Injurious insects occur among the following orders: _Coleoptera_, _Hymenoptera_, _Lepidoptera_, _Diptera_, _Hemiptera_ (both _heteroptera_ and _homoptera_), _Orthoptera_, _Neuroptera_ and _Thysanoptera_. The order _Aptera_ also contains a few injurious species. [Illustration: FIG. 1.--A, Wireworm; B, pupa of Click Beetle; C, adult Click Beetle (_Agriotes lineatum_).] Among the _Coleoptera_ or beetles there is a group of world-wide pests, the _Elateridae_ or click beetles, the adults of the various "wireworms." The insects in the larval or wireworm stage attack the roots of plants, eating them away below the ground. The eggs deposited by the beetle in the ground develop into yellowish-brown wire-like grubs with six legs on the first three segments and a ventral prominence on the anal segment. The life of these subterranean pests differs in the various species; some undoubtedly (_Agriotes lineatum_) live for three or four years, during the greater part of which time they gnaw away at the roots of plants, carrying wholesale destruction before them. When mature they pass deep into the ground and pupate, appearing after a few months as the click beetles (fig. 1). Most crops are attacked by them, but they are particularly destructive to wheat and other cereals. With such subterranean pests little can be done beyond rolling the land to keep it firm, and thus preventing them from moving rapidly from plant to plant. A few crops, such as mustard, seem deleterious to them. By growing mustard and ploughing it in green the ground is made obnoxious to the wireworms, and may even be cleared of them. For root-feeders, bisulphide of carbon injected into the soil is of particular value. One ounce injected about 2 ft. from an apple tree on two sides has been found to destroy all the ground form of the woolly aphis. In garden cultivation it is most useful for wireworm, used at the rate of 1 ounce to every 4 sq. yds. It kills all root pests. In Great Britain the flea beetles (_Halticidae_) are one of the most serious enemies; one of these, the turnip flea (_Phyllotreta nemorum_), has in some years, notably 1881, caused more than £500,000 loss in England and Scotland alone by eating the young seedling turnips, cabbage and other _Cruciferae_. In some years three or four sowings have to be made before a "plant" is produced, enormous loss in labour and cost of seed alone being thus involved. These beetles, characterized by their skipping movements and enlarged hind femora, also attack the hop (_Haltica concinna_), the vine in America (_Graptodera chalybea_, Illig.), and numerous other species of plants, being specially harmful to seedlings and young growth. Soaking the seed in strong-smelling substances, such as paraffin and turpentine, has been found efficacious, and in some districts paraffin sprayed over the seedlings has been practised with decided success. This oil generally acts as an excellent preventive of this and other insect attacks.