INSECTS 75 



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 ani- 

 mals with and - . Why would it be ambig- 

 uous 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 (i) 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 FlG 

 joints of the. foot combined. Is the relative FLY, with climbing 

 length of the joints of the leg the same in P ads - 

 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 th'e 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, /, 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. 



