Transcription
Imagine waking up and your body had changed. Imagine parts of your old body were liquefied. Even parts of your brain were digested and used as energy to fuel the process of building a new you. This is the norm for the majority of animals on Earth. As most species transform their bodies through the biological process of metamorphosis.
Insects make up the majority of all named species of animal on Earth. And out of these creatures, the vast majority of them undergo complete metamorphosis, which is known as hol metabolism. This makes hol metabolis insects the normal ones and other animals like us the weird ones. This process is well known about, but what is often overlooked is just how strange and just how complicated it all is.
Although most insects transform their bodies through different life stages, some don't. Wingless insects like silverfish and bristle tails are descendants of an ancient lineage that last had a common ancestor with most other insects over 350 million years ago. However, they remained a lot more like this common ancestor, never evolving wings, but also they never evolved to transform their bodies with metamorphosis. When they hatch from their eggs, they just look like smaller versions of the adults and then they just grow in size.
There are other insects like dragonflies, termites, crickets, and cicardas that undergo metamorphosis, but it is not as transformative, known as incomplete metamorphosis. When these creatures hatch from their eggs, they live as a nymph that gradually forms wings and sexual organs over the course of several molds, transforming into an adult that sometimes lives in a similar habitat. This is known as hem metabolis.
Hol metabolis insects go through four distinct life stages. And the most significant for their survival strategy is the puper stage where they usually wrap themselves in a protective covering and enter an immobile non-feeding state where they transform this body. This life stage allows these animals to undergo a profound reorganization of their body's tissues. It may seem strange to voluntarily enter a non-feeding state, but this has proven to be an overwhelmingly successful survival strategy. The koloptra which are the beetles, the lepodoptera, moths and butterflies, the true flies and the himanopta bees, wasps and ants are the four largest orders of insects by some margin and all live this way. Today these big four combined with a few other lineages of hollow metabolis insects make up around 80% of all insect life. As insects constitute the vast majority of animals already, this may be how most animals on Earth live today. What's more is that it is highly likely that this has only evolved once and every creature that enters their final life stage this way has inherited it from the same common ancestor.
This is significant because due to very quick generations, insects evolve very quickly and have many abilities that have adapted from multiple different lineages like silk production or forming colonies that have evolved multiple times independently. Yet, insects have only evolved complete metamorphosis once. As insects that change their body in a pupa cover most animal species alive today, there is an unbelievably high amount of diversity among the different groups and many differences for how each aspect of this change plays out. However, easily the most famous example of the transition are the way a winding bending caterpillar molds its body into a winged colorful butterfly.
If you were to cut open a caterpillar's chrysalis, you would not find a caterpillar butterfly hybrid, but actually be a barely recognizable selection of the most basic components of an animal, like a central nervous system and breathing apparatus and a digestive system being remolded. A caterpillar spends its time eating an exorbitant amount of food to fuel the change. for its final mol. Instead of forming a new exoskeleton, it forms a much harder chrysalis, which is actually a highly modified exoskeleton that has evolved to become much harder, and all the segments fuse together, creating a continuous structure. This only happens to butterflies, and many of the other creatures do this differently. For example, moths usually spin themselves in silk to create their protective barrier. Many species of fly also protect their bodies using their exoskeleton, but the process is different. Rather than molting to reveal their protective shell, their final exoskeleton hardens and becomes a shell, which when talking about flies is called a puparium. Other hol metabolis insects like beetles and mosquitoes pupate in a completely different way.
Inside the chrysalis, hormones trigger a massrogrammed cell death and the laral body starts to break down into a biological soup. This is a lot of the reason why a protective casing is necessary as this fluid needs to be contained while such an extensive remodeling of the body is occurring. There are parts of the body that remain intact throughout the process, but major sections are completely done away with. A caterpillar's legs are entirely broken down. The salivory glands that allow caterpillars to produce silk are completely dismantled, and most muscles throughout their body are destroyed. Even a caterpillar's eyes are completely removed and broken down. Caterpillar eyesight is very poor and are unable to form images and only really detect light intensity or basic movement caused from shadows. Butterflies, on the other hand, have very specialized compound eyes that give them 360° vision, being able to see a large variety of colors for foraging on blooming flowers. No part of the caterpillar's eyes are maintained in the process. They are completely broken down, and butterfly eyes are created a new.
