Credit: A. STARK/C. DAUGERON

Males of a recently identified species of dance fly captured on Mount Fuji in Japan may be using strangely enlarged leg segments to court females.

Christophe Daugeron at the National Museum of Natural History in Paris and his colleagues found that some male specimens of Empis jaschhoforum caught in their traps had normal limbs (pictured below) but roughly an equal number had one forelimb with a huge final segment (top). One fly even had modifications to both forelimbs.

Such a high prevalence of asymmetry in a population with symmetrical features is unprecedented, say the researchers, who ruled out parasites and genetic differences between cells as causes. The engorged limb-tip is probably a sexual signal and the costs associated with this unwieldy feature may explain why slim-legged flies still exist.

Biol. Lett. 7, 11–14 (2011)