A female fly on a male Gurania spinulosa flower
Experimental feature

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Experimental feature

The dazzling diversity of insect life in the tropics is even richer and more specialised than appears at first sight. A study at one site in the Peruvian Amazon has shown that just two species of flowering vine host 14 fly species that live on the plants and 14 parasitic wasp species that prey on the flies.

The fly and wasp species are almost indistinguishable by eye – new techniques of molecular biology are needed to tell them apart and reveal their startling interdependency, which researchers from Cornell College in Iowa and the University of Iowa describe in the journal Science. They were amazed to see so many species occupying what looked like the same niche, says study leader Marty Condon, because this went against ecological theory. The explanation lies in extreme specialisation. Each fly species eats the flowers of just one of the two closely related Gurania vines. It will eat either the male or the female flowers of its chosen species but not both.

The tiny wasps lay their eggs in fly larvae, inside which their own larvae hatch and emerge – killing their host in the process. Each wasp species preys on just one of the 14 fly species. If it lays its eggs in one of the other 13, the wasp offspring die.

“The reason so many species coexist appears to be that each fly has a very specialised ability to escape all but one parasitic wasp species, and each wasp has a very specialised ability to kill one specific fly species,” says Andrew Forbes, co-author. “Specialised interactions between species allow more species to live in the same place.”

The researchers found that the wasps could not always tell the flies apart. They sometimes laid their eggs in the “wrong” fly species, whose immune systems would destroy the young wasp larvae.

Far more research will be required to explain how such an intricate and relatively self-contained community has arisen in such a small ecological space. Some sort of evolutionary “arms race” is probably involved. Natural selection may sometimes prompt the appearance of a fly that resists attack by all existing wasps; then one of the wasps will evolve in a way that enables it to parasitise the new fly.

Charles Godfray, zoology professor at Oxford university, comments on the study in Science: “Condon et al provide a wonderful example of a ‘society where none intrudes’,” he writes, quoting Lord Byron. “The web of interactions they reveal is complex and raises fascinating hypotheses about how different species influence each other’s dynamics, both directly and indirectly. These ideas need to be tested experimentally.”

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