop on and bite people. We know that they harbor a slew of pathogens – a recent study conducted in Gorongosa National Park on bats Rhinolophus landeri and Hipposideros caffer showed that flies living on these animals are vectors of Trypanosoma species that are ancestral to those that cause Chagas disease. Add to this the fact that one of the first cases of Marburg disease in Zimbabwe (caused by a virus related to Ebola) was caused by a bite of an arthropod (by default all unidentified bites seem to be classified by the medical community as “spider bites” and spiders in the area were tested, predictably unsuccessfully, for Marburg). It is far more likely that the bite was caused by a fly that fell off a bat.

A friend of mine recently expressed her dismay at “lowly” parasites. I beg to differ – if anything, parasites, including bat flies, are incredible examples of evolution at its best, organisms capable of both adapting to life in the most hostile of environments (the very substrate you live on wants you dead!) and resisting diseases that live inside your body. I cannot promise that I will not try to smack the next tsetse fly that lands on me but at least I promise that I will do it in the most respectful, considerate way.

Louse flies (Hippoboscidae) are close, equally modified for ectoparasitic lifestyle family of flies. This Lipoptena sp. was collected from a Nyala antelope while it was fitted with a GPS collar. Louse flies are parasites of large mammals and birds, and some are considered serious pests of sheep.

Louse flies (Hippoboscidae) are closely related to bat flies and equally modified for ectoparasitic lifestyle. This Lipoptena sp. was collected from a Nyala antelope while it was being fitted with a GPS collar. Louse flies are parasites of large mammals and birds, and some are considered serious pests of sheep.

My crumbling beliefs

A posion arrow frog (Dendrobates pumilio) next to a timber fly (Pantophthalmus sp.) on a palm leaf at La Selva Biological Station, Costa Rica.

A poison arrow frog (Oophaga pumilio) next to a timber fly (Pantophthalmus sp.) on a palm leaf at La Selva Biological Station, Costa Rica.

I always assumed that there existed at least a few immutable truths about the natural world, dogmas that had no exceptions, no matter how hard you looked for them. One of them, I thought, was the rule that flies, which cartoons and children books made me believe were the favorite food of frogs, should be significantly smaller than the frogs. And I pretty much accepted this gospel, until one day I found in the rainforest of Costa Rica, sitting on a palm leaf, a frog and a fly. This singular sighting shattered my entire belief system, or at least the part that pertained to frogs and flies. Because you see, the fly was a timber fly, a member of the family Pantophthalmidae, which are the largest flies in the world, some reaching the wingspan of 100 mm. Next to one of these monsters an adult poison arrow frog looks almost like a dog next to a horse.

A female Pantophthalmus bellardii from Paloverde, Costa Rica; notice the partially retracted, telescopic ovipositor.

A female Pantophthalmus bellardii from Paloverde, Costa Rica; notice the partially retracted, telescopic ovipositor.

Timber flies are a small family, consisting of only 2 genera and 22 species, all found in the lowland rainforests of Central and South America. In addition to their unholy size they differ from other flies in that their larvae are wood burrowers, something that traditionally has been the domain of longhorns and other beetles. There are other flies that feed on wood (some Syrphidae and Asilidae), but those are incapable of drilling their own tunnels in the wood and can only use those already created by beetles or other insects.

Little is known about the behavior of adult timber flies. Nobody is really sure if they feed at this stage, and if so, on what. They have never been seen mating, although oviposition has been observed. Females have a long, telescopic ovipositor, which they use to deposit eggs in the cracks of dead and live wood, depending on the species. These insects are not common – in all my years in the tropics I have only seen them four times, but each time my faith in the unshakable laws of nature suffered a bit more. What’s next, a land arthropod as big as a cat?

A portrait of Pantophthalmus cf. pictus from Guanacaste, Costa Rica.

A portrait of Pantophthalmus cf. pictus from Guanacaste, Costa Rica.

A female Pantophthalmus bellardii.

A female Pantophthalmus bellardii.

There is a fly in my car

Although the larvae of bluebottle flies (Calliphora vomitoria) feed on decaying flesh, adults are often attracted to flowers and feed on nectar.

Although the larvae of bluebottle flies (Calliphora vomitoria) feed on decaying flesh, adults are often attracted to flowers and feed on nectar.

“Honey, what died in my car?” I called my wife a few days ago, after the stench had become overwhelming. For a few days after returning from a long trip to Mozambique I had tried to pretend that the god-awful smell in my car was just a figment of my imagination. Alas, opening windows or cranking up the radio wasn’t helping, and it was time to face the reality. On the other end of the line my wife softly cleared her throat and in the tone of voice that I was hoping never to hear (“That kid of ours – your son – well, he isn’t yours”) she whispered “I kind of, sort of, hit a bird. He might have been sucked in under the hood.”

