Hymenoptera

Hymenoptera, (order Hymenoptera), the third largest—and perhaps the most beneficial to humans—of all insect orders. More than 150,000 species have been described, including ants, bees, wasps (including parasitoids such as ichneumons and chalcids), sawflies, and lesser-known types. Except in the polar regions, they are abundant in most habitats, particularly in tropical and subtropical regions.

Collectively, the Hymenoptera are most important to humans as pollinators of wild and cultivated flowering plants, as parasites of destructive insects, and as makers of honey. The Hymenoptera are divided into two suborders: Symphyta (mainly sawflies and horntails) and Apocrita (wasps, ants, bees, and most parasitic forms).

The order includes the best known of the social insects—ants and some species of bees and wasps. Most species, however, are solitary in habit. Hymenopterans may be parasitic or nonparasitic, carnivorous, phytophagous, or omnivorous.

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General features

Hymenopterans are chiefly small to medium-sized insects, usually with four membranous wings and a narrow waist that sets off the abdomen from the thorax, or middle region of the body. The mouthparts may be either of the biting type or of the biting-sucking type. In the higher evolutionary forms—bees, for example—mouthparts are modified into a sucking apparatus, although many also retain biting mandibles. The ovipositor, or egg-laying organ in the female, is often very long and may be modified for piercing, sawing, or stinging. Metamorphosis is complete; i.e., the insect develops through four distinct stages: egg, larva, pupa, and adult. Sex is usually determined by whether or not an egg is fertilized, with fertilized eggs developing into females and unfertilized ones becoming males.

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Hymenoptera display an array of interesting behavioral characteristics, particularly in social species. The dancelike movements of honeybees communicate information from one individual to another about the location, distance, quantity, and quality of a particular food source. These movements were carefully studied and described by the German entomologist Karl von Frisch. Trophallaxis, or the mutual exchange of food between larvae and adults of bees, ants, and wasps, has been of special interest to hymenopterists. Hyperparasitism—the parasitic habit of one species upon another parasitic species—has also attracted attention. Polyembryony, the development of many individuals (as many as 1,000) from a single egg, is an unusual phenomenon occurring in some members of the families Chalcididae and Proctotrupidae. Parthenogenesis (production of young by females that are not fertilized by males) also occurs in some forms.

Certain ants are remarkable for their relationship with insects such as aphids and scales that provide honeydew or other sweet fluids. Ants that obtain sweet fluids from the caterpillars of certain species of blue butterflies (Lycaenidae) reciprocate by allowing the caterpillars to devour ant larvae. The honey ant (Myrmecocystus in the United States, Plagiolepis in Africa) has in the nest a division of worker ants known as repletes, which are fed sugary secretions. As a consequence of high food intake, the abdomens of repletes swell into globules up to 1 cm (about 0.4 inch) in diameter.

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Hymenoptera, even those equipped with a sting, are sought as food by other animals. Skunks, badgers, field mice, shrews, and other animals attack bee nests for the insects as well as for the honey. The larvae of the wax moth (Galleria mellonella) live in bee nests, where they eat beeswax, thus damaging the nest. The cuckoo bumblebees, which are close relatives of non-parasitic bumblebees, lays their eggs in bumblebee nests, where the larvae are cared for and nourished by bumblebee workers.

Size range and diversity of structure

Hymenopterans range in size from the smallest fairyflies (Mymaridae), which are about 0.21 mm (about 0.008 inch) in length, to the largest of the Pelecinidae, which may exceed 5 cm (about 2 inches) in length.

Principal differences in structure within the order include the presence or absence of wings that, when present, consist of two pairs; the presence or absence of a “waist”; modifications of the ovipositor; and adaptations of mouthparts for specific eating habits. Polymorphism, the occurrence of two or more forms of a species, is highly developed in some of the social Hymenoptera. A honeybee colony, for example, contains at least one queen, plus workers and drones, with each form being structurally and physiologically different from the others.

Economic importance

The European honeybee has been valued since ancient times for its honey and beeswax. In the Americas, stingless bees have been kept for their honey and wax since pre-colonial times. Beekeeping in modern times has become a lucrative and highly developed enterprise. Royal jelly, produced by honeybee workers, has enjoyed some popularity as a cosmetic, although its beneficial properties in this respect have not been satisfactorily demonstrated.

Certain parasitic forms are valuable control agents against insect pests. Notable among these are the parasitic wood wasps that attack wood-boring beetles; braconids that parasitize many Lepidoptera and wood-boring beetles; eulophids that parasitize scale insects; pteromalids that parasitize several crop pests; chalcids and trichogrammatids that parasitize a variety of orchard pests; and tiphiids that parasitize the Japanese beetle. Fig wasps (Agaonidae) are valuable as the only pollinators of the Smyrna fig, an important crop in the Western United States.

Relatively few Hymenoptera species are serious economic pests. Chief among these, however, are the wheatstem sawfly (Cephidae); some seed chalcids that are also pests of wheat; the larch sawfly (Pristiphora erichsonii), which destroyed much of the larch forests in Britain and North America late in the 19th century; and the European spruce sawfly (Gilpinia), which was once a serious pest in North America. In order to control the European spruce sawfly, parasitic wasps were introduced from Europe.

The fire ant (Solenopsis saevissima), accidentally introduced into the United States from South America, feeds on young plants and seeds and is known to attack young mammals. The destructive habit of legionary ants, or army ants (Dorylinae), is of particular importance in South and Central America. Eciton army ants (Eciton burchellii), for example, roam in hordes of as many as 1,500,000 insects, destroying almost all animal life they encounter. At the same time, however, they are critical to the survival of hundreds of animal species and support rainforest biodiversity. Leaf-cutting ants (Atta) are serious pests, especially in Brazil, where they may ravage extensive plantings of cultivated plants overnight. These insects are used locally as a source of food by people in South America.