Insect

• 1. Introduction

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  Insects represent more than half of all animal species on Earth. More than a million have been described and named, and estimates suggest perhaps 5.5 million exist in total. They are the largest group within the arthropod phylum, defined by a chitinous exoskeleton, a body split into head, thorax and abdomen, three pairs of jointed legs, compound eyes, and a single pair of antennae. The word itself, from the Latin insectum, means cut up, because the body appears divided into parts. Yet that simple anatomy hides extraordinary range. How did one lineage of six-legged invertebrates come to outnumber every other form of animal life? Why are some adapted to the Sonoran Desert while others flourish in the Arctic? And how does a creature that breathes without lungs, carries blood that does not deliver oxygen, and must shed its skeleton to grow, conduct a life as complex as building a colony or speaking in symbols? The answers begin with a body plan refined over hundreds of millions of years.

• 2. A Name Cut In Three

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  Pliny the Elder introduced the Latin word insectum, formed from in and seco, meaning cut up, because insects appear to be cut into three parts. He calqued it from the Ancient Greek entomon, the root that survives in entomology. That Greek term, entomos, meaning cut in pieces, was Aristotle's own word for this class of life in his biology, a reference to their notched bodies. The English word insect first appears in 1601, in Philemon Holland's translation of Pliny. In common speech, insects and other terrestrial arthropods are often called bugs or creepy crawlies. Entomologists reserve the word bugs more narrowly, for the true bugs of the order Hemiptera, such as cicadas and shield bugs. Centipedes, millipedes, woodlice, spiders, mites and scorpions are sometimes mistaken for insects because they too have a jointed exoskeleton. The real test is the three-part body with head, thorax and abdomen, and three pairs of legs on the thorax. Adult insects are also the only arthropods that ever have wings, with up to two pairs.

• 3. Half The Living World

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  Around half of all eukaryote species, a group that includes animals, plants and fungi, are insects. The most diverse orders each carry more than 100,000 described species: the Hemiptera or true bugs, the Lepidoptera or butterflies and moths, the Diptera or true flies, the Hymenoptera of wasps, ants and bees, and the Coleoptera, the beetles. Every continent and nearly every terrestrial habitat holds them, with far more species in the tropics, especially rainforests, than in temperate zones. The world's regions have been studied unevenly. The British Isles have been surveyed so thoroughly that Gullan and Cranston in 2014 judged the total of about 22,500 species to be within 5% of the true figure. They considered Canada's list of 30,000 described species to be surely over half of the actual total, and the 3,000 species of the American Arctic broadly accurate. By contrast, most tropical and southern hemisphere insects remain undescribed. Some 30 to 40,000 species live in freshwater, while perhaps only a hundred are marine. Snow scorpionflies thrive in cold places including the Arctic and at high altitude, while desert locusts, ants, beetles and termites endure some of the hottest, driest ground on earth.

• 4. Deep Time And Coevolution

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  Leverhulmia, from the Early Devonian Windyfield chert, is the oldest fossil that may be a primitive wingless insect. The oldest known flying insects date to the mid-Carboniferous, around 328 to 324 million years ago, after which the group underwent rapid explosive diversification. Claims of a far earlier origin in the Silurian or Devonian, some 400 million years ago, rest on molecular clock estimates and are unlikely to be correct given the fossil record. Four large-scale radiations followed: beetles from about 300 million years ago, flies from about 250 million years ago, and moths and wasps both from about 150 million years ago. The Hymenoptera appeared around 200 million years ago in the Triassic, but reached their wide diversity more recently in the Cenozoic, which began 66 million years ago. Some highly successful groups evolved in conjunction with flowering plants, a powerful illustration of coevolution. Insects were among the earliest terrestrial herbivores and acted as major selection agents on plants. Plants evolved chemical defenses, and the insects evolved mechanisms to handle plant toxins. Many insects now use those very toxins to protect themselves, often advertising the danger with warning colors.