All of these major body parts are broken down into a fluid that surrounds the bug, consisting of mainly insect blood, but also proteins and fats from the stripped away cells. Almost immediately as the curtains of the crysalis close and the body is broken down, the insect releases digestive enzymes into the mix as well. As the highly nutritious fluid is digested to fuel the formation of a new body. This part of the process sounds crazy, but oddly out of the whole undertaking, this is one of the most familiar to humans and all other vertebrates. Programmed cell death is known as apoptosis, which is a fundamental biological process found throughout the animal kingdom. The way bugs break down their cells, digest them, and then use them as energy to create new cells is also a normal thing that all animals, including humans, do all the time. What makes hol metabolis insects special is that they do a whole lot of it in a very short time frame.
As these creatures trigger a highly programmed breakdown of the tissue, they are able to digest unneeded cells while keeping specific body parts with surgical precision. While many significant sections of the body are removed, there are many sections that remain throughout the process. The insects are able to regulate hormones throughout, which is crucial for regulating the whole process. And they can still get oxygen, which is also important as this is very energyintensive. The digestive system is cut at both ends, shrunk, and completely remolded. A caterpillar's gut is changed from being good for large quantities of solid food to an adult diet of liquid nectar. When this drastic remolding is occurring, the microbiome is significantly reduced in diversity and quantity, but some survive the complete process. Some species of gut bacteria are just exceptionally hardy, while others migrate to different tissues while the transformation is taking place.
Their brain and nervous system is changed and expanded as their behaviors need to be altered to live as an adult like the ability to navigate while flying. However, the nervous system is not completely replaced, and it is known that many laral neurons persist through metamorphosis and get integrated into adult systems. A study in 2008 subjected moths to certain odor at the same time as an electric shock, as a negative stimuli, both in caterpillar form and as an adult. The study found that the adult moths that were subjected to the shock as caterpillars had developed an aversion to the odor even though they had not been exposed to the stimuli at any point as adults. This shows that as a caterpillar, they do have some memories carried over from their laral form.
As the body is broken down, certain organized groups of cells called imaginal discs survive the digestive process, and these are where the insect grows its new body from. The imaginal discs are actually formed while the bug is still in the egg, and they are present throughout their time as a lava, roughly in the position they will be as an adult. Once in the puper stage, they rapidly grow into adult body parts like wings, legs, antenni, eyes, and genitalia and eventually reconnect. Each body part is grown from an imaginal disc. So there are two for the eyes and four for all four wings in butterflies and moths. Once the crysalis has formed, they grow rapidly, but the imaginal discs have already started forming into wings and legs in some species of caterpillar before this point, but they are hidden under the skin.
Hol metabolis insects are incredibly old and this complex process of insects transforming their body has been around since before the dinosaurs. The oldest fossil of these insects are known from the carbonifpherous period over 300 million years ago. Today, insects that form pupe are the more common type. But during the carbonifpherous, hem metabolis insects that undergo incomplete metamorphosis like dragonflies were considerably more dominant in the earth's ecosystems. There were a group of six-winged insects named the paleodictopa that were one of the most common insects alive at the time and are now completely extinct. Fossils of them found during their nymph stage shows they were hematabolis too.
The oldest hol metabolis insect fossils are of larae and fossil evidence of these creatures in their pup stage are much rarer. There is evidence of bore holes found in the triacic period around 200 million years ago that are likely made by pupating insects. These holes share a resemblance with various species of beetles that chew into wood as lava to make pupil chambers they use for their transformation. One of the earliest fossils of a pupating insect are of snake flies that have specimens trapped inside amber in Myanmar 100 million years ago stuck mid transformation. This lack of fossil evidence makes studying their evolution difficult. And while it isn't fully understood how hol metabolis insects evolved, it is highly likely that they are direct descendants of hem metabolis insects.
The most battle insects do not undergo metamorphosis at all. And around 300 to 400 million years ago, a lineage of insects called the terraota seem to have evolved the ability to fly and some form of metamorphosis around the same time. This change to flying insects is a much bigger deal than often realized as the legacy of the ancient non-flying insects like silverfish and bristle tails also molt throughout their life. This is similar to more primitive arropods like millipedes and centipedes that continue to mol into adulthood. Hem metabolis insects usually mol less and then stop completely once they reach their final adult stage. The one exception being the mayfly that mol one last time as a winged creature.
