Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the Earth, with over a million described species—more than half of all known living organisms[2][3]—with estimates of undescribed species as high as 30 million, thus potentially representing over 90% of the differing life forms on the planet.[4] Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.
There are approximately 2,000 praying mantis, 5,000 dragonfly species, 20,000 grasshopper, 82,000 true bug, 120,000 fly, 110,000 bee, wasp and ant, 170,000 butterfly and moth, and 360,000 beetle species described to date. Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower figure of about six to ten million.[2][5][6] Adult modern insects range in size from a 0.139 mm (0.00547 in) fairyfly (Dicopomorpha echmepterygis) to a 56.7 centimetres (22.3 in) long stick insect (Phobaeticus chani).[7] The heaviest documented insect was a Giant Weta of 70 g (2½ oz), but other possible candidates include the Goliath beetles Goliathus goliatus, Goliathus regius and Cerambycid beetles such as Titanus giganteus, though no one is certain which is truly the heaviest.[8]
The study of insects (from Latin insectus, meaning "cut into sections") is called entomology, from the Greek εντομον, also meaning "cut into sections".[9]
Sound production and hearing
Insects were the earliest organisms to produce sounds and to sense them. Soundmaking in insects is achieved mostly by mechanical action of appendages. In the grasshoppers and crickets this is achieved by stridulation. The cicadas have the loudest sounds among the insects and have special modifications to their body and musculature to produce and amplify sounds. Some species such as the African cicada, Brevisana brevis have been measured at 106.7 decibels at a distance of 50 cm (20 in).[8] Some insects, such as the hawk moths and Hedylid butterflies, can hear ultrasound and take evasive action when they sense detection by bats. Some moths produce ultrasound clicks and these were earlier thought to have a role in jamming the bat echolocation, but it was subsequently found that these are produced mostly by unpalatable moths to warn bats, just as warning colourations are used against predators that hunt by sight.[16] These calls are also made by other moths involved in mimicry.[17]
Very low sounds are also produced in various species of Neuroptera, Lepidoptera (butterflies and moths), Coleoptera and Hymenoptera produced by the mechanical actions of movement often aided by special microscopic stridulatory structures.
Most sound-making insects also have tympanal organs that can perceive airborne sounds. Most insects are also able to sense vibrations transmitted by the substrate. Communication using substrate-borne vibrational signals is more widespread among insects because of the size constraints in producing air-borne sounds.[18] Insects cannot effectively produce low-frequency sounds, and high-frequency sounds tend to disperse more in a dense environment (such as foliage), so insects living in such environments communicate primarily using substrate-borne vibrations.[19] The mechanisms of production of vibrational signals are just as diverse as those for producing sound in insects.
Some species use vibrations for communicating within members of the same species, such as to attract mates as in the songs of the shield bug Nezara viridula[20] while it can also be used to communicate between entirely different species, such as between ants and myrmecophilous lycaenid caterpillars.[21]
The Madagascar hissing cockroach has the ability to press air through the spiracles to make a hissing noise, and the Death's-head Hawkmoth makes a squeaking noise by forcing air out of their pharynx.
Flight
Insects are the only group of invertebrates to have developed flight. The evolution of insect wings has been a subject of debate. Some proponents suggest that the wings are para-notal in origin while others have suggested they are modified gills. In the Carboniferous age, some of the Meganeura dragonflies had as much as a 50 cm (20 in) wide wingspan. The appearance of gigantic insects has been found to be consistent with high atmospheric oxygen. The percentage of oxygen in the atmosphere found from ice core-samples was as high as 35% compared to the current 21%. The respiratory system of insects constrains their size, however the high oxygen in the atmosphere allowed larger sizes.[22] The largest flying insects today are much smaller and include several moth species such as the Atlas moth and the White Witch (Thysania agrippina).
Insect flight has been a topic of great interest in aerodynamics due partly to the inability of steady-state theories to explain the lift generated by the tiny wings of insects.
In addition to powered flight, many of the smaller insects are also dispersed by winds. These include the aphids, which are often transported long distances by low-level jet streams.[23]
