Groups of Invertebrata Represented in the Soil.
The various groups of invertebrates represented in the soil may be briefly referred to in zoological order.
Nematoda.—The Nematoda or thread-worms are chiefly animal parasites, nevertheless they usually lead an independent existence in the soil in certain stages of their development. The numerous small species belonging to the family Anguillididæ, or eel-worms, form a definite constituent of the soil fauna; they are generally free-living and non-parasitic. Certain members of this family, however, are enemies of cultivated plants.
Annelida.—Terrestrial Annelida are almost entirely confined to the order Oligochæta, the majority of which are earthworms (Terricolæ), whose whole life-cycle is passed within the confines of the soil. The small white worms of the family Enchytræidæ belong to the aquatic section (Limicolæ) of the order, but they have various representatives which are abundant in damp soil containing organic matter.
Mollusca.—The terrestrial Mollusca are included in the sub-order Pulmonata of the Gastropoda. These organisms, which include the snails (Helicidæ) and slugs (Limacidæ), regularly deposit their eggs in moist earth. Slugs adopt the soil as a frequent habitat, only leaving it for feeding purposes in the presence of sufficient moisture. They are frequent consumers of vegetation, with the exception of Testacella, which is carnivorous.
Crustacea.—The few species of Crustacea inhabiting the soil belong to the order Isopoda, family Oniscidæ, which are popularly referred to as “woodlice,” “slaters,” etc.
Myriapoda.—The Diplopoda or millipedes include enemies of various crops and are common denizens of the soil. The Chilopoda or centipedes are usually less abundant and are carnivorous. The minute Symphyla are often evident but are of minor importance.
Insecta.—Insects form the dominant element in the invertebrate fauna. Phytophagous species devour the subterranean parts of plants, and notable examples are afforded by the larvæ of Melolontha, Agriotes and Tipula. Saprophagous forms are abundantly represented by the Collembola, and by numerous larval Diptera and Coleoptera. Predaceous species preying upon other members of the soil fauna are exemplified by the Carabidæ and many larval Diptera. Parasitic species pass their larval stages on or within the bodies of other organisms. The groups of Hymenoptera, and the dipterous family Tachinidæ, which exhibit this habit, constitute, along with predaceous forms, one of the most important natural agencies controlling the multiplication of insect life. There are also insects (ants, and other of the aculeate Hymenoptera) which utilize the soil as a suitable medium wherein to construct their habitations or brood chambers, without necessarily deriving their food from the soil. Lastly, there are many insects, notably Lepidoptera, which only resort to the soil for the purpose of undergoing pupation. The insect fauna is, therefore, a closely inter-connected biological complex; for a discussion and an enumeration of its representatives reference may be made to papers by Cameron (1913, 1917), and Morris (1921, 1922 a).
Arachnida.—The two principal classes represented in the soil are the Areinida, or spiders, and the Acarina, or mites, and ticks. The Areinida, which are well-known to be carnivorous, are an unimportant constituent of the fauna. Acarina, on the other hand, are abundant, and exhibit a wide range of feeding habits; most of the soil forms are probably carnivorous, and either free-living or parasitic.