More than half of all known animal species wear exoskeletons
Insects—Latin insectum, cut into three parts—are arthropods with a head, thorax, abdomen, six legs, compound eyes, and antennae. Over a million species are described, more than half of all animals. They breathe through side spiracles, achieve the only sustained powered flight among invertebrates, and range from solitary hunters to vast ant colonies.
Aristotle placed insects above sponges but below snails; Linnaeus made Insecta a class in 1758 with seven wing-based orders. Modern phylogeny nests insects in Hexapoda beside Diplura; winged Pterygota split from silverfish-like groups. Flying insects appear in mid-Carboniferous fossils around 328–324 million years ago, followed by radiations of beetles, flies, moths, and wasps. Paleozoic dragonfly relatives reached wingspans of fifty-five to seventy centimetres.
Growth requires molting because the chitin exoskeleton cannot stretch. Many undergo complete metamorphosis with an immobile pupa; others pass through nymph stages. Insects colonize every continent except polar ice, thrive in rainforests and deserts alike, and include the only invertebrates with true flight. Communication spans moth pheromones, cricket stridulation, and firefly light signals.
Humans both battle and depend on them. Crop pests drive insecticide use; pollinators sustain flowering ecosystems. Silkworms and honey bees were domesticated centuries ago, and roughly three thousand ethnic groups eat insects across eighty percent of nations. Estimates suggest 5.5 million insect species exist, most tropical and undescribed. Human activity is now shrinking that diversity.
Entomologists reserve bug for true bugs of order Hemiptera, yet lay speech lumps in centipedes and spiders that lack the insect body plan of head, thorax, abdomen plus three leg pairs. Freshwater holds thirty to forty thousand species; snow scorpionflies thrive in Arctic cold while desert locusts and termites endure Sonoran heat. Hymenoptera coevolved with flowering plants, insects acting as early herbivores that pushed plant chemical defences and warning colours.
Source: Insect
