Insects (Insecta) are the most diverse of all animal groups. There are more species of insects than there are species of all other animals combined. Their numbers are nothing short of remarkable—both in terms of how many individual insects there are, as well as how many species of insects there are. In fact, there are so many insects that no one knows quite how to count them all. The best that can be done is to make estimates.
Scientists approximate that there may be as many as 30 million species of insects alive today. To date, over one million have been identified. At any one time, the number of individual insects alive on our planet is staggering—some scientists estimate that for ever human alive today there are 200 million insects.
The success of insects as a group is also reflected by the diversity of habitats in which they live. Insects are most numerous in terrestrial environments such as deserts, forests, and grasslands. They are likewise numerous in freshwater habitats such as ponds, lakes, streams, and wetlands. Insects are relatively scarce in marine habitats, but are more common in brackish waters such as salt marshes and mangroves.
The key characteristics of insects include:
- three main body parts
- three pairs of legs
- two pairs of wings
- compounds eyes
- complex mouth parts
- one pair of antennae
- small body size
Insects are classified within the following taxonomic hierarchy:
Insects are divided into the following taxonomic groups:
- Angel insects (Zoraptera) - There are about 30 species of angel insects alive today. Members of this group are small, hemimetabolis insects, which means they undergo a form of development that includes three stages (egg, nymph, and adult) but lacks a pupal stage. Angel insects are small and are most often found living under the bark of trees or in rotting wood.
- Barklice and booklice (Psocoptera) - There are about 3,200 species of barklice and booklice alive today. Members of this group include granary booklice, booklice, and common barklice. Barklice and booklice live in moist terrestrial habitats such as in leaf litter, under stones, or in the bark of trees.
- Bees, ants, and their relatives (Hymenoptera) - There are about 103,000 species of bees, ants, and their relatives alive today. Members of this group include bees, wasps, horntails, sawflies, and ants. Sawflies and horntails have a body that is joined by a broad section between their thorax and abdomen. Ants, bees, and wasps have a body that is joined by a narrow section between their thorax and abdomen.
- Beetles (Coleoptera) - There are more than 300,000 species of beetles alive today. Members of this group have a hard exoskeleton and a pair of rigid wings (called elytra) that serve as protective covers for their larger and more delicate hind wings. Beetles live in a wide variety of terrestrial and freshwater habitats. They are the most diverse group of insects alive today.
- Bristletails (Archaeognatha) - There are about 350 species of bristletails alive today. Members of this group do not undergo metamorphosis (immature bristletails resemble smaller versions of adults). Bristletails have a cylindrical body that tapers to a narrow bristle-like tail.
- Caddisflies (Trichoptera) - There are more than 7,000 species of caddisflies alive today. Members of this group have aquatic larvae that build a protective case in which they live. The case is constructed of silk produced by the larva and also incorporates other materials such as organic debris, leaves, and twigs. Adults are nocturnal and short-lived.
- Cockroaches (Blattodea) - There are about 4,000 species of cockroaches alive today. Members of this group include cockroaches and waterbugs. Cockroaches are are scavengers. They are most abundant in tropical and subtropical habitats although their distribution is worldwide.
- Crickets and grasshoppers (Orthoptera) - There are more than 20,000 species of crickets and grasshoppers alive today. Members of this group include crickets, grasshoppers, locusts, and katydids. Most are terrestrial herbivores and many species have powerful hind legs that are well-adapted for jumping.
- Damselflies and dragonflies (Odonata) - There are more than 5,000 species of damselflies and dragonflies alive today. Members of this group are predators in both the nymph and adult stages of their life cycles (damselflies and dragonflies are hemimetabolis insects and, as such, they lack the pupal stage in their development). Damselflies and dragonflies are skilled fliers that feed on smaller (and less skilled) flying insects such as mosquitos and gnats.
- Earwigs (Dermaptera) - There are about 1,800 species of earwigs alive today. Members of this group are nocturnal scavengers and herbivores. The adult form of many species of earwigs has cerci (the rear-most segment of their abdomen) that are modified into elongated pincers.
- Fleas (Siphonaptera) - There are about 2,400 species of fleas alive today. Members of this group include cat fleas, dog fleas, human fleas, rabbit fleas, oriental rat fleas, and many others. Fleas are blood-sucking parasites that prey primarily on mammals. A small percentage of flea species prey on birds.
- Flies (Diptera) - There are about 98,500 species of flies alive today. Members of this group include mosquitos, horse flies, deer flies, house flies, fruit flies, crane flies, midges, robber flies, bot flies, and many others. Although flies have one pair of wings (most flying insects have two pairs of wings), they are nevertheless highly-skilled fliers. Flies have the highest wing-beat frequency of any living animal.
