Hemiptera

The Hemiptera /hɛˈmɪptərə/ or true bugs are an order of insects comprising some 50,000 to 80,000 species of groups such as the cicadas, aphids, planthoppers, leafhoppers, and shield bugs. They range in size from 1 mm (0.04 in) to around 15 cm (6 in), and share a common arrangement of sucking mouthparts. The name 'true bugs' is sometimes limited to the suborder Heteroptera. Many insects commonly known as 'bugs' belong to other orders; for example, the lovebug is a fly, while the May bug and ladybug are beetles. Most hemipterans feed on plants, using their sucking and piercing mouthparts to extract plant sap. Some are parasitic while others are predators that feed on other insects or small invertebrates. They live in a wide variety of habitats, generally terrestrial, though some species 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 important agricultural pests, damaging crops by the direct action of sucking sap, but also harming them indirectly by being the vectors of serious viral diseases. Other species have been used for biological control of insect pests. Hemipterans have been cultivated for the extraction of the dyestuff cochineal (also known as carmine) and for shellac. The bed bug is a persistent parasite of humans. Cicadas have been used as food, and have appeared in literature from the Iliad in Ancient Greece. Hemiptera is the largest order of hemimetabolous insects (not undergoing complete metamorphosis; though some examples such as male scale insects do undergo a form of complete metamorphosis ) containing over 75,000 named species; insect orders with more species all have a pupal stage (i.e. they do undergo complete metamorphosis or 'holometabolism'), Coleoptera (370,000 described species), Lepidoptera (160,000), Diptera (100,000) and Hymenoptera (100,000). The group is very diverse. 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, pond skaters, and giant water bugs. The fossil record of hemipterans goes back to the Carboniferous (Moscovian). The oldest fossils are of the Archescytinidae from the Lower Permian and are thought to be basal to the Auchenorrhyncha. Fulguromorpha and Cicadomorpha appear in the Upper Permian, as do Sternorrhyncha of the Psylloidea and Aleurodoidea. 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 (sometimes referred to as Rhynchota) 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. The order is now divided into four or more suborders, after the 'Homoptera' were established as paraphyletic (now the Auchenorrhyncha and the Sternorrhyncha). The cladogram is based on one analysis of the phylogeny of the Paraneoptera by Hu Li and colleagues in 2015, using mitochondrial genome sequences and homogeneous models. It places the Sternorrhyncha as sister clade to the Thysanoptera and the lice, making the Hemiptera as traditionally understood non-monophyletic. However, when heterogeneous models were used, Hemiptera was found to be monophyletic. The result where Hemiptera was found to be non-monophyletic is likely due to phylogenetic artifacts, such as elevated substitution rates in Sternorrhyncha compared with the other suborders of Hemiptera. English names are given in parentheses where possible. 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 blood-suckers, and a few are predators. Both herbivorous and predatory hemipterans inject enzymes to begin digestion extraorally (before the food is taken into the body). These enzymes include amylase to hydrolyse starch, polygalacturonase to weaken the tough cell walls of plants, and proteinases to break down proteins.