• 5. A Body That Breathes Through Holes

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  Insect respiration works without lungs. Air enters through paired spiracles, openings along the sides of the abdomen and thorax, then travels through tracheae and tracheoles that deliver oxygen directly to the tissues that need it. This system limits how large an insect can grow. As insects get bigger, gas exchange through spiracles becomes less efficient, and so the heaviest insect today weighs less than 100 grams. In the late Paleozoic, higher atmospheric oxygen allowed larger forms, including dragonflies with wingspans of more than 2 feet. Because oxygen reaches the tissues directly, the circulatory system carries none of it and is greatly reduced. It is open, with no veins or arteries, amounting to little more than a single perforated dorsal tube that pulses peristaltically and drives the hemolymph forward from the rear of the body. That hemolymph is plasma holding suspended hemocytes, cells that handle immune responses and wound healing. The outer skeleton, the cuticle, has a thin waxy epicuticle without chitin and a thick chitinous procuticle beneath. As an adaptation to land, insects use an enzyme and atmospheric oxygen to harden the cuticle, unlike crustaceans, which rely on heavy calcium compounds, making the insect exoskeleton a lightweight material.

• 6. Senses And Signals

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  The antennae of male moths can detect the pheromones of female moths over distances of more than a kilometer. There is a trade-off between visual acuity and chemical or tactile acuity, so most insects with well-developed eyes have reduced or simple antennae. Bees can perceive ultraviolet wavelengths and detect polarized light. Phylogenetic analysis suggests UV-green-blue trichromacy existed from at least the Devonian Period, some 400 million years ago. Fruit flies carry photoreceptor cells beneath each immobile lens that move rapidly in and out of focus, photoreceptor microsaccades, giving them a clearer image of the world than once assumed. Hearing has evolved independently at least 19 times in different insect groups, and insects were the earliest organisms to produce and sense sounds. Their voices vary widely. Cicadas make the loudest sounds, amplifying them with body structures called tymbals; the African cicada Brevisana brevis has been measured at 106.7 decibels at a distance of 50 centimeters. Some moths produce ultrasonic clicks that warn bats of their unpalatability, acoustic aposematism, while palatable moths mimic those calls. Fireflies of the family Lampyridae flash to attract mates, while cave-dwelling larvae of Arachnocampa glow to lure small flying insects into sticky silk. Some ants and bees navigate by magnetoreception, and the Brazilian stingless bee senses magnetic fields through the hair-like sensilla on its antennae.

• 7. Molting, Metamorphosis And Mating

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  The inelastic exoskeleton forces insects to grow through a series of molts, because the hard covering cannot stretch. Nearly all hatch from eggs, and the immature stages often differ from adults in structure, habit and habitat. There are two routes to adulthood. In incomplete metamorphosis, or hemimetabolism, the young change gradually through stages called instars until they reach the adult form, with no pupa. In complete metamorphosis, or holometabolism, the insect passes through four stages: egg, larva, pupa and adult. A butterfly's larva grows, becomes a pupa marked by reduced movement, and undergoes considerable change before emerging as an adult. Temperature shapes the pace, since most insects are poikilothermic and warmth speeds their development. During summer heat they may enter aestivation, and during cold winters hibernation, both forms of diapause. Reproduction has its own strategies. Males deliver sperm through the aedeagus, but in bed bugs the male injects sperm directly through the female's cuticle, a process called traumatic insemination. Some insects are parthenogenetic, reproducing without fertilization, and many aphids alternate between asexual and sexual generations. Insects locate mates through sex pheromones, mass swarming known as leks, and nuptial gifts.

• 8. Living With Humans

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  Insect pollination of crops and fruit trees was valued in 2021 at about $34 billion in the United States alone. Bees, butterflies, flies and beetles carry pollen, and most flowering plants depend on an animal to do it. Beyond pollination, insects yield honey, wax, lacquer, silk and carmine. Beekeeping in pottery vessels began about 9,000 years ago in North Africa, and the silkworm shaped human history as silk-driven trade linked China with the rest of the world. Insects are consumed as food in 80% of the world's nations, by people in roughly 3,000 ethnic groups, and one-third of the world's population already eats them regularly. The Food and Agriculture Organization of the United Nations holds that people worldwide may have to eat insects as a food staple. In the laboratory, the common fruit fly Drosophila melanogaster serves as a model organism, its genome sequenced in 2000, with 70% found to be similar to the human genome. Not all the news is benign. At least 66 insect species extinctions have been recorded since 1500, many on oceanic islands. A 2020 meta-study of 166 long-term surveys found terrestrial insect populations falling by about 9% per decade. Even in human imagination they endure: the scarab beetle carried religious meaning in ancient Egypt, and among the San bush-men of the Kalahari the praying mantis holds significance tied to creation and patience.