There are approx 80,000 species in this order with approx 1,700 species in the UK
The characteristics of these insects is that they have a piercing beak (the rostrum) which is used to suck juices from plants or other animals. Most are considered pests causing plant damage or transmit diseases when moving between hosts. Most have two pairs of wings, the forewing being partially hardened although some do not have wings.
There are two distinct sub orders; Heteroptera and Homoptera. I shall mention in the family description text below as to which sub order the family belongs.
The largest family in the sub order Heteroptera with approx. 6,000 known species world-wide.
They mainly feed on developing fruit and seed. They also mainly over-winter as eggs.
Most adults and larvae are carnivorous. The larvae pupates openly on plants.
Shield bugs are in the superfamily Pentatomoidea) and are commonly also called Stink Bugs due to the glands in the thorax, between the first and second pair of legs, producing a foul smelling liquid for defence.
Pentatomidae is largest of the Shield Bug families. They can be recognised from other families by their five segmented antennae. These bugs hibernate over winter
Coreidae are a large family of predominantly herbivorous insects that belong in the hemipteran suborder Heteroptera. There are more than 1,800 species in over 250 genera. They vary in size from 7 to 45mm, making the larger species some of the biggest heteropterans. The body shape of coreids is quite variable, with some species broadly oval while others are slender. Coreids are found throughout the world but most species are found in the tropics and subtropics.
In North America they are colloquially called “squash bugs” because some species, such as Anasa tristis, are pests of squashes. They are also called “leaf-footed bugs” due to the leaf-like expansions some species have on their hindlegs.
In older literature, the family is sometimes called "Corizidae". They differ from the related coreids in lacking well-developed scent glands. They are usually light-colored and smaller than the coreids. Some are very similar to the orsilline lygaeids, but can be distinguished by the numerous veins in the membrane of the hemelytra. They live principally on weeds, but a few (including the box elder bug) are arboreal. All are plant feeders. Currently 18 genera and over 200 species of rhopalids are known..
Hydrometridae is a family of semi-aquatic insects, known as marsh treaders or water measurers. They have a characteristic elongated head and body which makes them resemble a yardstick for measuring the water surface.
Backswimmers are predators and attack prey as large as tadpoles and small fish, and can inflict a painful "bite" on a human being (actually, similar to a mosquito "bite", it is a stab with their tubular mouthpart). They inhabit still freshwater, e.g. lakes, pools, marshes, and are sometimes found in garden ponds. Although primarily aquatic, they can fly well and so can disperse easily to new habitats.
There are over 400 species. They are soft-bodied, elongate, winged terrestrial predators. Many damsel bugs catch and hold prey with their forelegs, similar to mantids.
Damsel bugs of the genus Nabis are the most common. They and other genera are most numerous in fields of legumes such as alfalfa, but they can be found in many other crops and in non-cultivated areas. They are yellow to tan in color and have large, bulbous eyes and stiltlike legs. They are generalist predators, catching almost any insect smaller than themselves, and cannibalizing each other when no other food is available.
Extract from Wikipedia.