Manual of the apiary

by Albert John Cook

The organs of flight are the most noticeable appendages of the thorax. The wings are usually four, though the Diptera have but two, and some insects—as the worker ants—have none. The front or primary wings ([Fig, 3, A]) are usually larger than the secondary or hind wings ([Fig. 3, B]), and thus the mesathoracic or middle ring of the thorax, to which they are attached, is usually larger than the metathorax or third ring. The wings consist of a broad frame-work of veins ([Fig, 3]), covered by a thin, tough membrane. The main ribs or veins are variable in number, while towards the extremity of the wing are more or less cross-veins, dividing this portion of the wings into more or less cells. In the higher groups these cells are few, and quite important in classifying. Especially useful are the cells in the second row, from the frontal or costal edge of the front wings, called the sub-costal cells. Thus in the genus Apis there are three such cells ([Fig, 3, A, 1, 2, 3]), while in the Melipona there are only two. The ribs or veins consist of a tube within a tube. The inner one forming an air tube, the outer one carrying blood. On the costal edge of the secondary wings we often find hooks, to attach it to the front wings ([Fig. 3, B, a]).

Fig. 7.

The wings are moved by powerful muscles, compactly located in the thorax ([Fig, 7, a, a, a]), whose strength, as well as the rapidity of the vibrations of the wings when flight is rapid are really beyond computation. Think of a tiny fly outstripping the fleetest horse in the chase, and then marvel at this wondrous mechanism.

The legs ([Fig, 2, g, g, g]) are six in number in all mature insects, two on the lower side of each ring of the thorax. These are long or short, weak or strong, according to the habit of the insect. Each leg consists of the following joints or parts: The coxæ (Fig. 24), which move like a ball and socket joint in the close-fitting coxal cavities of the body-rings. Next to these follow in order the broad tracanter, the large, broad femur ([Fig. 2, g′, 1]), the long, slim tibia ([Fig. 2, g′, 2]), frequently bearing strong spines at or near its end, called tibial spurs, and followed by the from one to five-jointed tarsi ([Fig. 2, g′, 3, 3, 3, 3, 3]). All these parts move freely upon each other, and will vary in form to agree with their use. At the end of the last tarsal joint are two hooked claws ([Fig. 2, g′, 4]), between which are the pulvilli, which are not air-pumps as usually described, but rather glands, which secrete a sticky substance which enables insects to stick to a smooth wall, even though it be above them. The legs, in fact the whole crust, is more or less dense and hard, owing to the deposit within the structure of a hard substance known as chitine.

INTERNAL ANATOMY OF INSECTS.

The muscles of insects are usually whitish. Sometimes I have noticed quite a pinkish hue about the muscles of the thorax. They vary in form and position to accord with their use. The mechanism of contraction is the same as in higher animals. The ultimate fibers of the voluntary muscles, when highly magnified, show the striæ or cross-lines the same as do the voluntary muscles of vertebrates, and are very beautiful as microscopic objects. The separate muscles are not bound together by a membrane as in higher animals. In insects the muscles are widely distributed, though, as we should expect, they are concentrated in the thorax and head. In insects of swiftest flight, like the bee, the thorax ([Fig, 7, a, a, a]) is almost entirely composed of muscles; the œsophagus, which carries the food to the stomach, being very small. At the base of the jaws, too, the muscles are large and firm. The number of muscles is astounding. Lyonnet counted over 3,000 in a single caterpillar, nearly eight times as many as are found in the human body. The strength, too, of insects is prodigious. There must be quality in muscles, for muscles as large as those of the elephant, and as strong as those of the flea, would not need the fulcrum which the old philosopher demanded, in order to move the world. Fleas have been made to draw miniature cannon, chains, and even wagons many hundred times heavier than themselves.

The nerves of insects are in no wise peculiar so far as known, except in position. As in our bodies, some are knotted or have ganglia, and some are not.

The main nervous cord runs along the under or ventral side of the body ([Fig, 8]), separates near the head, and after passing around the œsophagus, enlarges to form the largest of the ganglia, which serves as a brain. The minute nerves extend everywhere, and in squeezing out the viscera of an insect are easily visible.

The organs of circulation in insects are quite insignificant. The heart is a long tube situated along the back, and receives the blood at valvular openings along its sides which only permit the fluid to pass in, when by contraction it is forced towards the head and emptied into the general cavity. Thus the heart only serves to keep the blood in motion. According to the best authorities, there are no special vessels to carry the blood to various organs. Nor are they necessary, as this nutritive fluid everywhere bathes the alimentary canal, and thus easily receives nutriment, or gives waste by osmosis, everywhere surrounds the tracheæ or air-tubes—the insect's lungs—and thus receives that most needful of all food, oxygen, and gives the baneful carbonic acid, everywhere touches the various organs, and gives and takes as the vital operations of the animal require.