- Mantids (Mantodea) - There are about 1,800 species of mantids alive today. Members of this group have a triangular head, elongated bodies, and raptorial forelegs. Mantids are well-known for the prayer-like posture in which they hold their front legs. Mantids are predatory insects.
- Mayflies (Ephemeroptera) - There are more than 2,000 species of mayflies alive today. Members of this group are aquatic in the egg, nymph, and naiad (immature) stages of their life. Mayflies lack a pupal stage in their development. Adults have wings that do not fold flat over their back.
- Moths and butterflies (Lepidoptera) - There are more than 112,000 species of moths and butterflies alive today. Moths and butterflies are the second most diverse group of insects alive today. Members of this group include swallowtails, milkweed butterflies, skippers, clothes moths, clearwing moths, lappet moths, giant silk moths, hawk moths, and many others. Adult moths and butterflies have large wings that are covered with tiny scales. Many species have scales that are colorful and patterned with complex markings.
- Nerve-Winged Insects (Neuroptera) - There are about 5,500 species of nerve-winged insects alive today. Members of this group include dobsonflies, alderflies, snakeflies, green lacewings, brown lacewings, and antlions. Adult forms of nerve-winged insects have highly-branched venation in their wings. Many species of nerve-winged insects act as predators to agricultural pests, such as aphids and scale insects.
- Parasitic lice (Phthiraptera) - There are about 5,500 species of parasitic lice alive today. Members of this group include bird lice, body lice, pubic lice, poultry lice, ungulate lice, and mammal chewing lice. Parasitic lice lack wings and live as external parasites on mammals and birds.
- Rock crawlers (Grylloblattodea) - There are about 25 species of rock crawlers alive today. Members of this group lack wings as adults and have long antenae, a cylindrical body, and long tail bristles. Rock crawlers are among the least diverse of all insect groups. They live in high-elevation habitats.
- Scorpionflies (Mecoptera) - There are about 500 species of scorpionflies alive today. Members of this group include common scorpionflies and hanging scorpionflies. Most adult scorpionflies have a long slender head and narrow wings with highly-branched veination.
- Silverfish (Thysanura) - There are about 370 species of silverfish alive today. Members of this group have a flattened body that is covered with scales, Silverfish are so named for their fish-like appearance. They are wingless insects and have long antennae and cerci.
- Stoneflies (Plecoptera) - There are about 2,000 species of stoneflies alive today. Members of this group include common stoneflies, winter stoneflies, and spring stoneflies. Stoneflies are so named for the fact that as nymphs, they live beneath stones.. Stonefly nymphs require well-oxygenated water to survive and for this reason are found in swift-moving streams and rivers. Adults are terrestrial and live at the edges of streams and rivers where they feed on algae and lichens.
- Stick and leaf insects (Phasmatodea) - There are about 2,500 species of stick and leaf insects alive today. Members of this group are best known for the fact that they mimic the appearance of sticks, leaves, or twigs. Some species of stick and leaf insects are capable of changing color in response to alterations in light, humidity, or temperature.
- Termites (Isoptera) - There are about 2,300 species of termites alive today. Members of this group include termites, subterranean termites, rottenwood termites, drywood termites, and dampwood termites. Termites are social insects that live in large communal nests.
- Thrips (Thysanoptera) - There are more than 4,500 species of thrips alive today. Members of this group include predatory thrips, common thrips, and tube-tailed thrips. Thrips are much maligned as pests and are known to destroy a variety of grain, vegetable, and fruit crops.
- True Bugs (Hemiptera) - There are about 50,000 species of bugs alive today. Members of this group include plant bugs, seed bugs, and stink bugs. True bugs have distinct front wings that, when not in use, lie flat over the insect's back.
- Twisted-wing parasites (Strepsiptera) - There are about 532 species of twisted-wing parasites alive today. Members of this group are internal parasites during the larval and pupal stages of their development. They parasitize a variety of insects including grasshoppers, leafhoppers, bees, wasps, and many others. After pupating, adult male twisted-wing parasites leave their host. Adult females remain within the host and only partially emerge to mate and then return to the host while young develop inside the female's abdomen, emerging within the host later.
- Web-spinners (Embioptera) - There are about 200 species of web-spinners alive today. Members of this group are unique among insects in that they have silk glands in their front legs. Web-spinners also have enlarged hind legs that enable them to scurry backwards through the tunnels of their underground nests.
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