Jozef Oboňa & Peter Mikula
While many of us are not sure of the answer to the question there are organisms who are unmistakable about it for a long time. We are not going to deal with any anthropocentric issues which trouble certain part of population but with a tiny aberrant group of insect. Therefor several following lines will be dedicated to flies marked as “Stalk-eyed flies“ or if you prefer to flies with the eyes on stalks. The two-winged flies belong predominantly to the tropical Diopsidae family and they only compose a small family of flies with the size of several tens of species. Due to their life-cycle and their way of life their presence is mainly adherent to humid places with decomposing vegetation e.g. in the surroundings of streams and rivers. One representative of this group (scientifically Diopsis macrophthalma) present in Africa is actually considered pest in the paddyfields 1. If you are impressed by their appearance or their way of life and you would like to see them live there is no need to travel that far because one representative of the family was recently also discovered in Europe and withal in our southern neighbouring Hungary2.
In some respect we hoped that we would also find the bizarre creatures during our recent expedition in Kenya but we started being sceptic after the look at the dry-washes and the droughty land. Eventually, we were successful in the north Kenyan mountain Mt. Kulal in the Nanta Mesi locality. It is a non-drying lakelet under the waterfall which is intensive used as a watering-place by the local shepherds during the dry season. We had no idea about the presence of the flies with the eyes of stalks until one of them trapped in the entomologic net. And that was it, we searched carefully the whole locality with the result – two specimen caught (although it could be better …). They are two specimen of the same species (belonging to the large group of similar species Diopsis apicalis – determination : Hans Feijen).
The bizarre creatures can be best observed at twilight (although it was at midnoon in our case) when they cluster every time at the same places. However, while females cluster narrowly to each other males are spaced out and they “fight“ for their place under the sun (better under the moon in this case). It is a kind of ritual during which the males compare the length of their eyes stalks and their span with each other. In the pursuit of maximization of the eyes stalks span some of the males turn to tricks with straight forelegs which visually elongate the distance between the two eyes stalks. There is no wonder at using such “falsehood“ though. There is truly a lot at stake and the process is cogently simple and straightforward in this case – the male with longer eyes stalks simply wins and gains more females3. There is a dog‘s chance for males with shorter stalks span for a successful upright transfer of his own genes and the famous assertion that “even a little clown can play a great theatre“ is not accurate in this case.
Scientists still speculate about the reason. The results of their research bring every time almost the same result: the male attractiveness rises with the length of their eyes stalks. That may suggest females themselves support the development of the extreme morphology of the males‘ eyes stalks by their sexual selection4. We could think “poor male“ at the thought of neverending stress on the growth of the eyes stalks. (Un)luckily, the natural selection counterworks the effects of sexual selection rather firmly and it eliminates from the population the individuals with too excessive demonstration of the badge by means of reduced viability (e.g. particular individuals are clumsier or more noticeable and therefor become an easy meat for their predators). Like the other animal species the case of “stalk-eyed fly“ males is not concerned with a kind of ill-nature or torture of the mankind from the side of females. The female choice is even here a pragmatic matter as the length of the eye stalk proves to be closely associated for instance with total largeness, the wing length as well as vitality or male fertility. Longer length of the stalks finally means the larger body and increased vitality of the father which results in the production of robuster and more vigorous descendants5. Moreover, the scientists found out that the gene with coded information on the longer eye stalk is concerned with the gene that enables to beget more male descendants6.
To resume …
The length really matters at least in this case …
Translated by Mgr. Iveta Zubková.