With a small handful of exceptions, all insects begin life as an egg. Once they hatch, they go through several stages of physical development in their lifetime. The physiological changes that occur and differentiate one stage of an insect’s life from another is called metamorphosis. While some insect species go through what is called “incomplete metamorphosis,” most insects experience dramatic physical changes during metamorphosis.
Types of Insect Metamorphosis
In general, entomologists divide insects based on the kind of metamorphosis they go through. There are 3 main classifications of insects based on metamorphosis.
- Ametabolous organisms exhibit very little or no metamorphosis throughout their lives. As such, ametabolous insects go through gradual development through their lives and larvae and adults have more-or-less the same body plans. Examples of ametabolous insects include silverfish, springtails, and bristletails
- Hemimetabolous insects show simple or partial metamorphosis throughout their lives. Hemimetabolous insects exhibit gradual physical and behavioral changes through the stages of their life. Hemimetabolous nymphs often resemble adult specimens but may develop wings or extra appendages as they mature. Hemimetabolous insects include cockroaches, mantids, termites, grasshoppers, and dragonflies.
- Holometabolous insects undergo what is called “complete” metamorphosis where each stage of their life is marked by a distinct physical appearance. Holometabolous insect larvae generally bear no resemblance to their adult forms and may exhibit wildly different diets and behaviors. The most common examples of holometabolous insects are butterflies and moths. Both butterflies and moths begin life as caterpillars that have a drastically different appearance than their adult forms. As they mature, they go through metamorphosis into a pupal resting stage and then emerge as winged adults. While in the pupal stage, the insect’s tissues and organs completely liquify and rearrange into the adult form.
What Causes Metamorphosis?
In insects, metamorphosis is controlled by hormones produced by endocrine glands. Several hormones work together in feedback mechanisms to promote physical change, molting, and metabolic delay. Several insects display the ability to delay metamorphosis and physical development in unfavorable conditions.
While it is known that hormones play a large role in initiating and regulating insect metamorphosis, relatively little is known about the exact cellular mechanisms that drive metamorphosis. This is partly because organisms that go through metamorphosis are extremely diverse and exhibit several distinct cellular mechanisms and because there is much debate over what specific physical changes should count as metamorphosis.
Evolution of Metamorphosis
The evolutionary development of insect metamorphosis is a key question in modern-day biology. The earliest insects were ametabolous though almost all modern-day insect species are hemimetabolous or holometabolous. It is believed that the first species of genuinely holometabolous insects emerged around 300 million years ago.
Throughout history, many theories have been proposed to explain the emergence of insect metamorphosis, some outlandish by modern standards. Aristotle argued that insect larvae were a kind of “secondary embryo” that exists before the perfect adult specimen develops, and Donald Williamson famously argued that butterfly metamorphosis emerged from an accidental mating between a flying insect and a ground insect.
Nowadays, it is known that insect larvae contain imaginal discs—clusters of tissue present in larvae that remain dormant until hormones from metamorphosis cause them to rapidly develop into adult appendages.
Scientists also have a plausible explanation for the evolutionary utility of metamorphosis. Insects that undergo metamorphosis have an evolutionary advantage as young and old insects do not have to compete for the same stock of resources. For instance, caterpillars spend all their time eating on leaves while butterflies hunt for nectar and mating opportunities. Since adults and the young don’t have to share the same resources, they can co-exist more effectively.