STRUCTURE OP INSECTS. 033 



hip bone of quadrupeds, is a broad, but very short truncated 

 cone. The mode of its articulation with the trunk admits 

 of great variety; sometimes it is united by a ball and socket 

 joint, as in the CurciiUo and Ccramhyx; and it then has, of 

 course, great freedom of motion: at other times the joint is 

 of the hinge kind, as in the Mdolontha. The trochanter 

 (t,) and the femur (f,) though in reality distinct pieces, are 

 usually so firmly united as to compose only one division of 

 the limb. The articulation of this portion with the haunch 

 is always effected by a hinge-joint. Joints of this descrip- 

 tion, when formed, as they are in insects, by the apposition 

 of two tubular pieces, are constructed in the following man- 

 ner. 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 li- 

 gament. These articular tubercles and dejnessions are so 

 adjusted to one another, that the joint cannot be dislocated 

 without 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 whole 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 ioint and of the next 

 are placed at right angles to each other; so that there results, 

 from the combination of both, a caj)ability in the thigh of 

 executing a circular motion in a manner almost as perfect 

 as if it had revolved in a spherical socket. The principle 

 of this compound motion is the same as that employed on 

 ship-board for the mariner's compass, and other instruments 

 which require to be kept steady during the motion of the 

 ship. For this purpose what arc called gimhah arc used, 

 the parts of which have two axes of rotation, at right angles 

 Vol. I. 30 



