Although the insects form but a single class of the animal kingdom, they are yet so numerous in orders, families, genera, and species, their habits and transformations are so full of instruction to the biologist, and they affect human interests in such a variety of ways, that they have always attracted more attention from students than any other class of animals, the number of entomologists greatly surpassing that of ornithologists, ichthyologists, or the special students of any other class, while the literature has assumed immense proportions.
Insects form about four-fifths of the animal kingdom. There are about 250,000 species already named and contained in our museums, while the number of living and fossil species in all is estimated to amount to between one and two millions.
In their structure insects are perhaps more complicated than any other animals. This is partly due to the serial arrangement of the segments and the consequent segmental repetition of organs, especially of the external appendages, and of the muscles, the tracheæ, and the nerves. The brain is nearly or quite as complicated as that of the higher vertebrates, while the sense-organs, especially those of touch, sight, and smell are, as a rule, far more numerous and only less complex than those of vertebrates. Moreover, in their psychical development, certain insects are equal, or even superior, to any other animals, except birds and mammals.
The animal kingdom is primarily divided into two grand divisions, the one-celled (Protozoa) and many-celled animals (Metazoa). In the latter group the cells and tissues forming the body are arranged in three fundamental cell-layers; viz. the ectoderm or outer layer, the mesoderm, and endoderm. The series of branches, or phyla, comprised under the term Metazoa are the Porifera, Cœlenterata, Vermes, Echinodermata, Mollusca, Arthropoda, and Vertebrata. Their approximate relationships may be provisionally expressed by the following
Tabular View of the Eight Branches or Phyla of the Animal Kingdom.
RELATIONS OF INSECTS TO OTHER ARTHROPODA
The insects by general consent stand at the head of the Arthropoda. Their bodies are quite as much complicated or specialized, and indeed, when we consider the winged forms, more so, than any other class of the branch, and besides this they have wings, fitting them for an aërial life. It is with little doubt that to their power of flight, and thus of escaping the attacks of their creeping arthropod enemies, insects owe, so to speak, their success in life; i.e. their numerical superiority in individuals, species, and genera. It is also apparently their power of moving or swimming swiftly from one place to another which has led to the numerical superiority in species of fishes to other Vertebrata. Among terrestrial vertebrates, the birds, by virtue of their ability to fly, greatly surpass in number of species the reptiles and mammals.
The Arthropoda are in general characterized by having the body composed of segments (somites or arthromeres) bearing jointed appendages. They differ from the worms in having segmented appendages, i.e. antennæ, jaws, and legs, instead of the soft unjointed outgrowths of the annelid worms. Moreover, their bodies are composed of a more or less definite number of segments or rings, grouped either into a head-thorax (cephalothorax) and hind-body, as in Crustacea, or into a head differentiated from the rest of the body (trunk), the latter not being divided into a distinct thorax and abdomen, as in Myriopoda; or into three usually quite distinct regions—the head, thorax, and hind-body or abdomen, as in insects. In certain aberrant, modified forms, as the Tardigrada, or the Pantopoda, and the mites, the body is not differentiated into such definite regions.
In their internal organs arthropods agree in their general relations with the higher worms, hence most zoölogists agree that they have directly originated from the annelid worms.