As one of the most abundant and diverse groups of organisms on earth, insects have always been an important subject of entomological research due to the diversity in their life cycles. Throughout their development from egg to adulthood, insects exhibit astonishing metamorphic phenomena, with complete metamorphosis being one of the most typical and widespread types. This review aims to delve into the developmental process of holometabolous insects and present to readers this fascinating and intricate phenomenon of their life cycle.

Among the various types of insect development, complete metamorphosis stands out as a distinctive process, encompassing four developmental stages: egg, larva, pupa, and adult. This unique developmental pattern is widely distributed in the insect world and covers a large number of species, including many insects familiar to us in our daily lives, such as butterflies, bees, and beetles.

This review will first introduce the definition, characteristics, and classification of holometabolous insects, providing readers with a comprehensive background understanding. We will delve into each key stage of insect development, starting from mating and egg-laying, exploring the development and hatching of eggs, the growth and molting during the larval stage, the formation and development of the pupa, and finally, the emergence and eclosion of the adult. Through the analysis of these crucial stages, we hope readers will gain insight into the wonders of the complete metamorphosis process in insects and the significant roles each stage plays in the life of these fascinating creatures.

The life cycle and developmental process of holometabolous insects are of crucial significance for the stability of ecosystems and the reproduction of insect populations. Finally, we will explore the ecological significance and conservation value of insects with complete metamorphosis, aiming to enhance people's awareness and respect for these little creatures, and to better protect our shared home, the earth.

Through in-depth research on the development process of holometabolous insects, we hope this review can provide valuable references for entomological research and ecological conservation. We aim to stimulate further exploration of the diversity in insect life cycles, making contributions to humanity's exploration and understanding of the natural world.

1 Definition and Characteristics of Holometabolous Insects

1.1 Overview of holometabolism life cycle

Complete metamorphosis is one of the most common and typical developmental patterns in the life cycle of insects. It consists of four main stages of insect development: egg, larva, pupa, and adult. During this developmental process, the morphology and physiological characteristics of the insect undergo significant changes at each stage. The uniqueness of this life cycle lies in the fact that each stage has specific functions and survival strategies, allowing insects to adapt to their respective ecological environments (Llandres et al., 2015).

The complete metamorphosis of insects begins with the eggs laid by female insects. These eggs come in various shapes and sizes, some being attached to plant surfaces, some buried in the soil, and others suspended on the water's surface. Eggs mark the starting point of the insect life cycle and serve as the origin of the next generation of insects.

As the eggs hatch, insects enter the larval stage, which is a highly active phase in their lives. Larvae typically engage in active feeding and growth to acquire sufficient nutrients for molting and development. The appearance and behaviors of larvae vary significantly among different insect species; some have abundant spiky hairs, resembling caterpillars, while others move in a creeping, worm-like manner.

Next comes the pupal stage, which is one of the most mysterious and crucial processes in complete metamorphosis. The pupa is the transitional phase from larva to adult, during which insects undergo significant morphological and physiological transformations. Beneath the exterior of the pupa, the internal organs and tissues of the insect are undergoing essential changes, preparing for the emergence of the future adult.

Finally, the adult insect emerges from the pupa, becoming the ultimate form in the complete metamorphic life cycle (Figure 1). Adults are typically winged insects, capable of free flight in the air to search for food, mate, and find suitable habitats. The adult stage marks the final phase in the insect life cycle and is a crucial period for insect reproduction and propagation.

Figure 1 The process of a butterfly breaking through its pupa

1.2 The difference between complete metamorphosis and incomplete metamorphosis

Complete metamorphosis and incomplete metamorphosis are the two main categories of insect developmental patterns. The main difference between them lies in the presence of a pupal stage. In complete metamorphosis, insects undergo a transformation from larva to pupa and then to adult, while in incomplete metamorphosis, the larva goes through multiple molts but lacks a pupal stage.

