The factors responsible for the biological control of insect pests fall into three main groups. The first group includes the micro-organisms (Protozoa, Bacteria, and Fungi) producing what is ordinarily called disease; the second comprises the entomophagous insects which lead, during part or all of their life-history (generally during the larval stage only) a parasitic existence, while the third contains a great assemblage of animals of various kinds which agree in being carnivorous in habit and are known as predators. It is difficult to formulate definitions of parasite and predator which are applicable to all cases. The most satisfactory general distinction is perhaps the one recently suggested by E. Rabaud (1928), who restricts the term “parasite” to species which remain permanently attached to a living host, and considers as predators all species which come into contact with the host organism only at feeding time. Each of these categories can be subdivided, according to the effect the parasite or predator exerts upon its host, into sublethal or benignant and lethal or malignant species. Few, if any, of the parasitic entomophagous insects are of the sublethal type, the species whose habits correspond most closely to the definition being perhaps the bee-louse (Braula coeca, Nitzsch), which, however, is rather a commensal than a parasite. A few parasitic insects, such as some of the dipterous parasites of Orthoptera and Dermaptera (Thrixion halidayanum, Rond., on Leptynia hispanica, Bol.— v. Pant el 1897; Rhacodineura antiqua, Meig. on Forficula auricularia, L.—v. Thompson 1928) do not actually kill their host before issuing, but it dies usually within a relatively short time. In the majority of cases the host is destroyed before the parasite has completed its larval development. Benignant predacious entomophaga are also uncommon among the Arthropods. Certain Chironomids, Simuliids and even Culicids suck the blood of caterpillars, adult dragonflies, moths and butterflies, and other insects, without necessarily killing them (v. Edwards 1923) ; but, in general, the entomophagous insect and Arachnid predators, being as large as or larger than their hosts, cause death, as do the vertebrate predators.