Insect external structure
Insect internal structure
Development of insects
Relationships of insects
Among the larger groups of animals now recognized by science, the one known as the Chordata is naturally the most familiar, including the mammals, birds, reptiles, fishes, besides numerous forms less well known.
Another group, also familiar, and called the Mollusca, includes the snails, clams, etc., while a third, the Annulata, contains most of the more commonly seen worms. The starfish and sea urchins, often seen at the seashore, belong with other similar animals to a fourth group called the Echinodermata, and a multitude of tiny beings, almost all too small to be seen without the aid of a microscope, are included in the group Protozoa.
A sixth large group is composed mainly of soft, jelly-like animals, the more common larger members being called jellyfish, and to this the name Coelenterata is applied, and several other groups of less familiar forms are also known.
The largest group of all, however, is the Arthropoda, its members found in the seas, in fresh water, on land, or even flying freely; a group with remarkable differences of structure, and so abundant that all the other animals taken together are less than one-sixth as many as the arthropods. Well-known members of this group are the lobsters, crayfish and crabs; scorpions, spiders, mites, ticks and "daddy longlegs;" the centipedes and millipedes; and last, and most abundant of all, the insects.
No one feature will serve to separate the arthropods from all other animals, but the possession by an animal of several of those here described will enable the observer to determine in each case whether he is examining one of this group. In arthropods the body is composed of a series of more or less similar pieces or segments, placed one behind another, the line of attachment of these to each other being usually somewhat evident on parts of the body at least. This character is also shown, and indeed more clearly, in some members of the Annulata, such as the common earthworm. Another character of the arthropods is the presence of jointed legs (or appendages of some kind), as is indicated by the name of the group, and these are not possessed by annulates. The surface of the body is covered by a secretion which hardens on exposure to the air, forming an outside shell or external skeleton (exoskeleton), there being practically no internal supporting structures except as ingrowths from the outside. In the possession of this external skeleton these animals have a seeming resemblance to the shells (Mollusca), but the materials of which the skeleton is composed are quite different, being largely calcium carbonate in the Mollusca, and chitin which somewhat resembles horn in its nature, sometimes with calcareous salts deposited in it, in the Arthropoda. In its simplest members the arthropod body is also practically bilaterally symmetrical, though this condition is concealed somewhat by secondary changes in many of the group. The possession of a bilaterally symmetrical body consisting of a series of segments; an exoskeleton of chitin and the presence of jointed legs are, then, distinctive features of the arthropods.
To separate the various groups of arthropods, other characters must be used. Aside from several small sections not often seen, there are five large and important divisions which call for recognition. These are the Crustacea, including the lobster, crab, beach flea, sowbug and many others; the Diplopoda or millipedes; the Chilopoda or centipedes; the Hexapoda or insects; and the Arachnida, including the scorpions, pseudoscorpions, spiders, mites, ticks, etc.
The Crustacea (Fig. 1) are mainly water-inhabiting animals which breathe either by gills or, in the smaller forms, through the surface of the body. In those cases where its members live on land (Fig. 2) the gills are still present, though in a somewhat modified condition.They have numerous pairs of legs and generally two pairs of antennae (jointed "feelers"). Often some of the body segments are fused with the head to form a cephalothorax.
The Diplopoda (Fig. 3) are land animals breathing by air tubes opening on the sides of the body, these tubes carrying the air into all the internal parts of the animal. The head bears a pair of antennae and is followed by a series of segments all practically alike and each, except the first three, with two pairs of legs. The reproductive organs open far forward on the body. In most of the more common members of this group the body is quite cylindrical, and when disturbed the animal usually curls up in a sort of close spiral. Small diplopods about the diameter of the lead of a pencil and gray in color are often found boring into potatoes and roots in the ground in the fall and are sometimes wrongly called wireworms. The common name "millipede" refers to the large number of legs possessed by these animals.
The Chilopoda are also land animals (Fig. 4). Like the diplopods they have antennae and breathe by air tubes, and the body segments are practically all alike. The general form, however, is rather flattened; each segment bears only one pair of legs, and the reproductive organs open at the hinder end of the body. The front leg on each side is modified to serve as a poison claw. The numerous legs present in these animals have resulted in their receiving the common name "centipede."
The Arachnida (Figs. 5, 6, 7 and 8) generally have the segments of the body grouped into two sections called the cephalothorax and abdomen. No antennae are present and the eight legs are all attached to the firstnamed section. They breathe by air tubes somewhat similar to those of the other groups; by sacs containing many thin plates resembling leaves of a book, whence these structures take the name of book-lungs;
in the smallest forms, directly through the body surface. In the ites there is no evident division of the body into sections. Though lost of the group are land forms, a few are aquatic.
In the Hexapoda or insects (Fig. 9) the segments of the body are grouped in three distinct sections: the head, thorax and abdomen. A pair of antennae is (with rare exceptions) present on the head; the six legs are attached to the thorax, as are the four wings usually present; the animals breathe by air tubes; and, while living under a great diversity of conditions, the group as a whole is emphatically a terrestrial one, though in many cases their early life is spent in water.