Complete metamorphosis and incomplete metamorphosis also exhibit significant differences in morphology. In complete metamorphosis, the larval and adult forms have a striking contrast and are often almost entirely different organisms. However, in incomplete metamorphosis, the morphological differences between the larva and adult are relatively minor, and they resemble scaled-down versions of the adult form.

Additionally, complete metamorphosis and incomplete metamorphosis also differ in their ecological adaptation strategies. Due to the presence of pupal, holometabolism insects can reshape their morphology and physiology under the protection of pupae to adapt to different ecological environments. Non holometabolism insects need to have the ability to adapt to the environment in the larval stage, because they do not have the intermediate stage like pupae to provide protection.

1.3 Classification and examples of holometabolous insects

Complete metamorphosis is a widespread developmental type among insects, encompassing a vast array of insect species. According to the current classification system, holometabolous insects primarily belong to orders such as Coleoptera (beetles), Lepidoptera (butterflies and moths), Hymenoptera (ants, bees, and wasps), Diptera (flies), and Hemiptera (true bugs). Each of these orders includes thousands of different insect species with diverse morphological and ecological characteristics.

For instance, within the order Coleoptera, beetles like the ladybug (Figure 2) and ladybird beetle are representative examples of holometabolous insects. In the order Lepidoptera, butterflies and moths also exemplify typical complete metamorphosis in insects. Similarly, within the order Diptera, flies and mosquitoes, as well as within the order Hymenoptera, bees and wasps, are also insects with a complete metamorphic life cycle (Calma and Medina, 2020).

Figure 2 Representative of holometabolous insects - scarabaeidae

The classification and examples of holometabolous insects demonstrate the diversity and complexity of the insect world. Each species of insect has evolved a highly adaptive life cycle in different environments and ecological conditions. This ability to adapt and develop diverse life cycles is one of the crucial reasons why insects are able to thrive and proliferate so extensively across the earth.

Complete metamorphosis is one of the most common and typical developmental types in the insect world, comprising four stages: egg, larva, pupa, and adult. In contrast, incomplete metamorphosis lacks the pupal stage, and the larva directly undergoes multiple molts to become an adult. Holometabolous insects are classified into orders such as Coleoptera, Lepidoptera, Hymenoptera, Diptera, and Hemiptera. Within each order, there are thousands of insect species, showcasing the splendid diversity of insect life cycles. The unique life cycles of these insects provide effective adaptive strategies for their survival and reproduction in various ecological environments. They also serve as significant subjects for entomological research and ecological conservation. A comprehensive understanding of the process of complete metamorphosis in insects will offer us a deeper perspective to unravel the mysteries of insect biology and the stability of ecosystems.

2 Male and Female Mating and Egg Laying

2.1 Sexual maturity process of male and female insects

During the developmental process of the insect life cycle (Hong et al., 2016), both male and female insects need to undergo the process of sexual maturation to enable reproduction. Sexual maturation is a crucial stage in insect biology, as it signifies that insects have acquired the physiological and behavioral conditions necessary for mating and egg-laying.

For female insects, sexual maturation is closely linked to the development and maturation of eggs. During the pre-maturation phase, the reproductive system of female insects undergoes a series of changes and developments, including the enlargement of the ovaries and the formation of egg follicles. Over time, the egg cells within the egg follicles gradually mature into fully developed eggs. When the egg cells mature, and the egg follicles reach a certain size, female insects are in a state of sexual maturity and can engage in mating and egg-laying activities.

For male insects, sexual maturation is primarily associated with the development and functionality of their reproductive organs. During the process of sexual maturation, the reproductive organs of male insects undergo maturation and functional improvement. When the reproductive organs reach a mature state, male insects acquire the ability to engage in mating activities.

2.2 Mating behavior and its importance

Mating is an essential stage in the reproductive process of insects. The purpose of insect mating behavior is to unite the female's eggs with the male's sperm, completing the fertilization process and giving rise to new offspring (Vershinina and Kuznetsova, 2016).

