The rarity of the Stalk-eyed Flies
Flies of the family Diopsidae (Order Diptera), commonly referred to as Stalk-eyed Flies, sure look bizarre. 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.
Diopsidae occur mostly in Africa and SE Asia, although two species are found also in North America and one in Europe.
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.