As important components of ecosystems, the study of the ecological and biological characteristics of insects is crucial for maintaining ecological balance and biodiversity. Holometabolous insects, with their unique characteristics, require in-depth research on their reproductive behavior and population structure to gain a comprehensive understanding of their biology and ecological traits.

In recent years, with the in-depth study of ecology and evolutionary biology, the research on the reproductive behavior and population structure of holometabolous insects has become more and more in-depth. The reproductive behavior and population structure of holometabolous insects not only directly affect their number and distribution, but also have an important impact on the role and interaction of species in the ecosystem. Therefore, it is of great significance to deeply study the reproductive ecological characteristics of holometabolous insects for understanding the Ecological niche and interaction of species, as well as protecting the stability of ecosystems and species diversity. In practical research, various interdisciplinary approaches, such as ecology, genetics, behavior, and ecosystem modeling, are often employed to comprehensively understand the reproductive ecology of holometabolous insects. This enables a more comprehensive understanding of their reproductive ecology.

This review aims to explore the relationship between reproductive behavior and population structure in holometabolous insects, aiming to provide certain reference and inspiration for the field of insect ecology and biology. By summarizing and synthesizing relevant studies on the reproductive behavior and population structure of holometabolous insects, this review will delve into the connection between mating behavior and population structure, offering new ideas and perspectives for research in the field of insect ecology and biology. Additionally, it will investigate the influence of population structure on reproductive behavior in holometabolous insects and the impact of reproductive behavior on population structure. Through these investigations, a deeper understanding of the relationship between reproductive behavior and population structure in holometabolous insects will be achieved, providing a theoretical foundation and practical guidance for insect conservation and ecosystem maintenance.

1 Reproductive Behavior of Holometabolous Insects

1.1 Life cycle of holometabolous insects

The life cycle of holometabolous insects begins with eggs. After hatching, they develop into organisms called larvae. Larvae are the second stage of holometabolous insects and are characterized by their soft bodies, lack of wings and legs, and have significant differences from adults (Nel et al., 2013). Larvae continuously grow by feeding until they reach a sufficient size and then proceed to the next stage.

When the larvae are large enough,, they form a protective shell called a pupa, where internal tissues undergo reorganization and differentiation. During this stage, there are significant changes in the physical appearance of holometabolous insects, and it will form features such as wings and feet. The duration of the pupal stage varies among different species and can last from a few days to several months.

After completing the pupal stage, holometabolous insects will hatch into adults. Adults are the final stage of their life cycle, characterized by the presence of wings and legs, allowing them to fly and move freely. The adult stage is the reproductive stage of holometabolous insects, and their behavior and characteristics are of great significance in studying their ecological and biological traits.

1.2 Characteristics and behavior of male and female insects

There are significant differences in the characteristics and behavior between male and female holometabolous insects, primarily due to their different reproductive needs.

In holometabolous insects, males are typically larger than females, exhibiting a more robust body structure and more developed wings.. The primary function of male insects is to reproduce offspring, so they exhibit special behaviors during the mating season, such as wing fluttering, vocalization, and releasing pheromones, among others, to attract the attention of female insects. Some male insects compete for mating rights by establishing territories, engaging in fights, and displaying their reproductive abilities. Moreover, male insects often die after mating, resulting in shorter lifespans compared to females.

In contrast, female holometabolous insects are usually smaller in size, with smaller or absent wings. The main function of female insects is to lay eggs and hatch larvae, so they typically choose the most suitable mating partner. Some female insects attract the attention of male insects by releasing pheromones. Outside the breeding period, female insects often search for optimal breeding sites and protect and care for the growth and development of the larvae.

1.3 Mating behavior of holometabolous insects

Male insects usually employ a series of behaviors to attract female insects. For example, they may release specific chemicals or emit distinct sounds and vibrations to catch the attention of females. In some holometabolous insects, males engage in prolonged courtship processes to ensure successful mating.

In some holometabolous insects, males present gifts, such as food or other items, to females during the courtship process. These gifts may influence the choices of females and increase the likelihood of successful mating. When the number of females is small, males engage in more competition and rivalry behaviors. These behaviors may include competing for reproductive resources, fighting, and displaying physical traits. During mating, female insects typically remain stationary while males complete the mating process through specific postures and behaviors (Figure 1). For example, in some holometabolous insects, males insert their genitalia into the female's back to complete mating.

2 Population Structure of Holometabolous Insects

2.1 Definition of population structure

Population structure refers to the number of individuals and the interrelationships between individuals in an ecological system of a species, involving individual number, age and gender structure, spatial structure, and genetic structure, etc. Population structure is of great significance for the conservation and management of species in an ecological system.

