STRUCTURE OF INSECTS. 329 



with the haunch is always effected by a hinge- 

 joint. Joints of this description, when formed, 

 as they are in insects, by the apposition of two 

 tubular pieces, are constructed in the following 

 manner. One of the tubes has, at the end to be 

 articulated, two tubercles, which project from 

 the margin, and are applied to the adjacent end 

 of the other tube at two opposite points of its 

 circumference ; the line which passes through 

 those two points being the axis of motion. On 

 the side where the flexion is intended to be 

 made both tubes are deeply notched, in order to 

 admit of their being bent upon one another at a 

 very acute angle : and the space left by these 

 notches is filled up by a pliant membrane, which 

 performs the office of a ligament. These articular 

 tubercles and depressions are so adjusted to one 

 another, that the joint cannot be dislocated with- 

 out the fracture of some of its parts. As the 

 different axes of motion in the successive joints 

 are not coincident, but inclined at different 

 angles to one another, the extent of motion in 

 the Avhole limb is very greatly increased. 

 Thus in the cases where the articulation of the 

 haunch with the trunk is a hinge joint, the 

 axes of this joint and of the next are placed 

 at right angles to each other ; so that there 

 results, from the combination of both, a capa- 

 bility in the thigh of executing a circular mo- 

 tion in a manner almost as perfect as if it liad 



