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.
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. 122) is thickest? Slenderest?
Shortest? One joint (which?) of the foot
(Fig. 122) 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.
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Fig. 126.—Foot of Fly, with climbing pads.
