OTTAWA — Larval fruit flies, supposedly relentless devourers of rotting fruit, at times leave their regular laboratory food to stalk, kill and group-cannibalize some of their older, fatter fellows, scientists report.
This predatory cannibalism shows up in Drosophila melanogaster, the fly species that generations of biologists have grown in untold numbers, Roshan Vijendravarma of the University of Lausanne in Switzerland reported July 6 at the Evolution Ottawa scientific congress. He and Lausanne colleagues documented the behavior in both Canton S fruit flies, a strain raised in labs for more than six decades, and the Valais strain, brought into culture only in the last two years.
Because fruit fly genetics is known in such detail, Vijendravarma said his discovery may allow researchers to study the evolution of predatory cannibalism at the DNA level.
The closest reports Vijendravarma has found to what he’s witnessed describe larva of a different fruit fly, Drosophila hydei, dining on an already dead youngster of its own kind. What Vijendravarma reported is not just feeding on a happenstance free lunch, but hunting as well. He showed close-up videos of the dark, pronged mouthparts of a smaller larva scraping again and again against the wide, cream-colored body of a larger one. Finally the big larva’s body rips open, exposing softer flesh. Vijendravarma also showed photographs of clusters of small larvae side-by-side with their mouths against the flesh of a much larger one.
Those larger larvae represent the final phase of childhood for fruit flies. After a voracious race to eat and grow as much as possible, the big (in fruit fly terms) blobby almost-adults wander off to go through their last metamorphosis, which turns a pale wormy cylinder of a larva into a winged adult. That last larval phase appears to be a food bonus worth stalking, Vijendravarma said. In a feeding test, more than a third of the younger larvae survived by eating nothing but the older ones.
“I have never read or heard about this, and I was absolutely stunned that nobody has ever noticed this before,” said Thomas Flatt of the University of Veterinary Medicine in Vienna. “This story is to my mind of great biological interest, and it shows very clearly that there are many surprises left, even in a well-studied model organism.”