See text .
* Ectognatha * Entomida
INSECTS or INSECTA (from
Insects have a chitinous exoskeleton , a three-part body (head , thorax and abdomen ), three pairs of jointed legs , compound eyes and one pair of antennae . They are the most diverse group of animals on the planet, including more than a million described species and representing more than half of all known living organisms . The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the differing animal life forms on Earth. Insects may be found in nearly all environments , although only a small number of species reside in the oceans, a habitat dominated by another arthropod group, crustaceans .
The life cycles of insects vary but most hatch from eggs . Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts . The immature stages can differ from the adults in structure, habit and habitat, and can include a passive pupal stage in those groups that undergo four-stage metamorphosis (see holometabolism ). Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages. The higher level relationship of the Hexapoda is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants .
Humans regard certain insects as pests , and attempt to control them
using insecticides and a host of other techniques. Some insects damage
crops by feeding on sap, leaves, fruits, or, in the case of termites,
the wood itself. A few parasitic species are pathogenic . Some insects
perform complex ecological roles; blow-flies , for example, help
consume carrion but also spread diseases.
* 1 Etymology * 2 Definitions
* 3.1 Evolutionary relationships * 3.2 Taxonomy
* 4 Diversity
* 5 Morphology and physiology
* 5.1 External
* 5.2 Segmentation
* 5.2.1 Exoskeleton
* 5.3 Internal
* 5.3.2 Digestive system
* 5.3.3 Reproductive system
* 5.3.4 Respiratory system
* 6 Reproduction and development
* 6.1.1 Incomplete metamorphosis * 6.1.2 Complete metamorphosis
* 7 Senses and communication
* 7.1 Light production and vision * 7.2 Sound production and hearing * 7.3 Chemical communication
* 8 Social behavior
* 8.1 Care of young
* 9 Locomotion
* 9.1 Flight
* 9.2 Walking
* 9.2.1 Use in robotics
* 9.3 Swimming
* 10 Ecology
* 11 Relationship to humans
* 11.1 As pests * 11.2 In beneficial roles * 11.3 In research * 11.4 As food * 11.5 In culture
* 12 See also * 13 References * 14 Bibliography * 15 Further reading * 16 External links
The word "insect" comes from the
The precise definition of the taxon Insecta and the equivalent English name "insect" varies; three alternative definitions are shown in the table.
Definition of Insecta GROUP ALTERNATIVE DEFINITIONS
Archaeognatha (jumping bristletails) Insecta sensu stricto =Ectognatha
Pterygota (winged insects) Insecta sensu strictissimo
In the broadest circumscription , Insecta sensu lato consists of all hexapods . Traditionally, insects defined in this way were divided into "Apterygota" (the first five groups in the table) – the wingless insects – and Pterygota – the winged insects. However, modern phylogenetic studies have shown that "Apterygota" is not monophyletic, and so does not form a good taxon. A narrower circumscription restricts insects to those hexapods with external mouthparts, and comprises only the last three groups in the table. In this sense, Insecta sensu stricto is equivalent to Ectognatha. In the narrowest circumscription, insects are restricted to hexapods that are either winged or descended from winged ancestors. Insecta sensu strictissimo is then equivalent to Pterygota. For the purposes of this article, the middle definition is used; insects consist of two wingless taxa, Archaeognatha (jumping bristletails) and Zygentoma (silverfish), plus the winged or secondarily wingless Pterygota.
PHYLOGENY AND EVOLUTION
Evolution of insects
This section needs to be UPDATED. Please update this article to reflect recent events or newly available information. (July 2017)
Crustacea (crabs , shrimp , isopods , etc.)
Arachnida (spiders , scorpions , mites , ticks , etc.)
Eurypterida (sea scorpions: extinct)
Pycnogonida (sea spiders)
A phylogenetic tree of the arthropods and related groups
The evolutionary relationship of insects to other animal groups remains unclear.
Although traditionally grouped with millipedes and centipedes
—possibly on the basis of convergent adaptations to
terrestrialisation —evidence has emerged favoring closer
evolutionary ties with crustaceans . In the
insects, together with
A report in November 2014 unambiguously places the insects in one clade, with the crustaceans and myriapods , as the nearest sister clades. This study resolved insect phylogeny of all extant insect orders, and provides "a robust phylogenetic backbone tree and reliable time estimates of insect evolution".
Other terrestrial arthropods, such as centipedes, millipedes, scorpions , and spiders , are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointed exoskeleton. However, upon closer examination, their features differ significantly; most noticeably, they do not have the six-legged characteristic of adult insects.
The higher-level phylogeny of the arthropods continues to be a matter
of debate and research. In 2008, researchers at Tufts University
uncovered what they believe is the world's oldest known full-body
impression of a primitive flying insect, a 300-million-year-old
specimen from the
Carboniferous period . The oldest definitive insect
fossil is the
Four super radiations of insects have occurred: beetles (evolved about 300 million years ago), flies (evolved about 250 million years ago), and moths and wasps (evolved about 150 million years ago). These four groups account for the majority of described species. The flies and moths along with the fleas evolved from the Mecoptera .
The origins of insect flight remain obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects had an additional pair of winglets attaching to the first segment of the thorax, for a total of three pairs. As of 2009, no evidence suggests the insects were a particularly successful group of animals before they evolved to have wings.
Carboniferous and Early
The remarkably successful
Hymenoptera appeared as long as 146 million
years ago in the
Many modern insect genera developed during the Cenozoic. Insects from this period on are often found preserved in amber , often in perfect condition. The body plan, or morphology , of such specimens is thus easily compared with modern species. The study of fossilized insects is called paleoentomology .
Insects are prey for a variety of organisms, including terrestrial vertebrates. The earliest vertebrates on land existed 400 million years ago and were large amphibious piscivores . Through gradual evolutionary change, insectivory was the next diet type to evolve.
Insects were among the earliest terrestrial herbivores and acted as major selection agents on plants. Plants evolved chemical defenses against this herbivory and the insects, in turn, evolved mechanisms to deal with plant toxins. Many insects make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity using warning colors. This successful evolutionary pattern has also been used by mimics . Over time, this has led to complex groups of coevolved species. Conversely, some interactions between plants and insects, like pollination , are beneficial to both organisms. Coevolution has led to the development of very specific mutualisms in such systems.
- Archaeognatha - 470