The mating behavior of insects is highly diverse, with each species potentially having different mating methods and behaviors. Some insects engage in unique courtship dances to attract the opposite sex, while others release specific pheromones to attract potential mates. Mating behavior often requires a high level of coordination and cooperation between two individuals to ensure successful mating. For example, butterflies' mating takes place on the ground (Figure 3). Male butterflies first fly near food plants, waiting for the arrival of females. Once a female is detected, the male starts to pursue her while releasing chemical substances to attract her. If the female shows interest in the male's dance and chemical signals, she will halt and mate with him.

Figure 3 Butterflies in mating

Mating plays a crucial role in insect populations. Through mating, insects are able to increase genetic diversity, reduce the risk of inbreeding, and enhance the adaptability and survival ability of their offspring. In addition, mating also helps to strengthen social connections between individuals, maintain population stability, and promote reproduction.

2.3 Oviposition methods and factors influencing oviposition site selection

Oviposition is a critical stage in the life cycle of insects and an important step in reproduction. The oviposition methods and the selection of oviposition sites in insects are usually closely related to their species characteristics, ecological environment, and habits of life.

Some insects choose to scatter their eggs individually in suitable environments, which increases the survival chances of each egg. Other insects opt to lay their eggs on specific plants, providing an ample food resource for the larvae. There are also insects that choose to lay their eggs inside other insects, utilizing parasitic or spider-like biological characteristics for the egg's development. Social insects, such as ants, deposit their eggs in the nest. This provides a secure environment and abundant food resources for the larvae, supporting their growth and development (Figure 4).

Figure 4 Distribution of ant eggs

When selecting an oviposition site, insects are influenced by various factors. One of the most important factors is to ensure that the eggs and larvae can access sufficient food and protection for smooth development and survival. Additionally, insects also consider the suitability of the environment, including factors such as temperature, humidity, and light, to ensure that the eggs can hatch and grow under optimal conditions.

Mating and oviposition are crucial stages in the life cycle of insects. Sexual maturity enables insects to have the ability to reproduce, and mating and oviposition are essential steps to complete the reproductive process. Different insect species exhibit a wide range of behaviors and strategies during these stages, adapting to various ecological environments and survival needs. In-depth understanding of these processes will provide us with a more comprehensive knowledge of the biology and ecology of insects, and also offer essential insights for insect conservation and maintaining ecological balance.

3 Development and Hatching of Eggs

3.1 Structure and composition of eggs

Eggs are the starting point of the insect life cycle and are the reproductive cells produced by female insects after sexual maturity. Although the shape, size, and color of eggs may vary among different insect species, they all have a common structure and composition (Daimon et al., 2015).

Egg usually consists of two main parts: the yolk and the shell. The yolk is the primary source of nutrients in the egg, rich in proteins, fats, and other essential substances, providing energy and nourishment for the developing embryo. The shell serves to protect the embryo inside the egg, preventing harmful substances from the external environment from damaging the embryo. The eggshell is typically composed of one or more layers of protein, and its thickness and hardness vary depending on the insect species, ranging from thin as paper to hard as a shell.

3.2 The impact of the environment on eggs

The development and hatching of eggs are influenced by environmental conditions, which is an important adaptive strategy in the life cycle of insects. Environmental factors such as temperature, humidity, light, and climate all play a role in the development and hatching of eggs.

Temperature is one of the most important environmental factors that affects egg development and hatching. Different insect species have different temperature requirements, with some insect eggs requiring higher temperatures for development while others require lower temperatures. Temperature fluctuations can also affect the speed of egg development, with higher temperatures generally resulting in faster egg development.

Humidity is another important environmental factor that affects the evaporation and absorption of water inside eggs. When the humidity is appropriate, the moisture inside the eggs is less likely to be lost, which helps maintain the vitality and development of the eggs. For some insects, humidity also affects the permeability of the eggshell, thereby influencing the exchange of oxygen and carbon dioxide inside the eggs.

