Wasp is the common name applied to most species of hymenopteran insects, except bees and ants. Insects known as wasps include the sawflies, the parasitic wasps, and the stinging wasps, which are the best known. About 75,000 species of wasps have been identified, most of them parasitic.

Wasps are characterized by two pairs of membranous wings and an ovipositor (tube for laying eggs) that may be modified in various ways. In some species one sex may be wingless. Most stinging wasps are predators or scavengers; their ovipositors may be modified to inject venom used for killing prey or for defense.

No place to call home: Unlike social wasps, sawflies and parasitic wasps are free-living; that is, they do not build nests. After depositing their eggs on a host plant or animal, the adult wasps fly off in search of food for themselves or more hosts for their larvae. The eggs are left to develop and hatch on their own. Stinging wasps tend to live in hives and societies similar to bees and ants.

Various homes: The stinging wasps rely on a nest from which they conduct many of their activities, especially rearing young. Wasp nests may be as simple as a straight burrow in the ground, while others, such as those of mud daubers and potter wasps, are above ground, constructed of mud cavities attached to twigs, rocks, or human structures. The simplest mud nests contain only one or a few larval cells and are not used by the adults. Other mud nests contain many cells arranged side by side. Among the most intricate nests are those made of paper fibers collected from dry wood and bark and mixed with the wasps' saliva. The vespoid wasps (yellow jackets, hornets, and paper wasps) build nests of this type. In each paper-fiber nest there are one or more combs, or densely packed arrays of cells for larva. The adults may congregate on the combs, and some nests have an outer cover, forming a protective refuge for the whole colony. This is the familiar "hornet's nest" that may house hundreds or thousands of individuals.

An important link in the chain: Wasps are highly important to ecosystems. Sawflies consume vegetation and so limit plant growth. Most other wasps are either parasitic or predaceous and therefore play a vital role in limiting the populations of thousands of other insect species. All wasps are eaten by other species, thereby providing many links in the food web. Many parasitic wasps have been cultured and used in the biological control of agricultural pests. Although a few of the stinging wasps are considered nuisances, they also provide benefits. Yellow jackets and paper wasps, for example, prey on caterpillars and other larvae that can destroy crops. Some wasps feed on flower nectar and play a role in pollination.

Dangerous females: All female stinging wasps can defend themselves and their nests by using their ovipositor to inject venom. Males do not have a stinger. No species will attack a human except in defense. If the colonies of some yellow-jacket and hornet species are disturbed, they may respond by releasing more than 100 defending wasps, each capable of delivering several stings. The nests of these species should be left alone or removed professionally if they are considered a nuisance. Wasp venom contains factors that release histamine, which dissolves red blood cells. Most people can survive many stings, responding with only temporary pain and swelling, but to hyperallergic individuals--about 1 percent of the population--a single wasp sting can be fatal.

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