Insect
A leaf-footed bug climbs wind-blown grass and flies off, demonstrating the core traits that define its class. Insects are hexapod invertebrates with a chitinous exoskeleton supporting their three-part body plan of head, thorax, and abdomen. They possess three pairs of jointed legs attached to the thorax and a pair of antennae for sensing their environment. Compound eyes allow most insects to see, while small ocelli provide additional light detection. The insect nervous system consists of a brain connected to a ventral nerve cord running along the underside of the body. Air enters through paired openings called spiracles located along the sides of the thorax and abdomen. These openings connect to internal tubes known as tracheae that deliver oxygen directly to tissues. Blood does not carry oxygen; it circulates partially within vessels and partly in an open hemocoel cavity.
The oldest fossil that may be a primitive wingless insect is Leverhulmia from the Early Devonian Windyfield chert. Flying insects appeared much later during the mid-Carboniferous period around 328 to 324 million years ago. This group subsequently underwent rapid explosive diversification across geological time. Claims about origins dating back to the Silurian or Devonian some 400 million years ago based on molecular clock estimates are unlikely correct given the fossil record. Four large-scale radiations occurred including beetles starting about 300 million years ago and flies appearing roughly 250 million years ago. Moths and wasps both emerged approximately 150 million years ago. The remarkably successful Hymenoptera appeared some 200 million years ago in the Triassic Period but achieved wide diversity more recently in the Cenozoic era beginning 66 million years ago. Insects were among the earliest terrestrial herbivores acting as major selection agents on plants. Plants evolved chemical defenses against this herbivory while insects developed mechanisms to deal with plant toxins. Many insects make use of these toxins to protect themselves from predators using warning colors.
The majority of insects hatch from eggs enclosed by a shell called the chorion that consists of maternal tissue. Most insect eggs are drought resistant because two additional membranes develop inside the chorion: the amnion and the serosa. Some species like aphids and tsetse flies are ovoviviparous meaning their eggs develop entirely inside the female before hatching immediately upon being laid. Other species such as those in the cockroach genus Diploptera are viviparous gestating inside the mother and born alive. Development involves a series of molts since the exoskeleton does not stretch and would otherwise restrict growth. Hemimetabolous insects change gradually after hatching through stages called instars until reaching the final adult stage. Holometabolism or complete metamorphosis occurs where the insect changes in four distinct stages: egg, larva, pupa, and adult. An egg hatches to produce a larva which is generally worm-like in form. The larva grows and eventually becomes a pupa marked by reduced movement before emerging as an adult. Butterflies are well-known for undergoing complete metamorphosis while most insects use this life cycle.
Male moths can detect pheromones from female moths over distances exceeding one kilometer using antennae equipped with chemical receptors. Insects perceive sound through thin vibrating membranes known as tympana located on various parts of the body. Mosquitoes can hear frequencies up to 2 kilohertz while certain predatory insects detect characteristic sounds made by their prey. Some nocturnal moths perceive ultrasonic emissions from bats helping them avoid predation. Many species can detect light in infrared ultraviolet and visible wavelengths with color vision capabilities. Fruit flies have photoreceptor cells underneath each lens that move rapidly in and out of focus during movements called microsaccades. This gives them clearer images than previously assumed. Chemical communication via semiochemicals allows social insects like ants to coordinate complex behaviors within colonies. Honey bees evolved abstract symbolic communication where dance angles represent directions relative to the sun and dance lengths indicate flight distance.
Termites modify the environment around their nests encouraging grass growth while many beetles act as scavengers recycling biological materials. Dung beetles recycle nutrients into forms useful to other organisms contributing significantly to topsoil creation processes. Pollination transfers pollen enabling fertilization and sexual reproduction for most flowering plants requiring animal assistance. The majority of pollination is performed by insects receiving energy-rich nectar as a mutualistic benefit. Insects account for vast majority of insect consumption yet also provide critical pest control services. Ladybugs feed on aphids causing economic loss to agriculture reducing damage through natural predation methods. At least 66 insect species extinctions have been recorded since 1500 many occurring on oceanic islands. A larger 2020 meta-study analyzing data from 166 long-term surveys suggested populations of terrestrial insects are decreasing rapidly by about 9% per decade. Human activities including artificial lighting land use changes pesticide use and invasive species contribute to these declines.
The value of insect pollination of crops and fruit trees was estimated in 2021 to be approximately $34 billion in the US alone. Honey bees have been cultured by humans for thousands years with beekeeping beginning in pottery vessels about 9,000 years ago in North Africa. Silkworms were domesticated for silk over 5,000 years ago establishing trade relationships between China and 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. Locusts and termites serve as common traditional human food sources in parts of Africa while deep-fried cicadas are considered delicacies elsewhere. Fly larvae formerly treated wounds preventing gangrene by consuming dead flesh only finding modern usage in some hospitals today. Black soldier fly larvae provide protein and fats for cosmetics while superworms produce cooking oil and fatty alcohols. The Food and Agriculture Organization considers that people throughout the world may need to eat insects as a food staple due to abundance concerns and global shortages.
Common questions
What are the defining physical characteristics of an insect?
Insects are hexapod invertebrates with a chitinous exoskeleton supporting their three-part body plan of head, thorax, and abdomen. They possess three pairs of jointed legs attached to the thorax and a pair of antennae for sensing their environment.
When did flying insects first appear according to the fossil record?
Flying insects appeared much later during the mid-Carboniferous period around 328 to 324 million years ago. Claims about origins dating back to the Silurian or Devonian some 400 million years ago based on molecular clock estimates are unlikely correct given the fossil record.
How do insects breathe without lungs?
Air enters through paired openings called spiracles located along the sides of the thorax and abdomen. These openings connect to internal tubes known as tracheae that deliver oxygen directly to tissues.
What is the difference between hemimetabolous and holometabolous development in insects?
Hemimetabolous insects change gradually after hatching through stages called instars until reaching the final adult stage. Holometabolism or complete metamorphosis occurs where the insect changes in four distinct stages: egg, larva, pupa, and adult.
Why are insects important for global agriculture and food security?
Insects account for vast majority of insect consumption yet also provide critical pest control services. The value of insect pollination of crops and fruit trees was estimated in 2021 to be approximately $34 billion in the US alone.