Lighting conditions can also potentially impact the development and hatching of eggs. Some insect eggs are sensitive to light and require darkness for proper development, while others need exposure to light to stimulate hatching.

3.3 Incubation process and larval hatching method

Hatching is the process in which eggs transition from a dormant state to an active state, marking a critical moment in the insect's life cycle. The process of hatching typically involves the development and cell division of the embryo inside the egg, which is triggered by suitable environmental stimuli.

During the hatching process, the eggshell usually undergoes changes, becoming thinner and more flexible to facilitate the emergence of the larva. When hatching, the larva uses special structures such as its head or mouth parts to puncture the eggshell and then emerges from it. For some insects, the larva remains inside the eggshell after hatching to continue absorbing nutrients from the yolk until it completely leaves the eggshell. The method of larval emergence varies among different insect species. Some insects hatch by breaking through one or both sides of the eggshell, while others emerge from the top or bottom of the eggshell. The timing and speed of larval emergence also differ depending on the insect species. After hatching, the larva typically immediately searches for food and begins its growth and molting process. The larva's growth and development will continue until it reaches a certain stage, then it will enter the next stage, continuing the complete metamorphic development process of the insect.

The development and hatching of eggs are crucial stages in the life cycle of insects. The structure and composition of the eggs provide nutrients for the initial growth of the larvae, while environmental factors influence the development and hatching of the eggs. After hatching, the larvae emerge by breaking through the eggshell, starting the next stage of their insect life cycle. Understanding these processes in-depth helps us gain a better understanding of the biology and ecology of insects, providing a deeper perspective for entomological research and ecological conservation efforts.

4 Larval Stage

4.1 Characteristics and functions of larval stage

The larval stage is one of the most critical phases in the complete metamorphic development process of insects, typically spanning from hatching from the egg until the formation of the pupa. During this stage, insects usually exhibit unique characteristics that are distinct from the adult form. Larvae are often referred to as caterpillars or grubs, and they have distinctive appearances, such as having a segmented body and various appendages.

The larval stage is highly significant in the life cycle of insects. Firstly, it is a critical period for insect growth, as larvae continuously intake nutrients and increase in body weight, undergoing significant transformations from egg to adult form. Secondly, the larval stage is a time of molting for insects. Through a series of molting processes, larvae gradually grow and adapt to different life conditions at each stage. Additionally, the larval stage is a phase of behavioral and ecological adaptation for insects. Some insects develop specific predatory or evasive strategies during this stage to cope with environmental challenges.

4.2 Nutrient intake and growth

One of the primary tasks during the larval stage is to acquire sufficient nutrients to support rapid growth and development. Larvae typically select specific food sources, such as eating plant leaves, stems, roots, flower buds, wood, or even the bodily fluids of other insects or animals. Different insect species have distinct preferences and feeding habits, which contribute to the various ecological roles they play during the larval stage within the ecosystem.

Nutrient intake and growth in larvae are closely interconnected. By consuming an adequate amount of nutrients, the larvae's bodies continuously grow, and they undergo molting at the appropriate times to renew and facilitate their growth. During the larval stage, larvae typically feed frequently, and after each molting, they increase in weight and size.

4.3 Relationship between molting and larval stage

Molting is an important process between the larval stage and the adult stage in the complete metamorphic development of insects. During the process of complete metamorphosis, larvae undergo multiple molts, gradually becoming larger and approaching the form of the adult. Molting is a unique characteristic of the insect exoskeleton, which restricts the insect's size as it grows. Therefore, insects must undergo periodic molting to accommodate their increasing body size.

