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Just above the Hall of Saurischian Dinosaurs on the fourth floor of the American Museum of Natural History is the laboratory of entomologist David A. Grimaldi. Entering it, one finds microscopes and computers, a battered, overstuffed sofa and easy chair, and a row of cabinets with tiers of shallow drawers. These cabinets hold 15,000 pieces of translucent, orange-yellow amber containing fossils of ancient flies, ants, and mosquitoes that would have tormented some of the dinosaurs on the floor below. Amber is greatly compressed pitch or resin from various conifers as well as from some tropical broad-leaved trees. It very likely developed early in the Cretaceous Period (135-65 million years ago) as the trees' defense against an increase in herbivorous insects. It was to the trees what flypaper is to us--much the same color and, if anything, stickier.

Grimaldi studies insect fossils in both amber and stone. The stone fossils--mostly limestone--are the more abundant and cover a broader span in space and time. Fossils in amber date from no earlier than the Cretaceous and are limited to outcrops in a few places, such as Lebanon, Russia, Myanmar (formerly Burma), the Caribbean, and the east coast of the United States, from Martha's Vineyard to Georgia.

Some twenty years ago, while a graduate student at Cornell University, Grimaldi saw his first amber fossil. It immediately grabbed his attention. "I was amazed that more entomologists weren't studying amber fossils--I still don't understand it," he says. "You can study a cell's nucleus and mitochondria under an electron microscope, even its amino acids and maybe its DNA." His fascination hasn't diminished, and in 1996 he organized the Museum exhibition "Amber: Window on the Past."

Insects are descended from marine arthropods, he explains, and first appeared on land less than 400 million years ago as small, wingless bugs resembling silverfish. Between 345 and 280 million years ago, the forests buzzed with giant dragonflies. But Paleozoic insects such as these (along with 98 percent of all other life-forms) became casualties of the unprecedented mass extinction at the end of the Permian, and gradually, from the Triassic to the Cretaceous, insects evolved into more familiar forms. Says Grimaldi, "The Cretaceous interests me because that is when a lot of modern insect families evolved--and angiosperms, or flowering plants."

From his cabinets, Grimaldi produces two small bits of Cretaceous amber, one encasing a tiny bee, the other a small cluster of oak flowers. They might have come from Central Park, accidentally frozen into a clear orange popsicle. The 65-million-year-old fossilized bee has changed his thinking quite a bit: "We never knew or surmised there would be a bee from the Cretaceous. It's still the only bee we've found." This bee proves that bees had already evolved by then, but judging from their absence in the earlier fossil record, they were not yet plentiful. Flowering plants, on the other hand, were abundant: they appeared in the beginning of the Cretaceous, 135 million years ago, and explosively radiated 35 million years later, eventually becoming the dominant form of multi-cellular plant life on Earth.

Not only bees but also moths, butterflies, and long-tongued flies are relatively scarce in the Cretaceous fossil record, raising interesting questions: What was pollinating the flowers and causing their proliferation? What was decomposing the carcasses, given the total absence of fossils of the larger flies, such as flesh flies and blowflies (whose larvae, which we call maggots, feed on carrion)? And finally, since termite fossils are rare, what decomposed all the Cretaceous plant matter? We don't know.

In another cabinet drawer is a 90-million-year-old ant from an amber deposit in New Jersey; this ant defended itself not by stinging but by spraying formic acid. Grimaldi believes the existence of this fossil suggests that ants may have evolved as early as 110 million years ago.

Ants, like termites, were rare and primitive in the Cretaceous, but they were already social. Some of Grimaldi's pieces of amber have several ants in them--an unlikely occurrence unless the insects spent a lot of time together. And after identifying worker ants in several of the fossils, he believes that ants had already begun to specialize into castes. In addition, many species of Cretaceous ants and termites display a variety of specialized structures, another feature of social insects.

"For years, people who have studied social insects like ants, termites, bees, and certain kinds of wasps have said that sociality was critical to a group's abundance and a driving force in its ecological success and diversity," says Grimaldi. "But apparently not, for ants just squeaked along for their first 50 million years of existence, even though they were social. For some reason, at the beginning of the Tertiary Period, around the Paleocene or the lower Eocene, they became abundant and diverse. And the studies I am doing with termites reveal the exact same thing."

If sociality wasn't responsible for social insects' success, what about the effects of the cataclysmic impact that marked the end of the Cretaceous and the beginning of the Tertiary, an interval often called the K-T boundary? After all, if the resulting cold and darkness killed off dinosaurs and allowed our own rare and primitive shrewlike ancestors to radiate, might it not have performed the same service for the ants, termites, bees, and wasps by getting rid of their competitors?

Grimaldi thinks not. The fossil record for insects just before and just after the K-T boundary is virtually nonexistent. There is evidence over the broader sweep of time, however, that these insects made it through unscathed--and that the insects that might have been their competitors did not go extinct. Grimaldi attributes the increase of social insects in the Tertiary to climate change, their own evolution, and the further evolution of flowering plants. Meanwhile, he continues to search for new amber deposits, certain they will offer a window on insects' past.

Henry S.F. Cooper Jr., a former staff writer for the New Yorker, has been visiting the Museum since he was four years old, when his father sat him in a cavity of the Willamette meteorite.