The chest, or thorax, consists of three rings (Fig. [124]) called the front thorax (prothorax), middle thorax (mesothorax) and hind thorax (metathorax), or first, second, and third rings. The first ring bears the first pair of legs, the second ring bears the second pair of legs and the upper or front wings, and the third ring bears the third pair of legs and the under or hind wings.
Fig. 125.—Leg of Insect.
The six feet of insects are characteristic of them, since no other adult animals have that number, the spider having eight, the crayfish and crabs having ten, the centipedes still more, while birds and beasts have less than six. Hence the insects are sometimes called the Six-Footed class (Hexapoda). The insects are the only animals that have the body in three divisions. Man, beasts, and birds have only two divisions (head and trunk). Worms are not divided.
Define the class insecta by the two facts characteristic of them (i.e. possessed by them alone), viz.: Insects are animals with ———— and ————. Why would it be ambiguous to include “hard outer skeleton” in this definition? To include “bilateral symmetry”? “Segmented body”? The definition of a class must include all the individuals of the class, and exclude all the animals that do not belong to the class.
Fig. 126.—Foot of Fly, with climbing pads.
The leg of an insect (Fig. [125]) has five joints (two short joints, two long, and the foot). Named in order from above, they are (1) the hip (coxa), (2) thigh ring (trochanter), (3) thigh (femur), (4) the shin (tibia), (5) the foot, which has five parts. Which of the five joints of a wasp’s leg (Fig. [161]) is thickest? Slenderest? Shortest? One joint (which?) of the foot (Fig. [161]) is about as long as the other four joints of the foot combined. Is the relative length of the joints of the leg the same in grasshoppers, beetles, etc., as in the wasp (Figs.)? Figure 125 is a diagram of an insect’s leg cut lengthwise. The leg consists of thick-walled tubes (o, n) with their ends held together by thin, easy-wrinkling membranes which serve as joints. Thus motion is provided for at the expense of strength. When handling live insects they should never be held by the legs, as the legs come off very easily. Does the joint motion of insects most resemble the motion of hinge joints or ball-and-socket joints? Answer by tests of living insects. There are no muscles in the foot of an insect. The claw is moved by a muscle (m) in the thigh with which it is connected by the long tendon (z, s, t, v). In which part are the breathing muscles? As the wings are developed from folds of the dorsal skin, the wing has two layers, an upper and a lower layer. These inclose the so-called “nerves” or ribs of the wing, each of which consists of a blood tube inclosed in an air tube.
The abdomen in various species consists of from five to eleven overlapping rings with their fold-like joints between them. Does each ring overlap the ring in front or the one behind it?
