_Development._--In most Hemiptera the young insect (fig. 3) resembles its parents except for the absence of wings, and is active through all stages of its growth. In all Hemiptera the wing-rudiments develop externally on the nymphal cuticle, but in some families--the cicads for example--the young insect (fig. 10) is a larva differing markedly in form from its parent, and adapted for a different mode of life, while the nymph before the final moult is sluggish and inactive. In the male _Coccidae_ (Scale-insects) the nymph (fig. 4) remains passive and takes no food. The order of the Hemiptera affords, therefore, some interesting transition stages towards the complete metamorphosis of the higher insects.
[Illustration: After Riley and Howard, _Insect Life_, vol. i. (U.S. Dept. Agr.).
FIG. 4.--Passive Nymph or "Pupa" of male scale-insect (_Icerya_).]
_Distribution and Habits._--Hemiptera are widely distributed, and are plentiful in most quarters of the globe, though they probably have not penetrated as far into remote and inhospitable regions as have the Coleoptera, Diptera and Aptera. They feed entirely by suction, and the majority of the species pierce plant tissues and suck sap. The leaves of plants are for the most part the objects of attack, but many aphids and scale-insects pierce stems, and some go underground and feed on roots. The enormous rate at which aphids multiply under favourable conditions makes them of the greatest economic importance, since the growth of immense numbers of the same kind of plant in close proximity--as in ordinary farm-crops--is especially advantageous to the insects that feed on them. Several families of bugs are predaceous in habit, attacking other insects--often members of their own order--and sucking their juices. Others are scavengers feeding on decaying organic matter; the pond skaters, for example, live mostly on the juices of dead floating insects. And some, like the bed-bugs, are parasites of vertebrate animals, on whose bodies they live temporarily or permanently, and whose blood they suck.
The Hemiptera are especially interesting as an order from the variety of aquatic insects included therein. Some of these--the _Hydrometridae_ or pond-skaters, for example--move over the surface-film, on which they are supported by their elongated, slender legs, the body of the insect being raised clear of the water. They are covered with short hairs which form a velvet-like pile, so dense that water cannot penetrate. Consequently when the insect dives, an air-bubble forms around it, a supply of oxygen is thus secured for breathing and the water is kept away from the spiracles. In many of these insects, while most individuals of the species are wingless, winged specimens are now and then met with. The occasional development of wings is probably of service to the species in enabling the insects to reach new fresh-water breeding-grounds. This family of Hemiptera (the _Hydrometridae_) and the _Saldidae_ contain several insects that are marine, haunting the tidal margin. One genus of _Hydrometridae_ (_Halobates_) is even oceanic in its habit, the species being met with skimming over the surface of the sea hundreds of miles from land. Probably they dive when the surface becomes ruffled. In these marine genera the abdomen often undergoes excessive reduction (fig. 5).