Insecta

Insects are members of the arthropod class Insecta. They are the largest group of organisms on the planet and account for more than half of the named species known to science. Doubtless there are tens of thousands more species of insects waiting to be detected. More

Grasshopper

Lateral view

Abdomen

Thorax

Head

Forewing

Lateral view

Abdomen

Thorax

Head

Forewing

Insecta Thorax
Antenna

Legs

Body plan of a typical insect

MAJOR GROUPS OF INSECTA

The taxonomy of living insects includes 31 major groups or orders and more than 1,015 families within those groups. In addition, there are about 12 extinct groups that are known only from the fossil record. The following table includes an abridged list of the major groups of insects that account for about 75 percent of all living species. Those having roots in the Paleozoic Era are also indicated.

MAJOR GROUPS OF INSECTA

The taxonomy of living insects includes 31 major groups or orders and more than 1,015 families within those groups. In addition, there are about 12 extinct groups that are known only from the fossil record. The following table includes an abridged list of the major groups of insects that account for about 75 percent of all living species. Those having roots in the Paleozoic Era are also indicated.

Group

Characteristics

Members

Appearance in Fossil Record

Approximate Number of Named Living Species

Coleoptera

Two pairs of wings with front set armored to protect the rear set of flying wings; hard exoskeleton; biting and chewing jaws

Beetles, ladybugs, click beetles

Middle

Permian

360,000

Lepidoptera

Two pairs of large flying wings; hairy body; sucking mouth

Cretaceous

160,000

Diptera

Two pairs of wings, with front set for flying and rear set greatly reduced; piercing and sucking mouth

House flies, mosquitoes, gnats, botflies, fruit flies

Middle Triassic

125,000

Hymenoptera

Two pairs of flying wings; advanced eyes; mobile head; some possess stingers; chewing and sucking mouth

Bees, wasps, ants

Late Triassic

130,000

Hemiptera

Two pairs of wings or wingless; piercing and sucking mouth; live on plants or suck blood

True bugs, bedbugs, leafhoppers, aphids, cicadas

Permian

85,000

Orthoptera

Two pairs of wings or wingless; biting and chewing jaws; jumpers; some of the largest insects

Grasshoppers, crickets, katydids

Late Permian

22,500

Odonata

Two pairs of flying wings; long, slender torso; chewing jaws

Dragonflies, mayflies, damselflies

Early

Carboniferous

5,200

Blattaria

Two pairs of wings or wingless; biting and chewing jaws; jumpers

Carboniferous

4,000

Group

Characteristics

Members

Appearance in Fossil Record

Approximate Number of Named Living Species

Isoptera

Two pairs of wings but with wingless stages of development; chewing jaws; can eat wood

Cretaceous

2,100

Siphonaptera

Wingless; jumpers; piercing and sucking mouth; small

Fleas

Miocene

2,000

Archaeognatha and Zygentoma

Wingless; small, with three long caudal filaments; chewing jaws with external mouthparts

Bristletails, silverfish

Early Devonian

400

than 800,000 species of insects have been named. The physical traits that unite most insects include a three-part body that consists of the head, thorax, and abdomen; two pairs of wings; and three pairs of legs attached to the thorax. The head usually has one pair of uni-ramous (single-branched) antennae and three pairs of mouthparts that differ considerably with the type of insect. Some insects have jaws adapted for biting and chewing plants; others are equipped for piercing and sucking fluids from plants or animals.

The Early Devonian fossil deposits of Scotland known as the Rhynie Chert offer a unique window onto the world of early arthropods. It is in these deposits that many examples of the earliest known land plants and arthropods are found. The Rhynie Chert is especially important to the fossil record of insects because it is here that the earliest known forms are found. The first and best understood genus is Rhyniella, which was first named in 1926 based on fragmentary fossils of the insect's head. In subsequent years, other parts of this insect have been discovered, including legs, the thorax, and an abdomen discovered in 2004. Rhyniella was a small, wingless insect; it is believed to be related to modern springtails. Springtails are tiny, measuring only several millimeters long, and, like silver-fish, are among the most basal of insects.

Rhyniella may represent an important stage in the evolution of insects, the origins of which are little understood. Recent analysis of the genetic makeup—the genes—of these insects suggests that they represent an evolutionary stage that branched off from the crustaceans and led to modern winged insects. Some scientists do not actually classify springtails as insects. This is because springtails appear to represent an evolutionary line related to crustaceans that may be unconnected to true insects, despite such similarities as the springtails' six-legged anatomy.

If, as some scientists believe, springtails are not true insects, then another tiny fossil specimen discovered at the same time as the original Rhyniella specimen may hold the key to the origin of true insects. Rhyniognatha consists only of a pair of well-preserved mouthparts, called mandibles, that measure only nine-tenths of a millimeter long and that can be viewed only with a powerful microscope. At the time of their discovery in 1926, the mandibles were assigned to the new genus Rhyniognatha and called "insectlike," but no attempt was made to place them into any clearly defined insect group. Thus did Rhyniognatha disappear into obscurity, like so many extra bits of scrap shale tossed aside for later investigation. In subsequent years, other claims to the earliest known true insects were made from slightly younger fossil deposits, most notably some possible body fragments of a wingless insect from Late Devonian deposits in Gilboa, New York. In 2004, however, Michael Engel, an entomologist from the University of Kansas, and David Grimaldi, a zoologist from the American Museum of Natural History, revealed the results of a closer examination of the long-ignored Rhyniognatha specimen. What they determined is that Rhyniognatha has traits ascribable to winged insects even though no fossilized wings or related body parts were discovered. Rhyniognatha s mouthparts are consistent with the chewing lifestyle associated with winged insects. This discovery suggests that the development of wings occurred long before what is currently known from the first definitive fossils of winged insects from the Late Carboniferous Period.

After the Middle Devonian, the fossil record for insects goes nearly dark during the Late Devonian Epoch and Early Carboniferous Period. This is due to a lack of known terrestrial fossil deposits of the appropriate age and habitat. The abundance of fossils documenting the Late Carboniferous Period reveals a world full of many varieties and sizes of insects, however. Winged insects, cockroaches, mayflies, and true bugs took hold during the Late Paleozoic, giving root to many modern families of insects. A guide to the most prominent groups of Paleozoic insects follows.

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