One important aspect of population structure is the size and distribution of individuals, which can be determined by counting the number of individuals, estimating population density, and distribution range. Changes in the number of individuals can reflect changes in the ecological system, such as changes in the environment, food, and habitat supply (Liu et al., 2021; Ozerova and Gelfand, 2022). Population structure also involves the age and gender distribution of individuals, which can be determined by the number of individuals of different ages and genders, and can help us understand the life cycle, reproductive behavior, and reproductive success of the population.

Population structure includes the spatial distribution and interaction of individuals, which can be determined by statistics of individual spatial positions, territory, and habitat range, etc., and can help us understand the internal interaction and resource use of the population. Population structure also involves the genetic relationship among individuals, which can be determined by analyzing the genetic variation and kinship within the population, and can help us understand the evolutionary history and adaptability of the population.

2.2 Classification of population structure

According to the distribution of the number and proportion of individuals in different age groups in the population, the population structure is divided into three types: juvenile structure, adult structure, and elderly structure. According to the ratio of males and females in the population, the population structure is divided into three types: balanced sex ratio, male-biased, and female-biased. According to the reproductive status of individuals in the population, the population structure is divided into three types: reproductive structure, non-reproductive structure, and seasonal reproductive structure. According to the survival status of individuals in the population, the population structure is divided into three types: stable structure, unstable structure, and recovery structure. According to the number and proportion of individuals of different genotypes in the population, the population structure is divided into three types: heterozygous structure, homozygous structure, and diversified structure.

In holometabolous insects, due to the particularity of their life cycle, their population structure may change due to environmental factors (Yu et al., 2021). For example, the population structure during the larval period may be affected by factors such as food, temperature, and humidity, while the population structure during the adult period may be affected by factors such as mating behavior and reproductive ability.

2.3 Influence factors of population structure

The population structure of holometabolous insects may be affected by multiple factors, which may interact with each other and may have different effects in different stages of the life cycle.

Different stages of the life cycle of holometabolous insects may be affected by different environmental factors (Wassmer and Armstrong, 2023). For example, environmental factors such as temperature, humidity, food, and sunlight may affect the survival and development of eggs and larvae, while environmental factors such as light, temperature, and climate may affect the reproductive and transmission ability of adults.

Holometabolous insects usually require a large amount of food for growth and development during the larval period, so the abundance of food resources may affect the growth and number of larvae, thereby affecting the population structure. During the adult period, holometabolous insects usually equire mating to reproduce offspring. At this time, the mating behavior and reproductive ability of adults may affect the population structure, such as the number of eggs laid by female insects and the mating success rate of male insects.

Different types of holometabolous insects have different biological characteristics, such as lifespan, reproductive cycle, and fertility, which may also affect the population structure. In addition, factors such as competition and predation relationships may also affect the population structure. Holometabolous insects often compete for food and living space with other organisms in nature, and may also become prey for other organisms. These competition and predation relationships may also affect the population structure, such as leading to a decrease in population size or a decrease in the number of individuals of certain age groups.

3 The Association between the Reproductive Behavior and Population Structure of Holometabolous Insects

3.1 The impact of holometabolous insect mating behavior on population structure

The mating behavior of holometabolous insects includes various forms such as single mating, multiple mating, and polyandry. These mating forms also have different impacts on population structure. For example, some species of holometabolous insects adopt a single mating method, which may lead to genetic loss and fixation in the species population, affecting genetic diversity and adaptability. On the other hand, species that use multiple mating and polyandry may produce more diverse genotypes and phenotypes, thus increasing their adaptability and survival ability.

In some species of holometabolous insects, female individuals choose different mating partners to ensure the quality and quantity of their offspring. For example, in some species of moths, female moths choose to mate with males of the same species with higher quality, thus improving the survival and reproductive ability of their offspring. This selection behavior may lead to changes in the number and proportion of different types of individuals in the species population, thus affecting the species' role and interaction in the ecosystem.

The phenotypic diversity and ecological niche differentiation of holometabolous insects also have an impact on population structure. For example, in some butterfly species, there may be differences in the ecological niche between larvae and adults, leading to changes in the number and proportion of different types of larvae and adult individuals under different environmental conditions (Figure 2) (Huang et al., 2003). This ecological niche differentiation may promote the species' role and interaction in the ecosystem and affect its role in species diversity and ecosystem stability.