That explained a lot – the vile reek, the blowflies in my car. I decided to investigate it further, but found nothing. There was no decaying corpse stuck to the grill of the car, and no feathers anywhere I looked. And yet the stink was still lingering, and more and more flies were thrashing against the windows of my car.

I have nothing against blow flies. As a matter of fact, I like them a lot. One species of these insects, the greenbottle fly (Lucilia sericata), has recently become important in treating wounds infected with Methicillin-resistant Staphylococcus, especially in cases where antibiotic treatment or surgery are ineffective. Fly maggots applied to wounds help clean (debride) the dead tissue, speeding up recovery; they are also great at treating wounds infected with Gram-negative bacteria.

But the blow flies that were happily spawning inside my vehicle are known as the bluebottle flies, whose enchanting scientific name – Calliphora vomitoria – speaks volumes about both their appearance (in Greek “Calliphora” means “the one who carries beauty”) and lifestyle. I thought that they were pretty cool, but it was becoming difficult to breathe while driving, and the poor flies were  desperate to find food and water, and I was getting slightly annoyed with them trying to obtain both ingredients from my eyeballs. Something had to be done.

A jumping spider (Phidippus audax) with its blowfly meal.

A jumping spider (Phidippus audax) with its blowfly meal.

I followed my nose and zeroed in on the trunk of the car. Cringing in the anticipation of what I would find I gently lifted the lid, and a thick wall of funk hit my nostrils – the trunk was full of dog food. About 20 pounds of loose kibbles, originally dry but now wet and soggy, covered every surface. Now, some people say that processed food has little real animal protein in it, and that dog food is mostly ash and ground-up horse hooves, but the army of wiggly fly maggots in my car’s trunk proved them wrong. Bluebottle flies are carrion specialists, and if they found happiness in a pile of wet dog food that means that its animal protein content must be very high. It also must smell to them like a rotting corpse, which it certainly did to my unsophisticated, human olfactory organs.

I spent the next two hours scooping the dog food goo full of maggots from the car, spraying every surface with Lysos, vacuuming, and wiping. “Do you know anything about the dog food in the trunk?”, I inquired when I was done. “Oh, that thing”, my wife shrugged dismissively and walked away to tend her garden.

Stalk-eyed flies

Stalk-eyed fly (Diasemopsis  fasciata) from Gorongosa National Park in Mozambique [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Stalk-eyed fly (Diasemopsis fasciata) from Gorongosa National Park in Mozambique [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

I really don’t like when organisms that deviate from our narrow, anthropocentric perception of the natural world are described as “bizarre” but, let’s face it, flies of the family Diopsidae sure look that way. They are hypercephalic, which means that their head is extremely expanded in a way that places both their eyes and the antennae at the tips of very long, often almost horizontal stalks. This family of flies is not the only one that has eyes placed on long stalks, but in Diopsidae this feature is nearly universal. Diopsidae occur mostly in Africa and SE Asia, although two species are found also in North America and one in Europe. (One species is apparently quite common in Massachusetts, and I will definitely try to find it next summer.)

With eyes on such long stalks, keeping them clean is not an easy task. Not surprisingly, these flies spend a lot of time on personal hygiene. [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

With eyes on such long stalks, keeping them clean is not an easy task. Not surprisingly, diopsid flies spend a lot of time on personal hygiene. [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Although flies with hypercephalic features are considered a classic case of sexual selection driving the development of exaggerated morphological characters, Diopsidae don’t quite fit this explanation as both males and females have similar, greatly modified heads. In some species, however, sexual dimorphism exists, and males have longer eye-stalks than females. In such species females preferentially mate with males having the longest stalks, and these matings result in increased fitness of the females. Males engage in long, ritualized contests, where they clash with their heads, and the winner is almost always the individual with more widely separated eyes. This is because of a phenomenon known as hyperallometry – the larger the armament, the larger the body size, and thus the strength of the individual. Such contests serve as a simple way to assess the overall size of the rival, and smaller individuals will quickly give up the duel, sensing the strength of the larger rival.

It is not surprising that carrying your eyeballs at the ends of a long broomstick does not make your life any easier, and it has been shown that males who have particularly long stalks must also develop larger wings to compensate for the drag caused by their eyes. This, in turn, supports the idea of the “Handicap model” of sexual selection in these flies – because the long eye stalks make the male’s life more difficult, surely he must be a carrier of some excellent genetic material to be able to overcome the handicap of the gargantuan ornaments.

—————–

Update (22 Jan. 13): Thanks to Hans Feijen for identifying the fly species, which turned out to be Diasemopsis fasciata, and not Diopsis.

Stalk-eyed flies usually spend the night in large roosts close to small bodies of water (Guinea) [Nikon D1x, Sigma 180mm, flash Nikon SB-28DX]

Stalk-eyed flies usually spend the night in large roosts close to small bodies of water (Guinea) [Nikon D1x, Sigma 180mm, flash Nikon SB-28DX]