This is my first post to the insect museum blog, though I’ve been at NCSU for a couple years now. Actually, its the first time I’ve blogged about anything anywhere. I guess it took an insect this amazing to drag me into the 21st century:
What you see above are alternative male morphs of a new dance fly species, Empis jaschhoforum (family Empididae), just described by my colleague Christophe Daugeron) at the Paris Museum (with myself and two other co-authors). The specimens were collected in a malaise trap on Mt. Fuji, Japan in 1999 and later sent to Christophe. He was astounded to discover that males with strangely modified, balloon-like tarsi were identical in every other way to normal Empis males in the sample. There are lots of other dance flies with weird secondary sexual traits, but two things are unique about Empis jaschhoforum: first, that the modification is asymmetrical in most cases, and occurs on either side of the body; and second, that the modification is polymorphic – that is, not all males have modified legs. Only one specimen was found with both forelegs modified.
Since no one has studied these flies alive, we can’t be sure what the balloon legs are used for, but as in many other dance flies, they probably function to attract females. However, because they are not found on both forelegs or on all flies, there is probably some selective disadvantage to having grossly deformed legs. Maybe flies with one modified leg can’t chase females too well, and flies with two modified legs are hopelessly slow?
One thing is clear – somebody needs to go to Mt. Fuji and spend some time watching flies.
P.S. This is just one of the many really cool dance flies out there. You can find more at our project website, which I am hoping to move to insectmuseum.org in the next week or so.
C. Daugeron, A. Plant, I. Winkler, A. Stark, M. Baylac. 2010. Extreme male leg polymorphic asymmetry in a new empidine dance fly (Diptera: Empididae). Biology Letters, published online Sept. 22, doi:10.1098/rsbl.2010.0726.