The molting process is typically divided into four stages: the pre-molt stage, the molting stage, the post-molt expansion stage, and the hardening stage. During the pre-molt stage, the larva stops feeding and secretes a new inner layer under the exoskeleton. When molting, the larva vigorously twists its body to gradually shed the old exoskeleton. After molting, the insect's new body rapidly absorbs air, causing the new exoskeleton to fully expand and then gradually harden. Through molting, the larva can gradually increase in size and adapt to the requirements of different ecological environments. The molting process may also be related to the larva's behavior and ecological adaptation. Some insects undergo behavioral changes before and after molting to adapt to different stages of their lifestyle.

The larval stage is a crucial phase in the complete metamorphic development process of insects, during which larvae undergo rapid growth and development. By consuming sufficient nutrients and experiencing the molting process, the larvae gradually complete the transformation from egg to adult. The characteristics and functions of the larval stage, as well as the processes of nutrient intake, growth, and molting, are closely related to the insect's life cycle and ecology. Gaining a deeper understanding of these processes will provide us with a more comprehensive understanding of insect biology and ecology, and offer a more profound perspective for entomological research and ecological conservation efforts.

5 Formation and Development of Pupae

5.1 Transition from larva to pupa

The pupa is a crucial stage in the complete metamorphic development process of insects, representing the transitional phase between the larval stage and the adult form. The formation of the pupa usually involves a series of physiological and morphological changes.

When the larval stage comes to an end, the larva stops feeding and seeks an appropriate environment for pupation. During the process of pupation, significant physiological and morphological changes occur inside the insect's body. The tissues and organs of the larva undergo reorganization and reshaping to meet the requirements of the adult form.

5.2 Structure and physiological characteristics of pupae

Pupae typically have distinctive shapes and structures to protect the internal tissues and organs and provide suitable living conditions. The appearance of pupae can be spindle-shaped, oval, or similar to a cocoon-like structure. The outer shell of pupae is usually composed of proteins and hardening substances, forming a tough protective layer that prevents the invasion and damage of external substances.

In the pupal stage, insects are usually in a state of dormancy or quiescence, and their physiological activities are relatively low. During this stage, new tissues and organs of the insect gradually form and begin preparing for the functions and lifestyle of the adult stage.

5.3 Changes in tissues and organs within the pupa

During the process of pupation, the tissues and organs within an insect undergo significant changes and development (Nestel et al., 2016). These changes include processes such as cell proliferation, tissue differentiation, and organ formation. During the pupal stage, the insect's internal organs undergo gradual reorganization and remodeling. Some organs may degenerate, while others undergo further development and functional enhancement. For instance, the digestive system may undergo adjustments to adapt to the different dietary requirements of the adult stage. The nervous system, respiratory system, and reproductive system also undergo important changes to accommodate the lifestyle and reproductive needs of the adult insect. Pupal development also involves cell differentiation and restructuring. Cells within the larval body differentiate to form various types of tissues and organs. The differentiation and development of these cells and tissues are highly regulated and influenced by hormones within the insect's body and external environmental factors.

The pupa is an important stage in the complete metamorphic development of insects. During the transition from larva to pupa, significant physiological and morphological changes occur within the insect's body. The structure and characteristics of the pupa provide a protective and conducive environment for internal tissues and organs. During the pupal stage, the tissues and organs of the insect undergo reorganization, differentiation, and development to adapt to the form and functional requirements of the adult stage. A profound understanding of pupal formation and development contributes to a better comprehension of insect development processes and biological characteristics, offering a more comprehensive perspective for entomological research and ecological conservation.

6 Late Pupation and Eclosion

6.1 Preparation stage in the later stage of pupation

The late pupal stage is an important period in the complete metamorphic development of insects and a crucial phase in the transition from pupal to adult form. During the late pupal stage, insects go through a preparation phase, making necessary arrangements for eclosion (emergence from the pupal case) and the appearance of the adult.

During the preparation phase in the late pupal stage, physiological and morphological changes continue to occur within the insect's body. Internal tissues and organs may undergo final development and restructuring at this stage to meet the requirements of the adult form and function. At the same time, the insect may accumulate energy and nutrients to support eclosion and the initial stages of adult life.

