For most people, flies fall into the background noise. They rarely grab our attention, and unless they bite us or threaten our hygiene, we barely give them the time of day. Malaysian stalk-eyed flies (Cyrtodiopsis dalmanni) are no exception.
These tiny insects go about their business in the densely vegetated forest understory, where there is plenty of food in the form of rotting plant material. As far as flies go, they don’t even fly very much, or very well, preferring to scuttle around on spindly legs.
Stalk-eyed flies are handsome creatures, though – a warm russet brown with white blotches on their wings and a pair of spiny protuberances on their thorax.
But I’m skirting around the edges here, because by far the most obvious thing about these flies is their eyes, and particularly those of the males.
Why are they eyes of the stalk-eyed flies so incredibly weird - and big?
Not just because they are a rather splendid bright red and accessorise well with the rest of their bodies, but because they are positioned, along with a pair of stubby antennae, right at the ends of extraordinary peduncles that stick out on either side of their heads.
Small and easily missed they might be, but when your gaze finally meets theirs (it was an entomological seminal moment for me) it is a vision that can never be unseen. These flies’ eyes are so unusually spectacular that to say they stand out is an understatement.
Both the male and female ‘stalky’ (as entomologists are wont to call them) have these eye extensions, but those of the males are by far the most impressive, and it seems that the females agree. A ‘biggun’ can have an eyespan of some 17mm, which makes them wider apart than the body is long.
Once they’ve ‘hatched’ and before their exoskeleton hardens, the flies need to inflate their eye stalks – a process that resembles a balloon being inflated at a children’s birthday party.
However, rather than turning into a ‘poodle’, the male stalky metamorphoses into a living sex god – to female flies, that is. Like a stag’s antlers, the ornamentation advertises both strong genes and good nutrition, and he’ll fight with them if required.
Male stalk-eyed flies gather in loose groups, forming temporary territories known as leks, where they catch the eye of the females. At these lekking sites, several things happen simultaneously.
The males literally eye each other up, face to face, and compare the lengths of their ‘antlers’. Usually, the smaller male realises he is beaten before any blows can be exchanged and retreats, but when an evenly matched pair meet, there follows much front leg waving. If this still doesn’t sort things out, the adversaries may resort to physical slaps.
The most successful male – the one with the biggest eye-stalks – has the monopoly and control of the largest number of females. They select him for his vigour, and his eye-stalks are an outward expression of this, as well as a sign that he has fit and healthy sperm. When he roosts at night, he will be surrounded by a bevy of willing females just waiting to mate first thing in the morning.
While having such large eye-stalks is advantageous, it does have an impact on everyday activities. Flying is a challenge, for example. However, these males also have bigger wings and larger bodies to compensate, which are also appealing characteristics for onlooking females. The fact they secure the most mating opportunities also means the big eye-stalk genes are passed on to the next generation.
Like the antlers of deer, it’s a classic, textbook example of sexual selection, just
a little smaller.
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