Hemiptera.-- Most Hemiptera are phytophagous, but a large number of species in the more important families show a strong tendency toward entomophagy. In several instances of independent origin this has reached the point where plant feeding is abandoned entirely and predation is obligatory (Clausen 1940/1962). This occurs among both aquatic and terrestrial species. Although these species show certain preferences in choice of food, records exist to indicate that the majority is more or less omnivorous.
The life histories and behavior of many British Hemiptera and descriptions of immature stages were recorded by Butler (1923). These included all the predaceous groups as well as the wholly phytophagous representatives of the order. Elson (1937) made a comparative study of the anatomy of the order in order to correlate structure with food habits. He found that the predatory forms possess both external and internal modifications for predation. The front legs are frequently adapted for capturing and holding prey. The rostrum is usually short and stout, either straight or curved, but never angular, and capable of movement in all directions. The alimentary tract is longer than in other forms, and the salivary glands produce an alkaline secretion which is injected into the host's body, acting as a poison or paralyzing agent. Feeding by predaceous nymphs and adults is usually with the rostrum held in a horizontal plane to the body, while in strictly phytophagous forms it is held at right angles to the plane.
In addition to the families that will be discussed separately, Clausen (1940) noted a few such as Phymatidae and Enicocephalidae that are relatively small groups inhabiting foliage and flowers and feeding on a variety of insects. Members of the superfamily Gerroidea are mainly aquatic and the nymphs and adults of Gerridae, Vellidae, Naucoridae, Nepidae, Belostomatidae, Notonectidae feed principally on insects and a variety of smaller animal life found in ponds and streams. Some of the larger forms even prey on tadpoles and small fish. The Ochteridae and Nerthridae are littoral in habit and feed on such insects and other small animals that frequent the muddy margins of ponds and streams.
The true bugs include over 70,000 identified species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, etc. Their size is from 1 mm. to 17 cm. They all have sucking mouthparts.
The mouthparts where the mandibles and maxillae have become modified into a proboscis, enveloped in a sheath within a labium to form a rostrum, which is used for piercing and sucking tissues of plants or animals.
The forewings of are either membranous, or partially hardened. The name "Hemiptera" stems from the Greek hemi; "half") and pteron; "wing"), referring to the forewings of many species that are hardened near their bases but are membranous at the tips (= hemelytra). The forewings may be held over the body (typical of or held flat on the back, with the ends overlapping. The hindwings if present are entirely membranous and mostly shorter than the forewings. The antennae usually have five segments, though they may still be long, and the tarsi of the legs are three-segmented or shorter.
Although there is a large variation in form, the mouthparts are quite characteristic.
Hemiptera are hemimetabolous, and ther immatures are referred to as nymphs. These resemble adults. Hemiptera is the largest hemimetabolous insect order.
Aphids are usually hermaphroditic during part of the life cycle, so that females are able produce unfertilized eggs, that develop into viable individuals.
Most species are phytophagous, feeding on plant juices. Others are predatory, feeding on other insects, or small vertebrates. A few species feed on the blood of larger animals. These include bedbugs and the kissing bugs (Reduviidae), which can transmit potentially deadly Trypanosoma infections.
Several families of Hemiptera have water bugs that are adapted to an aquatic environment, such as the water boatmen and water scorpions. Most of these are predatory, and have legs which are modified as paddles to assist in movement through water. The "pondskaters" or "water striders" (Gerridae) are also associated with water, but use the surface tension of water to allow them to "walk" on the surface. Included is the marine genus Halobates. Many species of are injurious pests of crops and vegetables, including aphids and scale insects,
Some predatory species are useful as biological pest control agents. Included are species of Nabidae, and some primarily phytophagous species, such as the genus Geocoris (Lygaeidae). Other species have been used in the production of the cochineal dyes, shellac and crimson.
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