Los Amigos Biological Station sits within the Peruvian Amazon—one of the planet’s richest hotspots for life. Countless species fly, scurry, climb and burrow through the surrounding rainforest. To be at the station is to be surrounded by life at its most diverse and wondrous.

But you don’t have to go into the forest to find diversity.

The research station has a kilometre-long airstrip, and its borders are thick with climbing squash vines descending from the trees. A team of scientists led by Marty Condon from Cornell College collected some 3,600 flowers from these vines, all belonging to just two species. They found entire worlds.

The flowers were home to 14 species of fly, which lived nowhere else. “When we go out in the field, we collect every flower, fruit and stem of this group. These particular flies have only come out of these two plants,” says Condon. Most were even restricted to either the male or female flowers of their chosen plant.

There was more. Condon also found 18 species of parasitic wasps, which attack the fly larvae and lay eggs inside their bodies. Two of the wasps were generalists that attacked a wide variety of hosts. But the vast majority were specialists that targeted just one of the 14 available fly species, even though there were several possible targets around.

It’s difficult for us humans to appreciate just how restricted and specialised creatures like this can be. We are a global species. When you can travel to the other side of the planet in less than a day, it’s hard to imagine how dozens of species can exist nowhere else but on a single type of flower.

Most of these species all looked the same. Condon’s team used their DNA to tell them apart. Specifically, they sequenced a gene called COI in all of the flies. Your version of COI is a 98 percent match for a chimp’s, but none of the fly species from the squash flowers shared more than 96 percent of their COI sequences. “A 4 percent different is huge,” says Condon. Once the DNA had split the lookalike flies into different groups, it became easier to find more visible differences between them, from subtle physical traits to distinctive mating rituals. “These flies really are extraordinarily different.”

Of course, with over a million known insect species, and many millions more left to discover, we expect insects to be diverse. Even so, Condon was astonished by what she found. If two flies exploit exactly the same resource, you’d expect the more efficient competitor to eventually oust its rival. In this way, species partition themselves into distinct niches—each one specialised to a certain area, or food source, or time of day. They can co-exist because they each do their own thing.

“That’s the standard scenario: there should be one thing on each kind of resource,” says Condon. “But when we got our samples in, we thought: Whoa, this is not like that at all. We found multiple insects feeding on exactly the same tissues of exactly the same species.” For example, the male flowers of Gurania spinulosa are home to 9 of the flies and 12 of the wasps. “That was totally unexpected,” says Condon. This town ain’t big enough for two, let alone nine or twelve.

Why so much diversity? The team found a big clue when they dissected almost 400 fly pupae. They found that many wasps actually do lay eggs in the larvae of several fly species—it’s just that the wasp larvae can only survive in the right host. If they get implanted into the wrong one, they’re dead. The host kills them, perhaps by mounting some sort of immune defence.

That explains why there are so many wasps on the two flowers: the majority of them are adapted to parasitise a single fly species, and the others are inhospitable.

But why are so many species of flies? Condon’s hunch is that this group of flies is ancient and widespread. They have been feeding on the same sorts of flowers throughout South and Central America for around 6 million years. That period saw the rise of the Andes, and regular waves of drought and rain. These changing conditions would have regularly fragmented the local plant populations, and the flies could have adapted to these isolated pockets. “Then previously isolated plant habitats come back together and bingo, there are multiple fly species on same host plants,” says Condon.

Alternatively, the flies could be diversifying because of the wasps. Each adapts into an “enemy-free space”, becoming impervious to the existing wasps. The wasps counter-adapt, so that both parasites and hosts foster extra diversity in each other. They’re caught up in an evolutionary game of cat and mouse (or wasp and fly), diversifying into new forms as they play. As Andrew Forbes, who was involved in this study, has previously show, diversity can create itself.

It’s a fascinating study. Different species can be separated by physical barriers like mountains or by rivers, or because they’re active at different times of the day, or even because they harbour different gut microbes. But in this case, it seems that the tangled interactions between parasites and hosts create the barriers that keep species apart, and set up entire webs of life on single flowers.

That’s the basic idea, but there’s a lot still to discover about these insects. For example, how do the flies kill the wasps? And how do the generalist wasps manage to target so many different flies with impunity. And why would the specialist wasps ever occasionally lay eggs on the wrong fly? The team found that they only ever did this on plants that also contained the right host. So, Condon believes that the wasps are tracking down their hosts with some sort of chemical cue, but once they’re in the right ballpark, they sometimes get confused. “They think, ‘It looks like I can put my babies here somewhere’, and they make mistakes,” she says.

“I think it’s pretty outrageous how much diversity could be out there,” says Condon. If just two flowers could play host to so many species of insect, just think about how many more are lurking on or in the other plants of the rainforest. How many are there, and how would we ever find them all?

Reference: Condon, Scheffer, Lewis, Wharton, Adam & Forbes. 2014. Lethal Interactions Between Parasites and Prey Increase Niche Diversity in a Tropical Community. Science http://dx.doi.org/10.1126/science.1245007