6.2 Eclosion process and the role of eclosion fluid

Eclosion is a crucial process during the late pupal stage and an important transition from the pupal to the adult form in insects. During eclosion, the adult's wings gradually unfold and harden, and the insect's exoskeleton undergoes a series of adjustments and changes.

The process of eclosion is regulated by hormones within the insect's body and the external environment. Hormones in the insect's body trigger the contraction of wing muscles, gradually unfolding the wings. At the same time, during eclosion, the insect secretes eclosion fluid, a special biological fluid containing proteins and other substances. The eclosion fluid covers the wings and body surface and, through a chemical reaction with the exoskeleton, promotes its hardening and fixation. The role of eclosion fluid is to allow the wings to quickly harden and become sturdy after unfolding, enabling the adult to fly and seek food promptly after eclosion.

6.3 Adult emergence and hardening

After completing eclosion, the adult emerges within the exuviae (shed pupal skin). At this stage, the adult's exoskeleton may not be sufficiently sturdy and requires some time to harden and solidify. During the hardening process, the adult extends and spreads its wings, allowing them to dry and harden in the air. This process typically takes several hours to a day, during which the insect is relatively vulnerable and should avoid disturbances and predation from the external environment. Once the adult ecloses, the insect has completed its complete metamorphic development from an egg to an adult. At this point, the insect possesses fully formed organs and biological functions and can autonomously engage in feeding, reproduction, and searching for habitats.

The late pupal stage and eclosion are two important phases in the complete metamorphic development of insects. During the late pupal stage, insects make necessary preparations for eclosion and the appearance of the adult, undergoing a series of physiological and morphological changes. During eclosion, the adult's wings gradually unfold and harden, aided by the action of eclosion fluid, allowing the wings to harden and fix quickly. Once eclosion is complete, the adult emerges within the exuviae and undergoes the process of exoskeleton hardening. Understanding the processes of the late pupal stage and eclosion is of significant importance for gaining a comprehensive understanding of insect development and ecology. It also provides valuable insights for entomological research and ecological conservation.

7 Adult Stage

7.1 Characteristics and functions of adults

The adult stage is the final phase in the complete metamorphic development of insects and represents the mature stage of insects. During the adult stage, insects possess fully formed organs and biological functions, exhibiting specific morphological features and behavioral traits. Adults typically have prominent external characteristics, such as wings, antennae, compound eyes, and various body colors. Their exoskeletons are usually rigid and sturdy, providing protection and support. Furthermore, adults often have well-developed sensory and locomotor organs to adapt to their respective ecological environments.

In the adult stage, the primary functions of insects are feeding and reproduction. They obtain energy and nutrients through feeding to sustain their physiological activities and survival needs. At the same time, adults have the capacity for reproduction, engaging in sexual mating and egg-laying to pass on their genetic material.

7.2 Feeding and reproductive behavior

The feeding behavior of adults is closely related to their ecological roles. Different species of adults may have different feeding habits and dietary preferences. Some adults are herbivorous, feeding on nectar, leaves, pollen, or bark, while others are carnivorous, consuming other insects, carrion, or blood. Some adults also exhibit specialized feeding behaviors, such as bees collecting pollen and nectar or spiders preying on insects.

Reproduction is one of the essential tasks during the adult stage. Adults reproduce by mating and laying eggs to produce offspring. Mating is achieved through pairing between female and male adults, who are attracted to each other through specific behaviors and chemical signals. Adults also select suitable environments and resources for egg-laying to ensure the survival and development of their offspring.

7.3 Lifespan and reproductive cycle

The lifespan and reproductive cycle of adults vary depending on the insect species. Some insects may have a very short adult stage, lasting only a few days or even a few hours, while others can live for several months or even longer. The length of their lifespan is influenced by various factors, including environmental conditions, nutritional status, genetic factors, and more.

