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Hemiptera is an order of insects most often known as the true bugs (cf. bug), comprising around 50,000–80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others. They range in size from 1 mm to around 15 cm, and share a common arrangement of sucking mouthparts. Sometimes, the name true bugs is applied more narrowly still to insects of the suborder Heteroptera only.

Characteristics
The defining feature of hemipterans is their possession of mouthparts where the mandibles and maxillae are sheathed within a modified labium to form a "beak" or "rostrum", called a proboscis, which is capable of piercing tissues (usually plant tissues) and sucking out the liquids – typically sap. The forewings of Hemiptera are either entirely membranous, as in the Sternorrhyncha and Auchenorrhyncha, or partially hardened, as in most Heteroptera. The name "Hemiptera" is from the Greek ἡμι- (hemi; "half") and πτερόν (pteron; "wing"), referring to the forewings of many heteropterans which are hardened near the base, but membranous at the ends. Wings modified in this manner are termed hemelytra (singular: hemelytron), by analogy with the completely hardened elytra of beetles, and occur only in the suborder Heteroptera. The forewings may be held "roofwise" over the body (typical of Sternorrhyncha and Auchenorrhyncha), or held flat on the back, with the ends overlapping (typical of Heteroptera). In all suborders, the hindwings – if present at all – are entirely membranous and usually shorter than the forewings.

The antennae in Hemiptera are typically five-segmented, although they can still be quite long, and the tarsi of the legs are three-segmented or shorter.

Although hemipterans vary widely in their overall form, their mouthparts (formed into a "rostrum") are quite distinctive; the only orders with mouthparts modified in a similar manner are the Thysanoptera and some Phthiraptera, and these are generally easy to recognize as non-hemipteran for other reasons. Aside from the mouthparts, various insects can be confused with hemipterans, including cockroaches and psocids, both of which have longer many-segmented antennae, and some beetles, but these have fully hardened forewings which do not overlap.

Classification
The present members of the order Hemiptera were historically placed into two orders, Homoptera and Heteroptera/Hemiptera, based on the differences in wing structure and the position of the rostrum. These two orders were then combined into the single order Hemiptera by many authorities, with Homoptera and Heteroptera classified as suborders. The order is presently more often divided into four or more suborders, after it was established that the families grouped together as "Homoptera" are not as closely related as had previously been thought (see paraphyly). Auchenorrhyncha contains the cicadas, leafhoppers, treehoppers, planthoppers, and froghoppers....

The closest relatives of hemipterans are the thrips and lice, which collectively form the "hemipteroid assemblage" within the Exopterygota subclass of the Class Insecta.

The fossil record of hemipterans goes back to the Early Permian. Homopterans appeared first, with Heteroptera first appearing in the Triassic.

Life cycle and ecology
Hemipterans are hemimetabolous, meaning that they do not undergo metamorphosis between a larval phase and an adult phase. Instead, their young are called nymphs, and resemble the adults to a large degree, the final transformation involving little more than the development of functional wings (if they are present at all) and functioning sexual organs, with no intervening pupal stage as in holometabolous insects. Hemiptera is the largest insect order that is hemimetabolous; the orders with more species all have a pupal stage (Lepidoptera, Coleoptera, Diptera and Hymenoptera).

Many aphids are parthenogenetic during part of the life cycle, such that females can produce unfertilized eggs, which are clones of themselves.

Most hemipterans are phytophagous, feeding on plant sap, such as aphids, scale insects and cicadas. Most of the remainder are predatory, feeding on other insects, or even small vertebrates. A few, however, are parasites, feeding on the blood of larger animals. These include bedbugs and the kissing bugs of the family Reduviidae, which can transmit potentially deadly Trypanosoma infections.

Several families of Hemiptera are water bugs, adapted to an aquatic lifestyle, such as the water boatmen and water scorpions. They are mostly predatory, and have legs adapted as paddles to help the animal move through the water. The "pondskaters" or "water striders" of the family Gerridae are also associated with water, but use the surface tension of standing water to keep them above the surface; they include the genus Halobates which is the only group of insects to be truly marine.

Economic significance
Many species of Hemiptera are significant pests of crops and gardens, including many species of aphid and various scale insects, including the cottony cushion scale, a pest whose infestation of American citrus crops sparked one of the earliest biological pest control programmes, when the Australian beetle Rodolia cardinalis was introduced as a natural enemy of the scale insect.

Conversely, some predatory hemipterans are themselves biological pest control agents, such as various nabids, and even some members of families that are primarily phytophagous, such as the genus Geocoris in the family Lygaeidae. Other hemipterans have positive uses for humans, such as in the production of the dyestuffs cochineal and crimson, or shellac.