Hemiptera

Hemiptera is an order of insects, commonly called true bugs, comprising more than 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, bed bugs, and shield bugs. They range in size from to around, and share a common arrangement of piercing-sucking mouthparts. The name "true bugs" is sometimes limited to the suborder Heteroptera.
Most hemipterans feed on plants, using their sucking and piercing mouthparts to extract plant sap. Some are bloodsucking, or hematophagous, while others are predators that feed on other insects or small invertebrates. They live in a wide variety of habitats, generally terrestrial, though some are adapted to life in or on the surface of fresh water. Hemipterans are hemimetabolous, with young nymphs that somewhat resemble adults. Many aphids are capable of parthenogenesis, producing young from unfertilised eggs; this helps them to reproduce extremely rapidly in favourable conditions.
Humans have interacted with the Hemiptera for millennia. Some species, including many aphids, are significant agricultural pests, damaging crops by sucking the sap. Others harm humans more directly as vectors of serious viral diseases. The bed bug is a persistent parasite of humans, and some kissing bugs can transmit Chagas disease. Some species have been used for biological control of insect pests or of invasive plants. A few hemipterans have been cultivated for the extraction of dyestuffs such as cochineal and carmine, and for shellac. Cicadas have been used as food, and have appeared in literature since the Iliad in Ancient Greece.

Etymology

The name Hemiptera comes from the Greek words hemi and ptera, meaning "half-wings," since the forewings of many species are hardened and leathery at the base but membranous and somewhat transparent at the tips. Entomologists reserve the term bug for Hemiptera or Heteroptera, which does not include other arthropods or insects of other orders such as ants, bees, beetles, or butterflies. In some varieties of English, all terrestrial arthropods are colloquially called bugs. Many insects with "bug" in their common name are in other orders; for example, the lovebug is a fly, while the maybug and ladybug are beetles.

Diversity

Hemiptera is the largest order of hemimetabolous insects, containing more than 95,000 named species. Other insect orders with more species are all holometabolous, meaning they have a pupal stage and undergo complete metamorphosis. The majority of species are terrestrial, including a number of important agricultural pests, but some are found in freshwater habitats. These include the water boatmen, backswimmers, pond skaters, and giant water bugs.

Taxonomy and phylogeny

Hemiptera belong to the insect superorder Paraneoptera, which includes lice, thrips, and the true bugs of Hemiptera. Within Paraneoptera, Hemiptera is most closely related to the sister clade Thysanoptera.
File:Fulgoroidea Crato Formation Brazil.jpg|thumb|upright|Fossil planthopper from the Early Cretaceous Crato Formation of Brazil, c. 116 mya
The fossil record of hemipterans goes back to the Carboniferous. The oldest fossils are of the Archescytinidae from the Lower Permian and are thought to be basal to the Auchenorrhyncha. Fulgoromorpha and Cicadomorpha appear in the Upper Permian, as do Sternorrhyncha of the Psylloidea and Aleyrodoidea. Aphids and Coccoids appear in the Triassic. The Coleorrhyncha extend back to the Lower Jurassic. The Heteroptera first appeared in the Triassic.
The present members of the order Hemiptera were historically placed into two orders, the so-called Homoptera and Heteroptera/Hemiptera, based on differences in wing structure and the position of the rostrum. "Homoptera" was established as paraphyletic group and an obsolete name.
The order is now divided into four suborders, Heteroptera, Sternorrhyncha, Auchenorrhyncha, and Coleorrhyncha. The earlier work was based on nuclear DNA, but later phylogenetic analysis using mitochondrial DNA suggests that Homoptera may be monophyletic after all, a sister group of Heteroptera. The cause of the disparity in the analyses is suggested to be the long branch attraction effect in phylogenetic analysis, due to rapidly evolving DNA regions.
The cladogram shows Hemiptera's placement within Paraneoptera, as well as how Hemiptera's four suborders are related. English names are given in parentheses where possible.

