Insects are six-legged invertebrates that are members of the class Insecta within the phylum Arthropoda. They are distinguished by their three-part bodies covered by a hard, chitinous exoskeleton. The word ‘insect’ originates from the Latin word inseco, which means to ‘cut up,’ since an insect’s body looks like it has been cut into three separate segments.
Almost all insects are oviparous, meaning they lay eggs that progress through several developmental stages, known as metamorphosis, before reaching adulthood. It is estimated that some 5.5 million insect species exist on the Earth (only about a million are described) and account for more than 50% of all eukaryotic species.
The largest insect is Phryganistria chinensis, which measures 62.4 cm (24.6 inches) in body length, while the smallest one is Dicopomorpha echmepterygis, a species of parasitoid wasp, which is almost 186 μm long, on average.
Segmentation (Three-part Body)
An insect’s body is primarily divided into three interconnected segments, each containing an upper tergum and a lower sternum.
Exoskeleton
The body is covered by a cuticle, an exoskeleton comprising two layers: the outer waxy epicuticle (without chitin) and the inner chitinous procuticle. The latter further divides into an outer exocuticle and an inner endocuticle.
The endocuticle, which is tough and flexible, consists of many layers of fibrous chitin and proteins arranged in a criss-cross pattern, resembling a sandwich. In contrast, the exocuticle is hard and sclerotized, making it more rigid.
Insects have an open circulatory system lacking arteries and veins. Instead, they possess a dorsal blood vessel that divides inside the heart and aorta. This vessel circulates the body fluid (hemolymph), thus transporting nutrients, hormones, and wastes dissolved in it. The body fluid contains hemocytes suspended in plasma that keep up their immunity.
Most insects take up air through multiple paired openings called spiracles on the lateral sides of their thorax and abdomen. The air is then directed to a system of internal tubes and sacs called the trachea and tracheoles, through which oxygen either diffuses or is actively pumped inside the body tissues.
An insect’s digestive system comprises a gut that divides into three segments, foregut, midgut, and hindgut, along with salivary glands and reservoirs.
The foregut, lined with the protective cuticle, includes the mouth, the pharynx, and the storage organ, the crop. The prey is initially digested in the mouth using salivary enzymes and then stored in the crop before being passed to the midgut for further processing. A series of absorptive projections called microvilli help absorb the nutrients in the midgut.
The primary excretory organ found in insects is the Malpighian tubule, which could be up to 100 in number. Nitrogenous waste products are dumped directly into the junction between the midgut and hindgut.
An insect nervous system consists of a brain and a ventral nerve cord. Each of the six fused segments of the head capsule contains either a pair of ganglia or a cluster of nerve cells. While the first three pairs of ganglia fuse to the brain, the next three fuse into the subesophageal ganglia under the esophagus.
Depending on the species, insects have simple eyes (ocelli) or compound eyes with color vision. Many insects, such as bumblebees, can even detect ultraviolet rays and infrared light. Though the lens in the compound eyes of most insects is not adjustable, fruit flies possess photoreceptor cells under their lens, which can adjust focus through a series of movements called ‘photoreceptor microsaccades.’
Females possess a pair of ovaries, accessory glands, and one or more spermathecae (an organ for storing sperm). The accessory glands produce protective secretions to maintain the sperm in the female genital tract and protect the eggs.
The male reproductive system consists of one or two testes supported by the tracheae. The sperm tubes or follicles are encased in a membrane within the testes. A special sperm-delivering (intromittent) organ called the aedeagus is connected to the testes.
Aristotle was the first to consider insects as a group and placed them at the second-lowest level of animals, right below the hard-shelled marine snails. This classification was followed for centuries until Carl Linnaeus included insects under the class Insecta and divided them into seven orders based on their wing structure. These were the Aptera (wingless), the Diptera (two-winged), and five five-winged orders: the Coleoptera (fully-hardened forewings), the Hemiptera (partly-hardened forewings), the Lepidoptera (scaly wings), the Neuroptera (membranous wings but no sting), and the Hymenoptera (membranous wings and a sting). In 1809, Jean-Baptiste de Lamarck recognized insects as one of the nine invertebrate phyla.
Traditionally, the present-day subphylum Hexapoda was considered a superclass with four groups: insects (Ectognatha), Collembola, Protura, and Diplura. Based on phylogenetic data, insects are divided into two broad groups (historically treated as subclasses): wingless Apterygota and winged Pterygota. The former traditionally included the primitive orders Archaeognatha (jumping bristletails) and Zygentoma (silverfish). The latter was further divided into two groups: those with incomplete metamorphosis (Polyneoptera and Paraneoptera) in one and the rest of those with complete metamorphosis (Holometabola) in the other.
Insects are cosmopolitan in distribution but are most abundant on tropical continents. While they can thrive in almost every habitat, approximately 97% of insect species are found exclusively in terrestrial environments. About 30,000 to 40,000 species inhabit freshwater habitats, whereas barely a hundred are marine.
Desert locusts, ants, beetles, and termites are well-adapted to thrive in harsh desert environments, while snow scorpionflies live in high altitudes of the Arctic region.
Most insects are herbivores, feeding on leaves, roots, seeds, nectar, sap, or wood. In contrast, a few, like praying mantises, are predators, consuming other insects such as moths, caterpillars, flies, beetles, and spiders. In addition, fleas and lice are ectoparasites that live off vertebrate blood.
