Certain arachnids, such as mites and spiders, and many insects have profound effects on terrestrial ecosystems. No species of plant or fungus is known without at least one arthropod that feeds on it, and there are usually many more. Insects are the predominant herbivores in forests, and in most grasslands and savannas they consume more than herds of ungulates. Some introduced species have produced devastating effects, such as the gypsy moths (Lymantria dispar, introduced from Europe) on oak forests in eastern North America, and the so-called Colorado potato beetle (Leptinotarsa, introduced from North America) on potatoes in Europe. Predatory and parasitic arachnids and insects also are among the most significant selective agents in the control of such outbreaks. Without pollinating insects, approximately 90 percent of the world's flowering plants could not reproduce. Bees are the most important group of pollinators, but many flies, wasps, Lepidoptera, beetles, and thrips are also significant.
Insects have had dramatic impact even on human demographics, as vectors of plagues and other devastating diseases. Most such diseases are tropical to warm temperate, among the most significant being those caused by biting flies such as mosquitoes: malaria (caused by Plasmodium sporozoans), yellow fever and encephalitis viruses, and elephantiasis and other kinds of filiariasis (caused by nematode worms). Other important fly-transmitted diseases include leishmaniasis or "kala azar"(from phlebotomine sandflies), onchocerciasis or
Assorted Terrestrial Arthropods
A. Order Amblypygi; B. Order Mecoptera; C. Order Diptera; D. Order Heteroptera; E. Order Lepidoptera; F. Order Orthoptera; G. Order Mantodea; H. Order Pseudoscorpiones; I. Order Coleoptera; J. Order Hymenoptera
"river blindness" (a nematode transmitted by blackflies), and sleeping sickness (a try-panosome, transmitted by tsetse). Tens, perhaps hundreds of millions of people have died from these diseases over the past millennium alone. In South America, as many as 50,000 people die each year from Chagas's disease, another type of trypanosome, but one transmitted by blood-feeding triatomine assassin bugs. In areas of heavy infestation, the louse Pediculus humanus is a major vector of epidemic typhus. Ticks are important vectors of Rocky Mountain spotted fever (caused by a Rickettsia), Lyme disease (a Borellia spiro-chaete), Tularemia (a bacterium, Francisella), and others. Epidemics have not been entirely tropical: one of the most devastating plagues in history was the bubonic plague, or "black death" of medieval Europe, caused by a bacterium (Yersinia pestis) transmitted by Xenop-sylla fleas from rats. Despite intensive efforts to control these diseases through vaccines or extermination of the vectors, none have been extinguished. Some diseases, including malaria, are becoming notoriously difficult to control, because of rapid evolution of the parasites and insecticide resistance in the insect vectors.
In their 400 million years, insects have proven to be more enduring than many other groups of organisms. Mass extinctions that have dramatically affected or even exterminated other groups, such as the K/T extinction that marks the end of dinosaurs and ammonites, had little effect on insects. Despite this resilience, many insects are becoming extinct as a result of human activities. Best known are various butterflies, since these are most easily monitored in the field. Some bird-wing butterflies of the Asian tropics (Ornithoptera) have become extinct or highly endangered as a result of zealous collecting. Populations of others, such as the Karner Blue (a lycaenid butterfly), became endangered because of habitat loss. The caterpillar feeds on a lupine in pine barrens of the northeast United States, which are being lost to housing developments. Like many vertebrates endemic to islands, species of large, flightless insects have fallen easy prey to introduced rats and mongoose, such as the phasmid Dry-ococelus australis on Lord Howe Island, some large-headed stenopelmatid crickets (wetas) on New Zealand, and the giant earwig of St. Helena Island, all probably extinct. Most endangered and extinct insects were either narrowly specialized or distributed, like those in fragile ecosystems on distant islands.
Borror, Donald J., Charles A. Triplehorn, and Norman F. Johnson. 1989. An Introduction to the Study of Insects, 6th ed. New York: Saunders; Carpenter, Frank Morton. 1992. Part R, Arthropoda 4. Vols. 3 and 4: Superclass Hexapoda. Boulder, CO: Geological Society of America; CSIRO (Commonwealth Scientific and Industrial Research Organisation, Division of Entomology). 1991. The Insects of Australia. Vols. 1 and 2. Ithaca, NY: Cornell University Press; Foelix, Rainer F. 1982. Biology of Spiders. Cambridge: Harvard University Press; Gullen, Penny J., and Peter S. Cranston. 2000. The Insects: An Outline of Entomology, 2d ed. Oxford: Blackwell Science; Walter, David E., and Heather C. Proctor. 1999. Mites: Ecology, Evolution, and Behavior. New York: CABI; Wilson, Edward O. 1971. The Insect Societies. Cambridge: Harvard University Press.
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