INTRODUCTION. 



The parts of the knee joint are: — 



1. The hinge consisting of a region on both sides where hardened processes 

 from femur and tibia meet. The soft skin being very narrow at these points 

 and folds inwardly out of the way. In the more complex knee joints like 

 that of the grasshopper the hard parts also fold in making long hinges. 



2. The guide pieces of the femur, a flat lobe on either side at the tip below 

 the hinge and overlaying the base of the tibia when the knee is flexed. 



3. The collar at the base of the tibia beginning as a crease extending over 

 the back of the tibia from hinge to hinge and widening out in the middle like 

 the visor of a cap. 



4. The tendons, long slender infoldings of the soft skin both above and 

 below, at the end of the tibia serving respectively the extensor and flexor 

 muscles. These tendons usually reach almost the whole length of the femur. 



Figure 1. Foot of an ant. 



5. The knee caps usually present as two hardened plates above between the 

 ends of the femur and tibia. 



Almost as soon as the knee can be made out, the structures of the foot or 

 tarsus begin to show. In most legs there are flve tarsal joints, the fifth 

 bearing on the tip a pair of claws (ungues), between which there are a pair 

 of pulvilli, or more commonly a single empodium, or both. 



When the basal joint is enlarged it is commonly called the metatarsus. 



On the end of the tibia next to the foot there are usually one or more spurs. 



At the base of the leg there normally develops at least two articulations 

 allowing motion in two directions and separating off the coxa. Between 

 this and the femur there may be an additional segment or two, the trochanter. 



Before the legs there are four pairs of appendages exactly similar at first 

 to the rudimentary legs but each soon takes on a different shape, varying also 

 according to the group. These appendages arise from four primitive segments 

 of the head and are: — 



1. Antennae, usually long and many jointed. When the baiial joint is long 



