The next time you turn on a light and see a silverfish scampering away, be thankful evolution didn't bestow it with the same development as some of its more advanced insect cousins.
Four insect groups comprising beetles, bees and ants, moths and butterflies, and flies and mosquitoes make up nearly 60 percent of the more than 1 million known animal species. They are so prolific and exhibit great diversity because of metamorphosis, a process in which larval, pupal and adult stages differ greatly, allowing each to occupy a different habitat and consume different food sources.
Now two University of Washington zoology professors are proposing a novel hypothesis for how metamorphosis evolved. Their proposition suggests that a change in hormonal function during embryonic development led to the evolution of a unique larval stage, an innovation that allowed a virtual population explosion among these species in the last 250 million years.
"Metamorphosis really opened up niches that weren't available to insects before that," said UW zoologist James Truman, who along with UW zoologist Lynn Riddiford published their findings in the Sept. 30 issue of Nature.
The earliest insects, which strongly resembled today's silverfish, lacked metamorphosis and their juveniles looked very much like adults except that they didn't have functioning genitalia. After the evolution of flight, more advanced species, such as cockroaches and grasshoppers, developed incomplete metamorphosis. Their immature stages, called nymphs, still resembled the adults except that they lacked genitalia and bore wing buds that only transformed into functional wings during the molt to the adult stage. In both cases, the insects molt, or shed their external skeletons, several times as they grow to adults.
The higher insects, species with complete metamorphosis, spend their juvenile
life as larvae that bear no resemblance to the adults. What allows the change
from, say, a caterpillar into a butt
University of Washington