Insects Ascendant

By Carl ZimmerNov 1, 1993 6:00 AM


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Bugs don’t need flowers as much as flowers need bugs: insects were fruitful and diversified long before the first flower bloomed.

Over the past two summers of digging in Arizona’s Petrified Forest, Stephen Hasiotis and Russell Dubiel of the United States Geological Survey have unearthed numerous hunks of sandstone filled with miniature tunnels, ramps, and chambers--all trademarks of termite nests. What makes the nests remarkable is that they are 220 million years old. Until now the oldest evidence of termites, and for that matter of any insect with a complex social organization, had dated from 100 million years ago--just about the time when flowering plants began to evolve. That coincidence made sense: the abundance of food offered by flowering plants, many researchers figured, must have given insects the opportunity to create complex societies and thus to improve their chances of survival. We screwed up that picture, says Hasiotis.

Hasiotis and Dubiel’s discovery may turn out to be just one part of a massive revision in our view of insects and their relationship with flowering plants. Today insects are by far the most diverse land animals, comprising an estimated 5 million species (compared with 4,000 mammals). Most are exquisitely adapted to feeding on some particular group of flowering plant, or angiosperm. Thus many researchers have assumed that the rise of angiosperms sometime after 115 million years ago opened the way for the diversification of insects.

But recently Conrad Labandeira of the Smithsonian Institution’s Museum of Natural History and John Sepkoski of the University of Chicago have followed several lines of research to just the opposite conclusion: the success of insects, they say, cannot be attributed to flowering plants. Insects diversified and flourished long before flowers did.

I’ve never bought the idea that there was nothing interesting about insect life before angiosperms, Labandeira says. His gut feeling was reinforced by his dissertation project at Chicago, a study of the mouthparts of fossil insects. Insects have evolved a kitchenful of feeding utensils, including hooks for ripping into plant sacs, and siphons for sucking up nectar. If angiosperms had had such an important effect on insect evolution, the appearance of many diverse mouthparts presumably should have coincided with the appearance of angiosperms. Yet Labandeira found that 85 percent of the manifold types of insect mouthparts existed before flowering plants did.

Meanwhile Sepkoski was building a data base detailing the origins and extinctions of families of fossil insects. Labandeira helped him flesh it out. A lot of insect research, it turned out, was hiding in relative obscurity in old German and Russian journals. It took five or six months to shake every source in all the languages, says Labandeira. Eventually the researchers raised the number of families in the data base to 1,263. That’s when they began to see patterns in insect evolution that had been invisible before.

From their origin 390 million years ago, it seems, insects have steadily increased in diversity, as measured by the number of different families alive at any one time. Compared with other life-forms, insects are actually slow to evolve new families--but they are even slower to go extinct. Some 84 percent of the insect families alive today were alive 100 million years ago; for land vertebrates the figure is 20 percent. By the time angiosperms became successful, the rate of diversification in insects had in fact slowed down, possibly because they had begun to saturate their habitats.

According to Labandeira, these patterns imply that insects evolved to take advantage not of angiosperms but of ferns, cycads, and other primitive plants that were already around more than 200 million years ago. It was then that they developed their eating equipment and started to diversify and form societies. Rather than being crucial to the rise of insects, angiosperms may have actually depended on the previous success of bugs--which are now the angiosperms’ chief pollinators--for their own good fortune.

How could researchers have believed the opposite for so long? They lacked a complete data base on fossil insects, Labandeira says, and the early record isn’t great anyway. But they may also have been biased by their surroundings. We now live in a world dominated by one kind of plant, says Labandeira. It’s just hard to take those angiosperm glasses off and imagine a world before it.

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