The reproductive cycle of adults also varies among insect species. Some insects may have only one opportunity to reproduce, laying eggs and completing their life cycle in the final stage of life. On the other hand, other insects may have multiple opportunities to reproduce, engaging in multiple matings and egg-layings during their adult stage. In some insects, the adult stage may also involve other essential behaviors and ecological functions, such as migration, defense, and social behaviors. These unique behaviors and functions allow insects to play diverse roles in the ecosystem.

In summary, the adult stage is the final phase in the complete metamorphic development of insects. During this stage, insects possess fully formed organs and biological functions, engaging in feeding and reproductive behaviors. The feeding and reproductive behaviors of adults vary among insect species and ecological environments. The lifespan and reproductive cycle of adults also differ based on the insect species. Understanding the characteristics and functions of the adult stage helps us gain a comprehensive understanding of insect biology and ecology, providing valuable insights for entomological research and ecological conservation.

8 Ecological Significance and Protection Value of Holometabolous Insects

8.1 Ecological roles and interrelationships

Complete metamorphic insects play important roles in ecosystems, forming complex interactions with other organisms. Firstly, as crucial components of food chains, insects' feeding habits and food choices directly impact the survival and reproduction of other organisms. Herbivorous insects, such as butterfly larvae, influence plant growth and reproduction by consuming plant leaves. On the other hand, some carnivorous insects like dragonflies and spiders are primary predators of other insects and small animals, playing a crucial role in maintaining the balance of insect communities.

Complete metamorphic insects play a crucial role in the process of pollination. Insects such as bees, butterflies, and moths collect nectar from flowers while inadvertently picking up pollen on their bodies. They then transfer this pollen to other flowers, aiding in plant pollination and promoting plant reproduction. This pollination relationship is essential for maintaining plant diversity and the stability of ecosystems.

8.2 Impact on the ecosystems

Complete metamorphic insects play diverse roles in ecosystems, and their presence and activities have significant impacts on ecological balance and functioning. Firstly, insects contribute to energy transfer and nutrient cycling in ecosystems through the food chain. They serve as a source of food in the food chain, converting plant energy into the bioenergy of other organisms, thereby sustaining the material cycle in the ecosystem.

The biodiversity of complete metamorphic insects is crucial for maintaining the stability of ecosystems. Different insect species play various roles in the ecosystem, depending on each other and constraining each other, forming a complex ecological network. If certain insect species experience significant reductions in population or disappear, it can lead to disruptions in the food chain, loss of ecological functions, and affect the overall stability of the entire ecosystem.

Furthermore, the reproductive behavior and life history of complete metamorphic insects are of significant importance for the adaptation and stability of ecosystems. By laying eggs at appropriate times and ensuring the proper growth and development of their larvae, insects can avoid adverse environmental conditions and enhance the survival rate of their offspring. This adaptive reproductive strategy helps maintain the health and stability of insect populations.

8.3 Protection strategy and importance

The protection of holometabolous insects has important ecological significance and protection value. To implement effective protection measures, here are some key strategies and measures.

Protecting habitats to maintain the diversity and ecological functions of holometabolous insects is crucial. Different insect species live in different habitats, including forests, grasslands, wetlands, and so on. Protecting the integrity and stability of these habitats, including reducing habitat destruction and pollution, can provide suitable environments and resources for insects.

Controlling the use of pesticides. Excessive pesticide application may have severe adverse effects on insects, including poisoning and death. Therefore, reducing the quantity and frequency of pesticide use and adopting more ecologically friendly agricultural practices, such as organic farming and integrated pest management, help alleviate pressure on insect populations and ecological balance.

Promoting ecological agriculture. Ecological agriculture focuses on the sustainable development of ecosystems by conserving biodiversity and natural habitats to provide suitable habitats and food resources for insects. Ecological agriculture employs methods such as biodiversity maintenance, natural enemies' regulation, and the use of natural pesticides, which help reduce the negative impact on insects and provide favorable habitat conditions.

