Various reproductive modes of insects (hermaphroditism, parthenogenesis, oviparity)

Insects are dioecious animals, and dioecious animals should always reproduce sexually. However, in the diverse class of insects, there are several special reproductive methods, each reflecting different adaptation methods.

1. Hermaphroditic reproduction

Most species of insects reproduce sexually and lay eggs. Hermaphroditic reproduction requires male and female mating, and male individuals give birth to

Only after the fertilized sperm combines with the eggs produced by the female individuals can they develop normally into new individuals. The characteristics of insect hermaphroditic reproduction are:

The egg usually must receive sperm before the egg nucleus undergoes mature division (meiosis); while the sperm has already been reduced and divided by the time the male worm ejaculates. This is closely related to the differentiation of other reproductive methods.

2. Parthenogenesis

Among insects, it is not uncommon for eggs to develop into new individuals without fertilization. This phenomenon is collectively known as parthenogenesis. Parthenogenesis in insects can be roughly divided into three types:

① Occasional parthenogenesis, that is, bisexual reproduction occurs under normal circumstances, but occasionally unfertilized eggs may develop into new individuals. There are examples of this in moths, such as the familiar silkworm, which can carry out sporadic parthenogenesis.

② Frequent parthenogenesis, such as in Hymenoptera insects (such as bees), not all eggs are fertilized when the female bee ovulates. In this case, fertilized eggs develop into female wasps, and unfertilized eggs develop into male wasps, because the chromosomes of females of many Hymenoptera insects (including bees) are diploid, while the chromosomes of males are haploid. Male worms do not undergo meiosis when forming sperm. When the sperm combines with the egg nucleus that has undergone meiosis, it becomes diploid, so they all develop into females. There are also some insects that often reproduce parthenogenetically. There are very few male insects under natural circumstances, and some even have no males yet. Reproduction in these species occurs entirely or almost exclusively through parthenogenesis. For example, some sawflies, gall wasps (gall wasps), wasps, stick insects, whiteflies, scale insects, thrips, damselfish moths, etc. all have this kind of situation;

③Periodic parthenogenesis, that is, parthenogenesis and hermaphrodite reproduction alternate with seasonal changes. Aphids are the most familiar example. Many aphids only produce male aphids when winter is about to come, mate with males and females, and lay fertilized eggs to survive the winter. However, they reproduce through parthenogenesis for more than ten consecutive generations from spring to autumn, and during this period there are almost no male aphids. When an aphid reproduces parthenogenetically (except when giving birth to aphids), its offspring are all female. The eggs laid after mating between the two sexes will all develop into female aphids in the second year. Male aphids only appear when the aphids give birth. .

Parthenogenesis plays an important role in the widespread distribution of insects, because even if only one female is accidentally brought to a new area (such as wind blowing, human transmission), it is possible to reproduce in this area. Parthenogenetic insects are also more likely to retain their populations when encountering unsuitable environmental conditions that cause large numbers of deaths. So parthenogenesis can be considered a favorable adaptation to harsh environments and expanded distribution.

3. Polyembryonic reproduction

Polyembryony is a reproductive method in which one egg produces two or more embryos. This method of reproduction is common in some parasitic wasps of the order Hymenoptera, such as some species such as Braconidae, Braconidae, Braconidae, Braconidae, and Apidae. Polyembryonic reproduction is also performed in the order Torrionoptera. .

Polyembryonic parasitoids lay their eggs in the host's eggs, and the host larvae do not leave the host until they mature and pupate. In one host, 1-8 eggs can be laid (varying with species). Once eggs are laid, there can be fertilized eggs or unfertilized eggs. The former develops into female bees, and the latter develops into male bees. The polar bodies of these polyembryonic wasp eggs do not disappear when they mature and divide. Instead, they are concentrated at one end of the egg, continue to divide, and gradually develop into a trophic amniotic membrane wrapped outside the embryo. The embryo absorbs the nutrients it needs directly from the host's body through the nourishing amniotic membrane. Therefore, some people call nourishing amniotic membrane nutrient membrane. The mature and divided egg nucleus is located at the rear end of the egg - the end opposite to the polar body. As it divides again and again, the rear end of the egg swells. If it divides only once, two embryos will be produced in the future. However, judging from the known species, there are very few embryos that divide once. Most of them need to divide multiple times. The most can produce 1,600 to 1,800 daughter nuclei. In the future, Each daughter nucleus forms an embryo. The number of embryos produced often depends on the host's tolerance. Polyembryony can be seen as an adaptation to parasitism in living creatures. Because these parasitic insects often cannot find their corresponding hosts for all individuals. Polyembryony ensures that once a host is found, more offspring can be produced.

4. Viviparous and larval reproduction

The vast majority of insects lay eggs. But there are also some insects that can produce larvae or nymphs directly from the mother. This method of reproduction is called viviparous birth. There are also a few insects that reproduce in the larval stage before their mother reaches the adult stage, which is called larval reproduction. All insects that undergo larval reproduction produce larvae instead of eggs, so larval reproduction can be considered a form of viviparous reproduction. Since the mother of larval reproduction has not developed to the adult stage, of course there is no talk of sexual mating, so larval reproduction can be regarded as a type of parthenogenesis.

Viviparous birth can be thought of as an adaptation to egg protection. And due to the lack of a separate egg stage, the time required to complete one generation is also relatively short. Larval reproduction is also parthenogenetic, so it is also an adaptation that is conducive to widespread distribution and maintaining population survival under adverse environmental conditions.

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