Insect external structure
Insect internal structure
Development of insects
Relationships of insects<
Insect identification > Relationships of insects
Relationships of insects
Classification may be defined as the orderly arrangement of different objects into groups. Any articles can be classified in one way or another: chairs, for example, can be brought into groups according to the kinds of wood of which they are made; or whether they are upholstered or not; or according to their price; and any of these might be equally useful. With living things, however, the problem becomes one of a "natural" as opposed to an "artificial" classification:
It is now the general belief that the first animals were extremely simple in structure, and that in the course of generations (and centuries) variation in their descendants led to the production of different forms, and finally to all the multitudes of kinds now in existence. This development has often been pictured as a tree, the trunk representing the original animals, which, varying as individuals of the same kind always do, began after a time to show several distinct lines along which the variation took place. This would be represented in the tree by the lowest branching of the trunk. Each main limb under the influence of the same conditions would fork in its turn, perhaps into two, perhaps more, and this process repeated again and again would finally produce the terminal twigs - the present animals. Thus each twig would represent all the individuals of the same kind; i.e., a single species; those nearest it the other species most closely related to it; and those on another part of the tree, though species and also related, would only be distantly so and, of course, quite different.
A natural classification of animals, therefore, is an attempt to express the actual relationships of the animals, placing nearest each other those most closely related. To do this, the total of their differences and resemblances must be taken into account. Classification based on a single character, then, is almost always unreliable. The division of insects into three main groups based on their metamorphosis is an example of this, for, while it is entirely correct as a statement of facts, a classification using this character would bring near together many insects which in reality are only distantly related.
The largest limb of the animal tree represents the original insects, not because they were so numerous at first, but because insects now form such a large part of animal life. This limb is usually called a class, while the still more comprehensive groups considered are called phyla. These are the main divisions of the tree. In this case the Hexapoda is the name given to the insect class.
From all the evidence available, the original insects were at least comparatively small, wingless and with practically no metamorphosis. After a time many of their descendants began to develop wings, and a fork of the class was produced, one branch (or subclass), the Apterygota, apparently retaining much of its former character, while the other subclass became the Pterygota or winged insects. These have increased greatly in abundance, and their variations have resulted in the production of many branches passing outward toward the twigs, named in sequence, orders, families, genera and species. Intermediate branchings between these often also need recognition and are called suborders, superfamilies, etc., as may be necessary. The twigs each represent a single species, but here we may recognize subspecies, varieties, races, etc., among which the individuals which together constitute the species are distributed.
In any consideration of the different groups of insects one must necessarily follow after another through the book, and when four groups, for example, are equally near relatives the first and fourth treated may thereby appear more distant than is really the case.
Between the fork of the insect limb which produced the Apterygota and the Pterygota, and the twigs representing the species, the actual divisions of the branches are more or less uncertain. The species in general group themselves quite easily into different genera and these into families; but while these last can in most cases be definitely placed in their orders, their correct relation to each other is often debatable. The relation of the orders to each other is far from settled; and while some are evidently more closely allied than others, within certain limits one order could follow another in almost any sequence without any serious loss to the expression of relationships. Where orders appear to be closely allied to each other, this will be indicated in connection with their consideration.
With the relations between the orders and also the families within the orders, still uncertain in many cases, a tree showing these must of necessity express only the views of the individual who drew it. Such a tree carried to the species would be entirely too large for these pages (there are about 80 families of beetles, and many of the other orders have large numbers also), but one carried to the orders is given here (Fig. 35) simply to illustrate the general idea of a tree-like classification. All insects in the same order have the same kind of metamorphosis.