Suborder	No. of Species	First appearance	Examples	Characteristics
Auchenorrhyncha	more than 42,000	Lower Permian	cicadas, leafhoppers, treehoppers, planthoppers, froghoppers	plant-sucking bugs; many can jump; many make calls, some loud
Coleorrhyncha	fewer than 30	Lower Jurassic	moss bugs	small, rarely observed; found in/feed on moss; evolved before the splitting of Gondwana
Heteroptera	more than 45,000	Triassic	shield bugs, seed bugs, assassin bugs, flower bugs, leaf-footed bugs, water bugs, plant bugs	larger bugs; some are predatory, others are plant-sucking
Sternorrhyncha	12,500	Upper Permian	aphids, whiteflies, scale insects	plant-sucking bugs, some major horticultural pests; most are small and sedentary or fully sessile;

Biology

Mouthparts

The defining feature of hemipterans is their "beak" in which the modified mandibles and maxillae form a "stylet" which is sheathed within a modified labium. The stylet is capable of piercing tissues and sucking liquids, while the labium supports it. The stylet contains a channel for the outward movement of saliva and another for the inward movement of liquid food. A salivary pump drives saliva into the prey; a cibarial pump extracts liquid from the prey. Both pumps are powered by substantial dilator muscles in the head. The beak is usually folded under the body when not in use. The diet is typically plant sap, but some hemipterans such as assassin bugs are predators.
Both herbivorous and predatory hemipterans inject enzymes to begin digestion extra-orally. These enzymes include amylase to hydrolyse starch, polygalacturonase to weaken the tough cell walls of plants, and proteinases to break down proteins.
Although the Hemiptera vary widely in their overall form, their mouthparts form a distinctive "rostrum". Other insect orders with mouthparts modified into anything like the rostrum and stylets of the Hemiptera include some Phthiraptera, but for other reasons they generally are easy to recognize as non-hemipteran. Similarly, the mouthparts of Siphonaptera, some Diptera and Thysanoptera superficially resemble the rostrum of the Hemiptera, but on closer inspection the differences are considerable. Aside from the mouthparts, various other insects can be confused with Hemiptera, but they all have biting mandibles and maxillae instead of the rostrum. Examples include cockroaches and psocids, both of which have longer, many-segmented antennae, and some beetles, but these have fully hardened forewings which do not overlap.

Wing structure

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- and πτερόν, 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, by analogy with the completely hardened elytra of beetles, and occur only in the suborder Heteroptera. In all suborders, the hindwings – if present at all – are entirely membranous and usually shorter than the forewings. The forewings may be held "roofwise" over the body, or held flat on the back, with the ends overlapping. The antennae in Hemiptera typically consist of four or five segments, although they can still be quite long, and the tarsi of the legs have two or three segments.

Sound production

Many hemipterans can produce sound for communication. The "song" of male cicadas, the loudest of any insect, is produced by tymbal organs on the underside of the abdomen, and is used to attract mates. The tymbals are drumlike disks of cuticle, which are clicked in and out repeatedly, making a sound in the same way as popping the metal lid of a jam jar in and out.
Stridulatory sounds are produced among the aquatic Corixidae and Notonectidae using tibial combs rubbed across rostral ridges.

Life cycle

Hemipterans are hemimetabolous, meaning that they do not undergo metamorphosis, the complete change of form between a larval phase and an adult phase. Instead, their young are called nymphs, and resemble the adults to a greater or lesser degree. The nymphs moult several times as they grow, and each instar resembles the adult more than the previous one. Wing buds grow in later stage nymphs; the final transformation involves little more than the development of functional wings and functioning sexual organs, with no intervening pupal stage as in holometabolous insects.

Parthenogenesis and viviparity

Many aphids are parthenogenetic during part of the life cycle, such that females can produce unfertilized eggs, which are clones of their mother. All such young are females, so 100% of the population at these times can produce more offspring. Many species of aphid are also viviparous: the young are born live rather than laid as eggs. These adaptations enable aphids to reproduce extremely rapidly when conditions are suitable.

Locomotion

Hemipterans make use of a variety of modes of locomotion including swimming, skating on a water surface and jumping, as well as walking and flying like other insects.

Swimming and skating

Several families of Heteroptera are water bugs, adapted to an aquatic lifestyle, such as the water boatmen, water scorpions, and backswimmers. They are mostly predatory, and have legs adapted as paddles to help the animal move through the water. The pondskaters or water striders are also associated with water, but use the surface tension of standing water to keep them above the surface; they include the sea skaters in the genus Halobates, the only truly marine group of insects.
File:MicroveliaIndia.jpg|thumb|upright=0.9|Adult and nymph Microvelia water bugs using Marangoni propulsion