The big benefit of living in a different way at different stages of life is that it stops juveniles and adults from having to compete for the same resources. The insect group paleopter which contains dragon flies, mayflies, and damsel flies have taken this further by adapting to aquatic creatures while in their nymph stage completely severing any relation between the habitats of adults. This basic principle behind why hollow metabolis insects evolved their four life stages takes this to an even bigger extreme. A bigger transformation at the pup stage means that lava can specialize even more into developing as fast as possible. They have evolved into eating machines that don't need to worry about much else. They lack reproductive organs and other structures that would be superolous and just focus on growth.
Why hollow metabolis insects evolved is very similar to their ancestors, but how they evolved is different and less well understood. Although the principle of specialization and avoiding competition are the same. Biologically, nymphs and larae are very different. For instance, hem metabolous insects have fairly stable nervous systems throughout their development. Whereas hollow metabolis insects undergo dramatic change with a lava nervous system to suit lava needs and an adult system to serve adult needs.
Genetic studies show that the earliest surviving lineage of hollow metabolis insects are actually bees, ants, and wasps. Even though these creatures have highly developed pupil stages, due to living in colonies, they quickly evolved highly modified pupé to suit the unique demands of this lifestyle. All the various Himanoptera groups evolved from solitary wasps, but they would have had to evolve unique adaptations to their life stages too, as they are parasites laying their eggs in a host animal. Although they actually come from a later lineage, the animals that have the most similar pup stage to their ancestors are probably snake flies.
The vast majority of hol metabolism insects are either immobile or can move a tiny bit while in the pupa stage. But snake flies can move to different locations while pupating and can even bite if threatened, all while their body is changing. Snake flies used to be more common and are a surviving remnant of a much larger prehistoric population. And it is likely that the earliest hol metabolis insects started out much more mobile in their pupa stage 2. The true flies and moths and butterflies don't have insect legs in their laval form and so duramorphosis have to grow them. But this is not the case for all hol metabolis insects. The wider group of insects that snake flies come from, but also many species of beetle look a lot more nymphike. During the metamorphosis, many beetle species don't have or need imaginal discs for their legs as they are never detached from the body and just keep their laval legs into adulthood. The common ancestor of the hollow metabolis insect also likely retained its insect legs throughout this process with insects like flies and butterflies being more advanced in their metamorphosis.
True flies don't have legs at all as maggots, but caterpillars do. However, these are not related to their insect legs they form as adults and are completely novel structures. Interestingly, research into caterpillars has shown their legs known as prolles may actually have ancient origins. Prehistoric arropods like trilabites and many other creatures had what are called beramus legs where they branched into two directions. One used to walk with and the other for gills and their inner branches are known as endides. Most terrestrial arthropods like arachnids, centipedes, millipedes, and insects have evolved to lose endites over time, but many crustaceians still have them. Recent research has shown that the caterpillar prolles have actually been built from an ancient genetic program that once produced endites that has been switched back on.
Most hem metabolisms go through a brief stage between embryo and nymph just before or after hatching, known as the prymph, they are usually non-feeding, immobile, and have softer skin while in this state. and it allows for slightly more development. Typically things that can't physically fit inside the egg, like the development of the full leg length. The most popular theory of how hollow metabolis insects evolved as the insect larae evolved from a prinful stage that gain the ability to feed and certain aspects of development were delayed until the adult stage. On the other side of the development, nymphs often stop moving in feeding and become lethargic before their final malt. sometimes referred to as the promolts that may have been the precursor to the pupa stage. This is the leading theory to explain how hol metabolis insects evolved. However, there are some details that aren't fully explained. One issue being that while in the egg, both insects that undergo complete and incomplete metamorphosis go through the same number of pre-hatch molds.
Sometimes when animals evolve new features or adaptations, they remain niche. But on other occasions, they are so successful that once they have evolved, you see it everywhere. From our perspective, the complete rearranging of your body to reach adulthood may seem like a strange way of life, but it is now one endured by the majority of species living today.
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