Once a caterpillar hatches from an egg, it spends its first few weeks of life eating as much as it physically can. It then hangs upside down from a leaf or stem and sheds its outer skin to expose the pupa. Inside, the caterpillar’s body breaks down and specialized cells called imaginal discs begin to form the skeleton of the butterfly that emerges. Within a few weeks they are ready to mate and begin the mating process again.
The process of metamorphosis is so strange that it seems like something out of science fiction. So how did such a unique life cycle evolve in the first place?
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But something changed about 400 million years ago, according to biologist and University of Washington professor emeritus James Truman. Small genetic mutations cause insects to look different in their adult and juvenile stages. This is a phenomenon called incomplete metamorphosis. Insects that undergo incomplete metamorphosis (called hemimetamorphic insects) begin their lives in a stage called the larval stage, rather than hatching as smaller versions of adults.
Nymphs still largely resemble their adult relatives, but they also have small pads on which their wings grow. Each time the nymph molts, the wing pads develop further, and the final molt reveals functional adult wings. Thanks to these wings, incomplete metamorphosis was a major evolutionary step for insects. Because wings are so delicate, it was difficult for insects to hatch with fully functional wings, so it was easier for insects to develop wings throughout their lives.
After about 50 million more years, more genetic mutations further altered the insect’s early stages, Truman said. These genetic changes produced holometamorphic insects, which are insects that undergo complete metamorphosis. Rather than hatching from eggs as nymphs, these insects began emerging as larvae, insect-like creatures that bore no resemblance to their parents.
“The true identity of the parents is [in] It is never reflected in the identity of young people. There’s no resemblance at all,” Truman told Live Science. [Latin] The term “larva” means “mask” and, in fact, the larval stage masks the adult stage. ”
evolutionary advantage
Currently, there are approximately 5.5 million species of insects on Earth, and more than 80% of them have undergone complete metamorphosis. The reason metamorphosis was so successful may be because it confers many evolutionary advantages to insects, the first of which is flight. Hemimetamorphic insects were the first animals to develop functional wings and took to the air much earlier than vertebrates.
“For 100 million years, insects have used the air as their playground,” Truman said. “This ability allows us to completely take over insects.”
Complete metamorphosis has additional benefits. Because the life stages of larvae and adults are so different, juveniles and adults can specialize in different things. Generally, larvae spend most of their time eating, while adults focus on breeding. In some cases, adults of species such as the luna moth (Actias luna) don’t even have a functioning mouth. After metamorphosis, they spend the rest of their short lives finding a mate and never eat again.
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Truman said metamorphosis also has benefits related to resource competition, since adults and larvae can eat very different diets. In many species, larvae often feed on short-lived resources such as carcasses and insects, whereas adults feed on long-lived resources such as nectar. This means that larger numbers of each age group can survive because larvae and young do not compete for the same food.
Although the evolutionary advantages of complete metamorphosis are clear, the details of how this complex process evolved in the first place remain unclear.
“There are two main schools of thought,” Xavier Vélez Ross, an honorary fellow at Spain’s National Research Council, told LiveScience in an email.
One idea, supported by Bellés Ros, proposes that complete metamorphosis evolved as the nymph stage split into larval and pupal stages. Opponents, supported by researchers including Truman, argue that the larval stage derives from an embryonic stage known as the pronymph, a short stage when the insect first begins to emerge from the egg.
Scientists know some of the key genes that control the larval, pupal, and adult stages of insects that undergo complete metamorphosis. “Each step appears to be controlled by a master regulatory gene,” Truman said. What is not yet clear is how the same genes function in simple insects that develop without such dramatic changes.
Still, researchers say the enduring mystery of metamorphosis is part of its appeal.
“After 30 years of working on this research (and I’m still working on it), I’ve only been able to solve a few mysteries,” Velez-Ross said. “There is still much work to be done, and this should be attractive to future generations of entomologists.”
Evolution Quiz: Can you choose the correct answer naturally?
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