This section is from the book "The Standard Cyclopedia Of Horticulture Vol2", by L. H. Bailey. See also: Western Garden Book: More than 8,000 Plants - The Right Plants for Your Climate - Tips from Western Garden Experts.
The animals which constitute the insect world play an important part in most horticultural operations. The busy bee is an indispensable aid in the production of many fruits, but the equally busy jaws of canker-worms or other insects oftentimes seriously interfere with man's plans for profitable crops. Horticulturists should become more intimately acquainted with their little friends and foes in the insect world. Not only from the economic standpoint is this knowledge necessary in the business of growing plants, but the striking peculiarities of form, coloring, structure, habits, and the wonderful transformations of insects afford one of the most interesting fields in nature. The life-stories of many insects, if told in detail, would rival in variety and interest many a famous fairy tale. The science that treats of insects, or entomology, has now reached the stage at which its devotees are no longer looked upon with ridicule in most communities. At the present time more than 350 trained men are officially employed in entomological work in the United States and Canada.
An insect is an animal which, in the adult stage, has its body divided into three distinct regions: the head, the thorax and the abdomen (Fig. 1293). The head bears one pair of antenna, and there are always three pairs of legs and usually either one or two pairs of wings attached to the thorax. By these characteristics one can usually readily distinguish an adult insect from any other animal. Among the near relatives of insects in the animal world are the crayfish, sow-bugs, and crabs, but these are mostly aquatic animals, breathing by true gills; they have two pairs of antennae, and at least five pairs of legs. Centipedes, or "hundred-legged worms," and millipedes, or "thousand-legged worms," are also nearly related to insects, but they have the thorax and abdomen forming a continuous region, and with six to 200 segments, each bearing one or two pairs of legs; they have one pair of antennae. The layman usually classes such animals as the spiders, mites and daddy - long - legs among the insects, but they form a distinct class, as they have the head and thorax grown together, no antennas, and have four pairs of legs.
Insects are constructed on an entirely different plan from the higher animals. Their supporting skeleton is outside, it being simply the skin hardened more or less by a horny substance, known as chitin. This firm outer wall, or skeleton, supports and protects the muscles, blood-vessels, nerves, and other organs within. The mouth-parts, antennae and eyes of an insect are attached to its head, and all are exceedingly useful organs, as will be shown later in discussing the feeling and the other sensations of an insect. An insect's wings and legs are always borne by the thorax. The wings are primarily organs of flight, but are used as musical organs by some of the grasshoppers and crickets. Female canker-worm moths, bed-bugs, and some other insects have practically no wings, and the house-flies, mosquitos, male bark lice, and similar insects have but one pair of wings. Insects use their legs primarily for locomotion; some have their front legs modified for catching other insects for food; others have hind legs fitted for jumping, while the honey-bee has little "pockets" on its hind legs for carrying pollen to feed its young.
The arrangement of the internal organs in insects is interesting and somewhat peculiar. The alimentary or food canal in larvae is a nearly straight tube, occupying the central portion of the body; in adult insects it is usually much longer than the body and is more or less folded; from the mouth the food passes through a pharynx, an esophagus, sometimes a crop and a gizzard, a stomach, and a small and large intestine. The nervous system of an insect is similar to that in the higher animals, but it extends along the venter instead of the back. There is a little brain in the upper part of the head, and two nerve cords extend from this around the food-canal to another ganglion or nerve center in the lower part of the head; two nerve cords then extend longitudinally along the venter and connect a series of nerve centers or ganglia, typically one for each segment of the body. From each of these ganglia or little brains nerves arise, which supply the adjacent organs and ramify throughout the body. In insects, all parts of the body cavity that are not occupied by the internal organs are filled with a rich, colorless or slightly greenish blood.
There is no system of tubes like our arteries and veins, in which the blood is con-
Fig. 1293. A beetle, showing the different parts.
Fig. 1294. Head of grasshopper.
Showing the great eye. A detail of a part of the surface of the compound eye is also shown.
fined and through which it flows. There is a so-called "heart" above the food-canal, along the middle line of the back; it is a tube consisting of several chambers communicating with each other and with the body cavity by valvular openings. The blood is forced through this heart into the head, where it escapes into the body cavity. It then flows to all parts of the body, even out into the appendages, in regular streams which have definite directions, but which are not confined in tubes. They, like the ocean currents, are definite streams with liquid shores. Insects do not breathe through the mouth, as many suppose, but through a series of holes along the sides of the body. These openings, or spiracles, lead into a system of air-tubes, called tracheae. These tracheae branch and finally ramify all through the insect. Insects have no lungs, but the tracheae sometimes connect with air-sacs or bladders in the body, which help to buoy up the insect when flying. Thus the relation between the circulation of the blood and respiration is not nearly so intimate in insects as in man. In insects the air is carried to all the tissues of the body in the tracheae and the blood simply bathes these tissues.
Just how the blood is purified and how the waste matter is disposed of in insects are not yet clearly understood. Aquatic insects breathe by either carrying down bubbles of air from the surface entangled under their wings, or they may be provided with organs known as tracheal gills; these are usually plate-like expansions of the body that are abundantly supplied with tracheae, in which the air is brought practically in contact with the air in water, and may thus be purified. More than 4,000 different muscles have been found in a single caterpillar. Notwithstanding their delicate appearance, these muscles are really very strong and their rapidity of action is wonderful; in certain gnats the muscles move or vibrate the wings 15,000 times a second.
Fig. 1295. Fossil dragon-fly, Petalia longialata. ( X 1/5