3.2 The impact of population structure on the reproductive behavior of holometabolous insect

Changes in population density and sex ratio can affect the reproductive behavior of holometabolous insects. For example, some studies have shown that in high-density environments, holometabolous insects may be more inclined to choose a single mating method. In this case, the choice of mating partner may be more random, and female individuals may be more inclined to choose a mating partner similar to themselves (Luo et al., 2017). In contrast, in low-density environments, holometabolous insects may adopt multiple mating and polyandry, and female individuals may be more inclined to choose a mating partner with a different genotype, thereby increasing genetic diversity in offspring.

Population structure can also affect the mating behavior and mating success rate of holometabolous insects. For example, in some holometabolous insects, female individuals need to perform certain cooperative behaviors before mating to achieve the mating status with their mating partners. This cooperative behavior may be influenced by factors such as population density, sex ratio, and individual behavior, thereby affecting the mating success rate and the number of offspring (Moreira and Hermes-Lima, 2022).

Population structure also affects the ecological role and interaction of holometabolous insects. For example, in some holometabolous insects, there may be differences in the ecological niche between larvae and adults, resulting in changes in the number and proportion of different types of larvae and adult individuals under different environmental conditions. This ecological niche differentiation may promote the species' role and interaction in the ecosystem and affect its role in species diversity and ecosystem stability.

3.3 The interaction between reproductive behavior and population structure of holometabolous insect

There is an interaction between holometabolous insect reproductive behavior and population structure. Reproductive behavior affects the genetic diversity and quantity of the species, while population structure affects the performance and effectiveness of reproductive behavior.

Reproductive behavior is influenced by population structure. For example, holometabolous insects may adopt different mating methods and female selection behaviors to adapt to different population structures and environmental conditions. In low-density environments, holometabolous insects may be more inclined to adopt multiple mating and polyandry to increase the genetic diversity and adaptability of offspring. In contrast, in high-density environments, holometabolous insects may be more inclined to choose a single mating method to reduce mating costs and avoid competition for mating partners.

Population structure is also influenced by reproductive behavior. For example, holometabolous insect mating behavior and mating success rate can affect the number and distribution of the species. In situations with low population density and high mating success rate, the number of species may increase, whereas in situations with high population density and low mating success rate, the number of species may decrease. In addition, holometabolous insect reproductive behavior may also lead to problems such as genetic loss, genetic fixation, and genetic drift, thereby affecting the genetic diversity and adaptability of the species.

Taking grasshoppers as an example (Figure 3), some studies have shown that the mating behavior of grasshoppers changes in high-density environments, and male individuals may be more inclined to choose short-term mating behavior to reduce mating costs and avoid competition. In contrast, in low-density environments, the mating behavior of grasshoppers may be more complex, and male individuals may adopt long-term cooperative behavior to achieve mating status with female individuals.

4 Conclusion

There is a close relationship between the reproductive behavior and population structure of holometabolous insects. Reproductive behavior is important for population size and structure, including reproductive rate, mating behavior, and reproductive strategy. Changes in reproductive rate may affect the number and survival of species, while changes in mating behavior and reproductive strategy may lead to changes in the number and proportion of different types of individuals in the population, thereby affecting the role and interaction of species in the ecosystem.

Future research can further explore the relationship between the reproductive behavior and population structure of holometabolous insects, and study the molecular mechanisms and evolutionary adaptability of reproductive behavior in depth. Multidisciplinary approaches such as ecology, genetics, behavior, and ecosystem models can be combined to explore the molecular mechanisms and evolutionary adaptability of these influencing factors. Genetic methods can be used to study genetic variation and gene flow between different populations, and how these variations and flows affect reproductive behavior and population structure. In addition, the impact of holometabolous insect reproductive behavior and population structure on ecosystem stability and ecological restoration can be explored to provide theoretical basis and practical guidance for ecosystem protection and restoration.

In summary, the reproductive behavior and population structure of holometabolous insects have an important impact on the role and interaction of species in the ecosystem. With the in-depth study of ecology and evolutionary biology, people's understanding of the reproductive ecology of holometabolous insects is also getting deeper. Future research can combine multidisciplinary approaches such as ecology and genetics to analyze the laws of changes in species number and population structure in different environments and ecosystems, and study these influencing laws in depth to improve our understanding and ability of species conservation and management, and provide theoretical support and practical guidance for species conservation and management.

Authors’ contribution

XMY was responsible for the literature review, organization, and initial drafting of this review. LCX participated in the discussions and paper revisions. XMY was the supervisor of this review, providing guidance in writing and revising the paper. Both authors read and approved the final manuscript.

Acknowledgement

This review was funded by the Zhejiang Province Basic Public Welfare Research Program and Natural Science Foundation Project. Thanks to Ms. Liu Chuchu for her criticism and guidance on the writing of this review.

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