The Hemiptera are the dominant group of exopterygote insects. They range in length from less than 1 to 90 mm, and comprise insects with a great range of different structural and behavioural features and occupying a wide variety of environments. The most characteristic feature is the structure of the mouth-parts, which are highly efficient for extracting the liquid contents of plants or of animal prey. The basic structure of the mouth-parts has remained unaltered throughout the Hemiptera, and is diagnostic of the order.
Most Hemiptera are terrestrial and phytophagous; only the Heteroptera include groups which are carnivorous, many of which are also aquatic.
This is an old order, since, by the lower Permian, the Archescytinidae (Fig. 6.25d, e) were represented. Heteropteran Progonocimicidae (Fig. 6.25k) were well established in the late Permian. Extensive diversification occurred in the Mesozoic in association with the emergence of the flowering plants.
Excellent accounts of the Hemiptera have been given by Pesson (1951) and Poisson (1951).