Flight
Insects are the only invertebrates that can fly. While dragonflies, damselflies, and mayflies control their wings directly through the paired muscles attached to the wing base, other insects achieve indirect flight through the rapid oscillation of the thorax caused by the flight muscles.
Although most insects gather an aerodynamic lift by creating a vortex at the leading edge of their wings, smaller ones, such as thrips, clap their wings together and pull them apart to create vortices at the tips of their wings.
Walking
Many adult insects, such as cockroaches and ants, use all six legs for walking with an alternating tripedal gait. In this locomotion pattern, three legs remain in contact with the ground, moving the insect forward while the other three remain raised. However, exceptions to this pattern are seen while running. For example, cockroaches and other insects that run exceptionally fast resort to a bipedal gait, using just two legs.
Water striders drive their middle legs on the water’s surface and recede their claws in a special groove, thus preventing them from piercing the water film.
Insects have developed various mechanisms to avoid predation or defend themselves against predators, including camouflage, mimicry, and chemical defense.
Camouflage
Insects resort to camouflaging against their backgrounds to avoid being noticed by potential predators. For example, leaf beetles and weevils blend into the vegetation on which they feed.
Mimicry
Stick insects (order Phasmatodea) mimic the different forms of twigs, sticks, and leaves, while edible species, like hoverflies, resemble inedibles or present themselves as toxic and unpalatable.
Chemical Defense
Insects belonging to the orders Coleoptera and Lepidoptera assimilate toxins from plant tissues into their bodies. When predators eat these toxic insects, they either vomit immediately or learn to avoid them altogether. Similarly, ground beetles belonging to the family Carabidae spray toxic chemicals from their abdomen on their enemies to protect themselves.
Termites, bees, wasps, and ants are eusocial insects living in highly organized and well-coordinated colonies. Bees communicate through a specific ‘waggle dance’ in which they move in a typical eight-shaped pattern. This specific dance represents their direction relative to the sun and conveys to the group members the location of food or enemies.
Insects have multiple modes of communication with their conspecifics, such as through vocalizations, emission of light, or production of signaling chemicals.
Sound
Grasshoppers, crickets, and cicadas rub their body parts to produce sound (stridulation). The African cicada (Brevisana brevis) sometimes emits sounds as intense as 106.7 decibels.
In contrast, insects from the Coleoptera, Hymenoptera, Lepidoptera, Mantodea, and Neuroptera orders produce sounds of very low intensity.
The shield bug (Nezara viridula) uses songs to attract their mates, while caterpillars of the family Lycaenidae communicate with the ants they are mutualistically associated with. As evident from its name, the Madagascar hissing cockroach presses air through its spiracles and hisses furiously at an aggressor.
Light
Fungus gnats and various beetles, such as lightning bugs, glowworms, click beetles, and rove beetles, emit light (bioluminescence). While some species, like lightning bugs, use this light to lure prey, others attract potential mates through vibrant, luminescent colors.
Chemicals
Insects produce pheromones to attract potential mates, mark their trails, and agitate their conspecifics or those belonging to other species. These chemicals are often produced using plant metabolites, which are processed inside the insect’s body to form the final product.
Most insects reproduce sexually through the fusion of male and female gametes (sperm and ova). However, some wasps and bees undergo parthenogenesis, where the female gives birth without the males fertilizing her eggs. Many aphids go through cyclic parthenogenesis, alternating between sexual and asexual modes of reproduction.
Almost all insects are oviparous, laying eggs with embryos enclosed in protective membranous layers, like chorion, amnion, and serosa. However, a few, like aphids and tsetse flies, are ovoviviparous, with their eggs developing completely inside the female and hatching immediately upon being laid. In contrast, cockroaches belonging to the genus Diploptera are viviparous, giving rise to live young that have developed inside the parent’s body.
After hatching, the egg undergoes multiple stages of development until it reaches adulthood. It may undergo either a complete (holometabolous) or incomplete (hemimetabolous) metamorphosis.
Complete
Most insects undergo complete metamorphosis, in which a worm-like larva emerges from the hatched egg, gradually developing into a pupa, followed by the adult stage or imago. Some beetles and flies exhibit a special type of holometabolism called hypermetabolism, in which the larval instars are structurally and functionally different from the other developmental stages.
Incomplete
In incomplete metamorphosis, the insect undergoes multiple developmental stages called instars until the final adult stage is reached. Initially, an insect’s epidermis secretes a fresh epicuticle layer above the old one. It then produces a mixture of digestive enzymes to break down the endocuticle. The insect then takes in large quantities of water and purposely swells to make the old cuticle split along the areas where it was the thinnest.
They are preyed upon by entomophagous (insect-eating) animals, like frogs, lizards, birds, and mammals, such as pangolins, bats, and sloth bears.
Insects play crucial roles in different ecosystems by assisting in pollination, soil aeration, scavenging, and pest control. For example, termites enrich the soil around their nests and improve grass growth around their colonies, while dung beetles recycle biominerals from dung and convert them into forms useful for other organisms. Similarly, most flowers are pollinated by insects, which carry the seeds far and wide, helping in dispersion.
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