"To me it was like seeing the Yeti," exclaims Dr Marcus Stensmyr, one of the team that made the find in an expedition to Grand Cayman in the Caribbean, the sole known home of this species, to resolve enduring questions about how the fly fits into the tree of life.
To the untrained eye, it looks like just another fruit fly, one of the Drosophilidae, a family consisting of about 3,000 species.
In fact most of the members feed on microbes, not fruit, and one of the more strange choices of habitat is made by this particular fly, Drosophila endobranchia, one of only three fruit flies known to have found a home on (and inside) land-crabs.
To the surprise of scientists, though probably not to most of us, these crab flies have been neglected in research since their description in a paper in 1966. In fact, D. endobranchia has actually not even been seen since its initial discovery.
To fill in this glaring gap in knowledge, scientists from Prof Bill Hansson's group at the Max Planck Institute for Chemical Ecology, Germany, set out last year to relocate these elusive flies on Grand Cayman.
Expedition members Dr Marcus Stensmyr and Regina Stieber pored over the original notebooks from Hampton Carson, who first found the crab flies in 1965 but were disappointed to find that all those sites had been developed.
Soon after, says Dr Stensmyr, came the Eureka moment, which led to the collection of 66 specimens so they could resolve detailed arguments about how to place the flies in the Drosophila family:
"One day driving around we came across a place which to me looked suitable to crab life, but which did not fit the original description of the habitat. We nevertheless decided to revisit the area after darkness. Upon arriving to the area we almost immediately saw in the headlights from our Jeep this enormous crab sitting by the roadside.
This was the first decent sized crab we had seen since arriving on the island. I dashed out of the car, ran up to the crab, and in the beam of the flashlight I saw flies scurrying over the back of the crabs! I could hardly believe it! The flies were real, they were still on the island and they actually lived on land crabs."
The flies strike a very peculiar sight in real life. They are absolutely reluctant to leave their crab hosts, no matter what," he says. "Even after you have picked up the crabs, something which the crabs obviously are not to happy about, the flies still sit tight.
"There are still many unanswered questions regarding the flies which we hopefully will be able to address in the coming years. A key mystery is what the heck the adult flies are doing on the crabs. When you see them they most of the time just sit there, they don't seem to be doing anything."