The New York Times, November 29, 2005
In the annals of life, insects are one of the great success stories.
A little over 400 million years ago, their six-legged ancestors came out of the water onto dry land. They have evolved into an estimated five million living species — dwarfing the diversity of all other animals combined. Even if you throw in all the known species of plants, fungi and protozoans, insects still win.
Insects are also a success in terms of sheer biomass. Put all of the insects on a giant scale, and they will outweigh all other animals, whales and elephants included.
And insects are also ecologically essential. If all humans decided to leave for Mars, taking all the vertebrates with them, the disruption to life on Earth would be incomparably less than the catastrophe that would ensue if insects disappeared. Forests would probably collapse, rivers and oceans would be poisoned, and many other animals would starve.
Two entomologists have now written the first book that chronicles this success story. “Evolution of the Insects,” published by Cambridge University Press, results from five years’ labor by David Grimaldi of the American Museum of Natural History and Michael Engel of the University of Kansas.
Dr. Grimaldi and Dr. Engel are well qualified for the job. Among their many accomplishments, they identified the oldest insect fossils from a 410-million-year-old rock in 2004. But to write “Evolution of the Insects,” they went well beyond their own research and synthesized the work of the armies of scientists who study living insects, dig up insect fossils and discover evolutionary secrets in insect DNA.
This effort has produced an increasingly clear picture of the rise of insects. Their success, scientists now recognize, did not occur overnight. The oldest living lineages of insects — which include bristletails and silverfish — number only 900 species today. These early insects may not have been able to become very diverse because they didn’t have wings. When insects later evolved the ability to fly, they gained the ability to explore more territory and find new kinds of food — giving rise to more species.
Early flying insects grew wings that stuck straight out — as illustrated by mayflies and dragonflies, the oldest flying insects alive today. By 300 million years ago, other insects had evolved folding wings. This innovation may have given a new boost to the diversity of insects. Such an insect could keep its wings safely tucked away as it crawled through leaf litter, squeezed under tree bark or even dived into water.
It is unlikely that insects would have enjoyed their great success without the success of plants, which evolved from algae about 450 million years ago. Plants set the table for insects. They provided a vast amount of food for the taking, spurring the evolution of all manner of insect mouthparts for nibbling, sucking and drilling.
The assault of insects prompted the evolution of sophisticated chemical weapons in plants. But the insects evolved defenses against them. Some insects, like monarch butterflies, can recycle poisons from plants they eat, making themselves poisonous to birds and other predators.
As Dr. Grimaldi and Dr. Engel make clear, though, vegetarianism is hardly the rule among insects. Some of the most successful lineages eat other insects, drink the blood of mammals and birds, or lay their eggs inside unlucky hosts.
Scientists are beginning to learn how some of these transformations took place. Take fleas. Recent research has revealed that fleas descend from mosquitolike insects called scorpionflies, which have long wings and powerful eyes, aiding them in finding insect carcasses for food.
A clue to how scorpionflies evolved into fleas comes from the fleas’ closest living relatives. Known as boreids or snow fleas, these 24 species walk across snow in late winter to feed on moss. Unlike other scorpionflies, snow fleas have tiny wings that are useless for flying. They don’t have the keen eyesight of other scorpionflies, probably because they need their eyes only to detect predators.
Once the ancestors of fleas split from snow fleas 160 million years ago, they continued this trend. They lost their wings altogether and their eyes became completely covered over. But at the same time, they were adapting to a new habitat, the hair of mammals and the feathers of birds. That shift turned out to be a key to evolutionary success. There are now 5,000 species of fleas, 200 times as many as their snow flea relatives.
Insects came on land more than 50 million years before our vertebrate ancestors arrived. They were barely touched by the mass extinctions that annihilated the dinosaurs and marine reptiles 65 million years ago. But Dr. Grimaldi and Dr. Engel believe that they now face a challenge almost without precedent in their history: humans.
Roaches, houseflies and a few other species have adapted well to a human-dominated world. Insects that eat crops have even evolved resistance to almost all the pesticides farmers have sprayed on them. But other species are not so fortunate.
Insects that live only in certain habitats can face extinction when their homes are taken over — when houses are built on coastal dunes, for example. Other insects depend entirely on a single species of plant for food, and if that plant disappears, they may disappear as well. While pests evolve resistance, pesticides are devastating neighboring insects that don’t feed on the crops. So we can expect more pests and fewer bees and butterflies.
“The behavior, life histories, ecological interactions and biology of most insects in our own yards and city parks are largely unknown, let alone the millions of species in remote regions,” Dr. Grimaldi and Dr. Engel write. “We will never know the full extent of what we are losing.”
“Evolution of the Insects” may at least show us what came before.
Copyright 2005 The New York Times Company. Reprinted with permission.