|Scientific classification |
- Symphypleona - globular springtails
Subclass Archaeognatha (jumping bristletails)
- Monura - extinct
- Thysanura (common bristletails)
Insects are invertebrate animals of the class Insecta, the largest and (on land) most widely distributed grouping within the Phylum Arthropoda. The study of insects is called entomology.
Insects comprise the most diverse group of animals on earth, with over 800,000 species described—more than all other animal groups combined. Insects may be found in nearly all environments on the planet, although only a small number of species have adapted to any kind of life in the oceans. There are approximately 5,000 dragonfly species, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 350,000 beetle, and 110,000 bee and ant species.
A few smaller groups with similar body plans, such as springtails (Collembola), are united with the insects as the Subphylum Hexapoda. The true insects (Class Insecta) are distinguished from other arthropods in part by having ectognathous, or exposed, mouthparts and by having eleven abdominal segments. Most species, but by no means all, have wings as adults. Terrestrial arthropods such as centipedes, millipedes, scorpions and spiders are often confused with insects due to the fact that both have similar body structures.
Insects are generally small in size and possess segmented bodies supported by an exoskeleton made mostly of chitin. The body is divided into a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes as well as the mouth; the thorax has six legs (one pair per segment) and wings (if present in the species). The abdomen has excretory and reproductive structures.
Insects have a complete digestive system. That is, their digestive system consists of a tube that runs from mouth to anus, contrasting the incomplete digestive system found in simpler invertebrates. The excretory system consists of Malpighian tubules for the removal of nitrogenous wastes and the hindgut for osmoregulation. At the end of their hindguts, insects are able to reabsorb water along with K+ and Na+. Therefore, insects don't usually excrete water along with their feces, which allows them to store water in their bodies. This process of reabsorption enable them to withstand hot and deserted environments.
As noted above, most insects have two pairs of wings located on the second and third thoracic segments. Insects are the only invertebrate group to have developed flight, and this has played an important part in their success of reproduction. The winged insects, and their secondarily wingless relatives, make up the Pterygota. Insect flight is not very well understood, relying heavily on turbulent atmospheric effects. In primitive insects it tends to rely heavily on direct flight muscles, which act upon the wing structure.
More advanced flyers, which make up the Neoptera, generally have wings that can be folded over their back, keeping them out of the way when not in use. In these, the wings are powered mainly by indirect flight muscles that move them by stressing the thorax wall. These muscles have adapted to contract when stretched without nervous impulses, allowing the wings to beat much faster than would be otherwise possible.
Insects do not breathe using lungs as terrestrial vertebrates do; instead they use tracheal respiration in order to transport oxygen through their bodies. Insects have openings on the surface of their bodies called spiracles that lead to their tracheal systems. The air goes into the tracheal tubes and passes through the system of branching trachea. The circulatory system of insects, like that of other arthropods, is open: the heart pumps the hemolymph through arteries to open spaces surrounding the organs; when the heart relaxes, the hemolymph seeps back into the heart.
Insects hatch from eggs, and undergo a series of moults as they develop and grow in size. In most types of insects, the young, called nymphs, are basically similar in form to the adults (grasshopper), though the wings are not yet developed. This is called incomplete metamorphosis. Complete metamorphosis distinguishes the Endopterygota, which include many of the most successful insect groups. In these, the egg hatches to produce a larva, which is generally worm-like in form and may be fairly helpless. This in turn becomes a pupa, which is often sealed within a cocoon or chrysalis, and undergoes considerable change in form before emerging as an adult.
Social insects, such as the ant or the bee, are the most familiar species of eusocial animal. They live together in large well-organized colonies that are so tightly integrated and genetically similar the colonies are sometimes considered superorganisms.
Many insects possess very refined organs of perception; in some cases, their senses can be more capable than humans. For example, bees can see in the ultraviolet spectrum, and male moths have a specialized sense of smell that enables them to detect the pheromones of female moths over many kilometers.
Insect roles in the environment and in human society
Many insects are considered pests, because they transmit diseases (mosquitos, flies), damage structures (termites) or destroy agricultural goods (locusts, weevils). Many entomologists are involved in various forms of pest control, often using insecticides, but more and more relying on methods of biocontrol.
Some larvae of some insects are leaf miners.
Although pest insects get the most attention, more insects are beneficial to the environment and to humans. Some pollinate flowering plants (for example wasps, bees, butterflies, ants). Insects have co-evolved in step with them. Pollination is a trade between plants which need to reproduce, and pollinators which receive rewards of nectar and pollen. A serious environmental problem today is the decline of populations of pollinator insects, and a number of species of insects are now cultured primarily for pollination management in order to have sufficient pollinators in the field, orchard or greenhouse at bloom time.
Insects also produce useful substances such as honey, wax, lacquer or silk. Honeybees, (pictured above) have been cultured by humans for thousands of years for honey, although contracting for crop pollination is becoming more significant for beekeepers. The silkworm has greatly affected human history as silk-driven trade established relationships between China and the rest of the world. Fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene, as they would only consume dead flesh. This treatment is finding modern usage in some hospitals.
In some parts of the world, insects are used for human food, while being a taboo in other places. There are proponents of developing this use to provide a major source of protein in human nutrition. Since it is impossible to entirely eliminate pest insects from the human food chain, insects already are present in many foods, especially grains. Most people do not realize that food laws in many countries do not prohibit insect parts in food, but rather limit the quantity. According to cultural materialist anthropologist Marvin Harris, the eating of insects is taboo in cultures that have protein sources that require less work like farm birds or cattle.
Many insects, especially beetles, are scavengers, feeding on dead animals and fallen trees, recycling the biological materials into forms found useful by other organisms. The ancient Egyptian religion adored beetles and represented them as scarabeums.
Although mostly unnoticed by most humans, arguably the most useful of all insects are 'insectivores', those that feed on other insects. Many insects, such as grasshoppers can potentially reproduce so fast that they could literally bury the earth in a single season. However there are hundreds of other insect species that feed on grasshopper eggs, and some that feed on grasshopper adults. This role in ecology is usually assumed to be primarily one of birds, but insects, though less glamorous, are much more significant. For any pest insect one can name, there is a species of wasp that is either a parasitoid or predator upon that pest, and plays a significant role in controlling it.
Human attempts to control pests by insecticides may backfire, because important, but unrecognized insects already helping to control the pest populations, are also killed by the insecticides, leading to later population explosions of the pests.
Insects first appear in the fossil record during the Carboniferous age, about 350 million years ago. Types included several orders now extinct, and some insects larger than any living today. The Permian, around 270 million years, saw the development of most extant orders. Many modern insect genera developed during the Cenozoic; from this period on we find insects preserved in amber, often in perfect condition and easily compared with modern species. The study of fossilized insects is called paleoentomology.
- "Something in the insect seems to be alien to the habits, morals, and psychology of this world, as if it had come from some other planet: more monstrous, more energetic, more insensate, more atrocious, more infernal than our own."
- —Maurice Maeterlinck (1862-1949)
- Charles A. Triplehorn and Norman F. Johnson, Borror and DeLong's Introduction to the Study of Insects, 7th edition (Thomas Brooks/Cole, 2005) - a classic textbook in North America
- Tree of Life Project (http://tolweb.org/tree?group=Insecta&contgroup=Hexapoda) – Insecta
- Insect pictures -- from Webster's 1911 (http://www.sru.edu/depts/cisba/compsci/dailey/public/insects.html)
- UF Book of Insect Records (http://ufbir.ifas.ufl.edu/), documenting "insect champions" in different categories
- Insect guide (http://www.insectguide.net) describing various insect categories