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_). [Illustration: Fig. 7. _Thorax of Bee magnified three times._ _a, a, a_--Muscles. _b, b_--Crust.] 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.