Before the Museum exhibition about Colour and Vision closes on 6 November, I thought I should write a piece about some of nature’s most amazing eyes (their patterns and shapes). I’m talking of course about those belonging to flies – the most enigmatic of all species on the planet – and specifically all the species referred to as stalk-eyed flies.
My first experience of stalk-eyed flies came while I was carrying out fieldwork in Costa Rica over 10 years ago and it can probably go down as one of my favourite fieldwork moments. So what happened?
I was walking along a path in the forest near the tropical field station La Suerte (where I was based for a month or so) when I saw this strange creature – a hammerhead fly – scuttling over the bark of a tree. I chased it round and round the tree and finally managed to catch it in my hands! It turned out to be this little beast – Richardia telescopica, one of the stalk-eyed species in the family Richardiidae.
It proved to be a great find, not just for me personally but also for the Museum as we have very few specimens of this species represented in the collection.
Fly families with elaborate headwear
It is not just the Richardiidae family that contain stalk-eyed species, in fact the stalk-eyed condition has evolved independently in at least eight families of flies. In these families, and generally only in males, the eyes have migrated away from the head and are now found on the end of stalks. Called hypercephalization, this condition also includes examples where elaborate structures such as antlers and horns protrude from the head.
Micropezidae, Diopsidae, Ottidae, Platystomatidae, Tephritidae, Richardiidae, Periscelididae and Drosophilidae all contain some species that exhibit some level of hypercephalization and, interestingly, the stalk-eyed condition has evolved independently more than once in Platystomatidae. This is a very good example of convergent evolution – where species that are not related have a very similar form (the hedgehog and the spiny anteater are another good example). It is also an example of recurrent evolution – where a similar feature evolves time and time again.
The ones that all have stalks
The most notable example where this occurs is the family of flies called Diopsidae. Every species has eye stalks and so they are the ones commonly called the stalk-eyed flies.
The Diopsidae are one of the acalyptrate families of flies, which is a subsection of flies that contains a huge array of families so diverse and varied in their feeding habits, ecology and morphology that it is hard to believe some of them are closely related to each other. Others that do appear to look very similar aren’t closely related, which further complicates their identification.
The Museum’s collection of Diopsidae contains about half of the world’s described species.
Most Diopsidae species are found in Southeast Asia and southern Africa. I was lucky to come across some ‘lekking’ in Ethiopia. Yes, lekking – as in the communal displays more commonly seen among deer species – is indeed practised by their ever so slightly smaller cousins, the flies.
Males come head to head and primarily compare stalk length. If there is a difference the male with the smaller stalks leaves and the larger male retains control of his territory (and often harem). However, when they judge themselves as equal they fight!
Most times this just involves squaring up to each other like alcohol-infused humans. But sometimes the fights become more physical, with males head butting each other or having fist fights. Yes, proper fly-on-fly boxing.
Nothing is more entertaining for a human female (me, at least) than to watch these males prance around, gesticulating with their wings and then going in for low punches (with their first pair of legs). So I can but imagine how much the females of their own species enjoy it!
More tube-eyed than stalk-eyed
Within the 23 species that exhibit hypercephalization in the Drosophilidae there are also some very unusual extra head modifications. The genus Zygothrica includes 15 species (out of a total of 62 species in this genus) that have hypercephalic males. Unusually, these males are also very variable. The most extreme example is Zygothrica latipanops, where there are two discrete classes of male – the α (extremely hypercephalic) and β (moderately hypercephalic) males. The former don’t have stalks so much as tubes.
Wise words in small form
Some Museum specimens come with stories to help us remember something about them and this is the case with the next family to have a species with stalked eyes.
Within Micropezidae there is only one species that exhibits the stalk-eyed condition: Anaeropsis guttipennis. And within our collection there is only one, slightly damaged specimen that only has one stalk! However beside this specimen is an essay – albeit the smallest of essays. Unwrapped, this essay (by Ernest Edward Austin – a great Dipterist and my forefather in curation terms) describes how several authors have moved the taxonomic placement of this species around. Ernest felt that any decisions to be made needed further proof and greater comparisons – wise words.
The most impressive stalk-eyed fly
Within the fruit fly family, Tephritidae, there are two species that have stalked eyes. One of them, Pelmatops ichneumonea, was once placed in a different genus and family. In fact, it is easy to see at first glance why this happened when you compare it with Achias, the genus it was originally described as. Achias is now within the Platystomatidae family and contains arguably the most impressive of all of the stalk-eyed flies – Achias rothschildii.
Why do the flies have stalked eyes?
So why do these flies have such strange protrusions from their heads? Surely it must be problematic coping with the everyday functions of life such as flying, eating and mating.
Well the answer is thought to go back to those males that I came across lekking.
We think that the flies have evolved this condition through sexual selection – many of the females show a greater preference for males with longer stalks. The males with the longer stalks are thought to be genetically and physically more superior than the smaller-stalked males, as it is an extra burden to support such large and cumbersome structures. But it’s not just her preference that has caused such stalks. The males and their interactions with each other have also impacted upon evolution. They use their eye stalks to determine their fitness when it comes to fighting over territories or lekking sites for the best access to the best females.
The traditional proverb is that the eyes are the window of the soul but in this case the eyes are the indication of future reproductive success.