zoology biology benno meyer rochow science blog life eyes flies Florian Nock

Horny Fly Eyes

“Fly eyes are the better to see you with”

When Little Red Riding Hood in the Brother Grimm’s fairy tale of the same name asked the big bad wolf: “But Grandmother, what big eyes you have,” the wolf replied: “The better to see you with, my dear”. But there was no suggestion that the wolf had horny eyes. And wolves in Malaysia (if there ever had been any) also would not have them. However amongst the many truly weird looking insects that occur in Malaysia there are the “Stalk-eyed Flies” and these flies, as their name suggests, do possess eyes at the end of surprisingly long stalks which project laterally from their heads like the horns of a Watusi cow.

Although stalk-eyed flies were known already to Linné in the eighteenth century and occur in the tropics of Africa and Asia, only the Malaysian species have been studied extensively with regard to their physiology and behaviour, reproduction and genetic characteristics. Following eclosion from the pupal case, the eyes of the young fly are still short and folded backwards, but like the wings of the fly its eyes also become extended within the hour due to the insect’s haemolymph being pumped into them.

Sexual maturity is reached in 12 days, but individuals may live up to one and a half years. With a body length of only approximately 8 mm the flies are not exactly giants, but the distance between the left and right eye tips in males can reach 12 mm ! Their eyes consist of over 2,000 facets each and achieving resolutions of 1° of arc and flicker fusion frequencies of over 200 are comparable to those of honey bees and dragonflies when it comes to performance. And performance does matter: a flicker fusion frequency of 200 means that the impression of continuous movement only occurs if there are more than 200 separate images per second. Humans already begin to see a continuous movement when just 14 pictures (or frames in a movie) per second are viewed. And a resolution of 1° of arc gives the fly a fair amount of detail to see.

Malaysian stalk-eyed flies see well during the day but not at night, which is why they have resting places each night, which often are vertical aerial roots of jungle epiphytes at least two meters above ground and which are the same places every night. Each “sleeping thread” has a population of a male with its harem of female flies. A dominant male frequently engages in ritual fights with other males, but this has its risks, because while occupied with the serious business of establishing dominance, so-called sneaker males hiding amongst the females may grab the opportunity (and a nearby female fly) to attempt copulations.

The unusual lateral eye projections, which give the males their horny appearance and led to their English name, are now thought to have evolved as a response to male combat, for it could be shown by Burkhardt and de la Motte that males with the widest eye-span would appear most intimidating to others and also sire most offspring (females love their males’ horny eyes). Of course one could speculate how useful it must be to have a periscope on the head with which to peer around corners or from behind a leaf. But a weapon to butt the competition with or used to overpower prey, these eyes are certainly not, because all stalk-eyed flies are harmless plant-juice lickers.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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