Establishing insect reserves and nature conservation areas. These protected areas can provide secure locations for insect breeding and habitat, safeguarding insect species and their ecological environment. By setting up these reserves, human activities' interference with insects and their habitats can be restricted, offering relatively safe living conditions for insects.

Strengthening scientific research and education in the field of entomology. In-depth understanding of insect ecological characteristics, species diversity, and ecological functions can provide scientific basis and guidance for insect conservation. Furthermore, through scientific research and education, raising public awareness and importance of insect conservation can be achieved, promoting conservation efforts.

Holometabolous insects play a crucial ecological role in ecosystems, and their conservation is essential for maintaining biodiversity and ecological balance. By protecting their habitats, controlling pesticide use, promoting ecological agriculture, establishing reserves, and enhancing scientific research and education, we can effectively conserve holometabolous insects, ensuring their diversity persists, and safeguarding the health and stability of ecosystems. The implementation of these conservation measures is of significant importance for maintaining ecological balance and sustainable development.

9 Conclusion and Outlook

Holometabolous development is one of the most common and typical types in the insect developmental process, consisting of four stages: egg, larva, pupa, and adult. In the egg stage, the insect's life begins, and the eggs typically have different shapes and colors. After a period of incubation, the eggs hatch into larvae. Larvae are the feeding and growing stage in the insect's life cycle, undergoing multiple molts during which their body size gradually increases. In the pupa stage, the larva enters a relatively inactive state, undergoing significant tissue and organ restructuring, eventually transforming into an adult. Adults are the final stage in the holometabolous development of insects, possessing complete organs and biological functions, including feeding, reproduction, and passing on their genes.

Holometabolous insects play a crucial ecological role in ecosystems, forming complex relationships with other organisms, including the composition of food chains, pollination, and biodiversity maintenance. The conservation of insects not only helps maintain their own diversity but also fulfills the needs of protecting the entire ecosystem. Understanding the life cycle and developmental process of holometabolous insects enables us to gain deeper insights into their biological characteristics and ecological roles, providing scientific basis and guidance for ecological conservation and biodiversity protection.

In the future, there are many directions worth exploring in the study of holometabolism insects. Firstly, we can further investigate the molecular regulatory mechanisms during insect development, exploring the regulatory networks of hormones and genes at different stages of development (Li, 2015). This can help reveal the genetic and biochemical mechanisms underlying insect development. Secondly, with the development of biotechnology and genetics, research on insect gene editing and genetic modification can be carried out. The application of these technologies can help elucidate the relationship between insect genes and developmental traits, and provide new ideas and methods for insect ecological protection and agricultural pest control. In addition, research on the ecological functions and effects of insects in ecosystems needs to be strengthened. Understanding the ecological roles and relationships of insects can help predict and evaluate the ecological risks of insect species, providing a scientific basis for ecological conservation and environmental management.

Future research can focus on the molecular regulatory mechanisms of insect development (Chai et al., 2015), gene editing and genetic modification, insect ecological functions and ecological effects, among other aspects. At the same time, strengthening public outreach and education, and promoting public participation in insect conservation efforts, is also an important direction for future research. Through these efforts, we can better understand and protect holometabolous insects, and maintain the stability and sustainable development of ecosystems.

Author’s contributions

GTX was responsible for conducting relevant literature research, organizing, and writing the initial draft of this review, and participated in discussions and paper revisions. GTX was the lead author of this review, responsible for completing the paper writing and revisions. Author read and approved the final manuscript.

Acknowledgement

Author would like to express her gratitude to Ms. Luo Mengting for her guidance during the writing process of this review. Her detailed feedback and suggestions on our manuscript were invaluable. The illustrations used in this review are sourced from the internet. If there are any concerns regarding copyright or usage permissions, please contact the author to ensure proper acknowledgment of rights. Thank you for your understanding and support.

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