In English, insects are called Objects or Insecta, of Latin origin. Insects are a class of six-legged invertebrate arthropods. In the various literatures of different periods, the definition of insects and the scope of coverage vary greatly. In general, all kinds of animals in the arthropoda class Lnsecta are called insects.
Insects have chitinous exoskeletons (chitinous exoskeletons) and the body is divided into 3 parts, namely the head, chest and abdomen. There are 3 pairs of segmented feet under the thorax, namely 1 pair of forefoot, 1 pair of midfoot and 1 pair of hind feet. Many adult insects have 2 pairs of wings, which grow on the back of the middle and posterior thoraxes. The head of an insect has 1 pair of compound eyes and a pair of antennaes. In the animal kingdom, insects are the largest taxa. To date, more than 1 million species of insects have been discovered and described. Of all the organisms identified, insects account for about half of the total number of species. So how many species of insects are there in nature? Entomologists estimate that there may be as many as 6 to 10 million species. Of all animal life forms on Earth, more than 90% of the species are insects. Although only a very few species of insects survive in the oceans, insects are almost all over the planet. In the vast ocean, there is another type of arthropod, crustaceans.
It is widely believed that insects have existed on Earth for 396 million years, that is, in the paleozoic era of the paleozoic era, in the early Devonian period. Therefore, insects are also a very old type of creature. Among the fossil insects found, the most ancient giant dragonflies have a wingspan of up to 55 to 70 cm wide. In the history of biological evolution, many insects have evolved in tandem with flowering plants.
Figure 1. Starting from the top left, clockwise:
(1) Dance fly (pronounced méng);
②象甲（long-nosed weevil， ）；
③蝼蛄（mole cricket， ）；
(4) German wasp ( ) ;
(5) Heavenly silkworm moth (eucalyptus moth), emperor gum moth;
(6) Hunting bugs (Harpactorinae).
Taxonomic status of insects:
Animalia
Euarthropoda
Pancrustacea
Hexapoda
Insecta
Almost all insects are oviparous. They hatch from the eggs and, during growth, a series of molting processes are required in order to break the confines of the inelastic shell, the chitinous exoskeleton. There will be significant differences in morphological structure, habits and living environment between insects in the immature stage of larvae or nymphs and adults in the mature stage. Some insects undergo 4 different insect stages of metamorphosis during their lifetime, including a pupal stage in which they do not eat or move. Other insects, on the other hand, undergo only 3 stages of metamorphosis, none of which pupal. We call the former class of insects completely metamorphic insects, and the latter class of incomplete perverted insects. The immature stage of incompletely metamorphosed insects is called nymphal stages. The four stages (insect stage or insect state) of the life of a completely metamorphosed insect are: egg (stage) → larval (stage) → pupae (stage) → adult (stage). The 3 stages in the life of incompletely metamorphic insects are: eggs (stage) → larvae or nymphs (stage) → adults (stage).
Adult insects move on foot or crawling, flying, and some can swim in the water. When adults move rapidly and steadily on the ground, they often walk in a tripedal gait. The so-called three-legged gait is that when the adult insect walks, it always keeps 3 feet on the ground to support the body. The forefoot and hind feet on one side cross with the midfoot on the other side take turns lifting and landing on the ground. Among invertebrates, only insects have evolved flying species, and all winged insect species share a common ancestor. Many insects live in water at some point in their lives. In order to adapt to the aquatic environment, some larvae have evolved to form gills for breathing in the water, and adults can swim freely in the water. Some species of aquatic insects, such as water shrews,[pronounced mǐn] (water striders), can walk or paddle freely on the surface of water. Most insects are solitary or scattered, but many species are gregarious. Some bee, ant and termite species are highly organized social insects. Some species of insects, such as the flyfish [pronounced qúsōu] (earwigs), female adults can show a degree of maternal care for their offspring, and they will carefully care for their eggs and larvae.
Figure 2. Adult water stride
Figure 3. Common earwig (female)
The ways in which insects interact with insects are also diverse. Male moths can receive and feel sex pheromones released by female moths at long distances. Some insects rely on sound or vibration to communicate with each other, such as male crickets that can attract female mates or drive out other male competitors by rubbing sounds (stridulate) or rubbing their wings. Lampyrid beetles of the family yíng can communicate with each other by light.
Figure 4. Field cricket (field cricket) nymph
Figure 5. Firefly (firefly)
Some species of insects are considered to be important pests, such as some locusts, bugs, leafhoppers, planthoppers, leaf beetles, moths, flies and mosquitoes, which are often seriously harmful to animals and plants, bringing certain economic losses to people. For these insects, people always try their best to control them, such as the application of pesticides, physical methods or other measures. Some insects feed on plant roots, stems, leaves, flowers, fruits, seeds, woody fibers or sap, and some insects can also transmit pathogens from animals and plants. Some insects have complex and important ecological functions in the environment, such as in nature, blow-flies contribute to the consumption and decomposition of decomposing flesh or decomposing corpses, but also spread certain pathogenic microorganisms. In the biosphere, most organisms, including humans, depend to some extent on plants for their survival, and many of these flowering plants rely entirely or partially on certain insects to pollinate them during their life cycle. Without insects, the terrestrial part of Earth's biosphere would be destroyed. Many insects are ecologically or economically beneficial, and they can directly or indirectly bring certain benefits to humans. Silkworms provide silk for humans, and honey bees provide honey for humans, both of which have formed large economic industries in human society, namely sericulture and bee industry. In addition, insects can be used as food for humans and are a valuable source of protein and energy. It is estimated that about 3,000 ethnic groups in 80% of the world's countries or regions eat certain insects.
Figure 6. A pair of silkworm moths (adults), males and females.
Figure 7. V. age larvae of silkworms