related with those of the others. During the activities of these organisms various organic and mineral substances are decomposed or transformed into materials, such as humus and the inorganic compounds of nitrogen, phosphorus, potassium, etc., necessary or helpful for the growth of plants.

The protozoa (see Chapter I.) are the lowest and simplest forms of animal life, being mere specks of living matter. Three different groups of soil protozoa occur (Fig. 2). Some, like the amœba, progress by streaming movements, extruding temporary extensions of their substance in the form of finger or thread-like processes; the bodies of such protozoa may be naked, or enclosed in a shell-like covering secreted by the organism itself, or protected by an accumulation of particles of foreign matter. Some have a body of more definite shape and progress by means of the whip-like action of one or two thread-like processes, or flagella, arising from one end of the body. Such forms are the most numerous in the soil. Others, also of definite shape, control their movements by means of short, hair-like processes, or cilia, either distributed over the body or restricted to definite regions.

The protozoa are widely distributed, being most abundant in the richer types of soil, especially during the spring and autumn. A great amount of research has been undertaken at Rothamsted, England, and elsewhere, on the part played by protozoa in soil fertility; the evidence thus secured points to the probability that some of these organisms may be detrimental in that they devour certain kinds of bacteria responsible for the production of nitrates and other substances of nutritive value to plants. The extent of this may be realised from the fact that in a definite weight of soil (about 1-28th of an ounce) the micro-population was calculated to include not only about 1,550,000 protozoa, of which 430,000 were amœbæ (Fig. 2), but also some 6,000,000,000 bacteria. Observations showed that a single bacteria-destroying amœba required about 400 organisms for its nourishment, so that the amœbæ, to say nothing of the other protozoa, present in the weight of soil above-mentioned, would be capable of destroying about 172,000,000 of the bacterial population. Since the partial sterilisation of soil by steam results in an increase of fertility, it is thought, on account of the sterilisation destroying the protozoa, being more susceptible, and not the bacteria, that protozoa inhibit the activities of the bacteria to such an extent as to reduce the fertility of the soil; but this is a subject as yet open to argument. Apart from the bacteria-destroying protozoa, there are other forms that are thought to have something to do with the decomposition of organic substances.

The fungi, algæ and bacteria are amongst the lowest forms of plant life, and hold somewhat the same position in the plant kingdom as the protozoa do among animals; they are, especially the fungi and bacteria, of primary importance in the maintenance of soil fertility. The role of algæ lies mainly in increasing the organic content of the soil, and they are invaluable in developing favourable conditions for the establishment of vegetation on purely mineral soils. The fungi and bacteria are responsible for setting up the intricate reactions involved in the decomposition of organic matter, the bacteria being concerned in practically all of the chemical processes going on in the soil. Both fungi and bacteria are of two kinds: those that bring about decomposition, and those that live in a reciprocal relationship with plants upon the roots of the latter. Such relationship, which benefits both organisms and plants, is called symbiosis, the fungi being known as mycorrhiza, while the bacteria form nodules on the roots of such plants as the legumes.

CHAPTER III.

Structure of Insects.

Although insects present a great variety of forms, they nevertheless agree in general features; thus by studying the structure of some generalised species, which will give a broad idea of the main characteristics, one is enabled to recognise different structural modifications assumed by various species. For this purpose a weta, grasshopper, or cockroach may be taken as a type.

Just as in the case of the crayfish, so the body of an insect is completely covered and protected by a continuous “shell,” very solid in some insects, more or less pliable in others, but even in the most delicate forms tending to become rigid and brittle after death. This shell acts as a skeleton and as a very effective armour-plating, protecting and supporting the soft body within. Unlike the shell of the crayfish, which is mainly calcareous, that of insects consists of a horny substance called chitin, secreted by the underlying skin, and constitutes what is known as a cuticle. It is due to this horny cuticle or shell that the form and colour of most insects are preserved after death, though the enclosed body tissues decay unless preserved in some suitable medium.

The cuticle, though forming a complete covering, does not enclose the body in an inflexible shell; flexibility is allowed by the cuticle being formed of a segmented series of strongly-chitinised sections alternating with skin-like, feebly-chitinised, and very elastic sections; this arrangement gives freedom of movement to the enclosed body, as is readily seen in the movements of a caterpillar.

There are three distinctly separated divisions of the insect body—​head, thorax, and abdomen—​each consisting of a varying number of segments (Fig. 3). The head segments are so closely fused as to be practically untraceable, the cuticle forming a rigid capsule; the thorax, to which the head is attached, carries the wings (when present) and the legs, and consists of three segments; posterior to the thorax is the abdomen, comprised of several segments, which show the typical segmentation of insects better than any other part of the body.

The head capsule is more or less freely movable on the thorax, and bears certain sensory organs, together with the mouth appendages. The sensory organs are the eyes and the feelers, or antennæ. On each side is a compound eye of varying size, according to the insect; each eye consists of a variable number (from a comparative few to several thousand) of microscopic, hexagonal lenses, each of which records a separate image. Between the compound eyes, on top of the head, are three simple eyes in some insects, but in others one or all of these may be absent. Between the compound eyes on the front aspect of the head is a pair of feelers, or antennæ; they consist of a variable number of joints, are freely movable and highly sensory, thread-like or hair-like, short, or longer even than the whole body, and may be bare or clothed to a varying degree with hair or bristles. On the antennæ are the organs of touch, smell, and sometimes hearing.

FIG. 3.