SOME POIXTS IN IXSECT MECHANICS. Ill 



10. Some Points in Insect Mechanics. 

 By H. R. A. Mallock, F.R.S., F.Z.S. 



[Received March 18, 1919 : Read March 18, 1919.] 



(Text-figures 1-8.) 



In books on Natural History and on the anatomy of animals 

 the descriptions are chiefly concerned with generic or specific 

 characters, and little attention is paid to the mechanical features 

 of the various parts of the organisms. The mechaiiics of joints 

 and muscles, however, are not without interest, although the 

 similarities are so close over a wide range of orders that they do 

 not — with few exceptions — form a suflicient basis for purposes of 

 classification. 



Taking the whole range of animal life there is a broad dis- 

 tinction, in the mechanical sense, between those types which have 

 internal skeletons (i. e., vertebrates) and those where the skeleton 

 is external (i. e., insects and arthropods). In both the chief use 

 of the skeleton is to form a more or less rigid base for the attach- 

 ment of the muscles (especially the muscles of locomotion), but 

 the form of the joints on which the muscles operate is very 

 dijfferent in the two cases. 



Joints may be conveniently classified by their degrees of 

 freedom. The most general freedom which any two connected 

 parts of a structure can have with reference to one another is 

 that each can be both relatively displaced, and also turn about 

 three axes at right angles to one another. With joints only the 

 rotational freedom need be considered, and therefore the greatest 

 number of degrees of fi'eedom for a joint is three. Such joints 

 are met with in the case of A'ertebrates, as, for example, at the 

 shoulder, which allows the arm to be raised or lowered in a for- 

 ward or sideways direction, and also to turn about its own axis 

 relatively to the shoulder. The elbow-joint has two degrees of 

 freedom, namely, so as to alter the angle between the fore-arm 

 and the humerus, and to turn the wrist about the mean axis 

 of the radius and ulna. The last two joints of the fingers are 

 •examples of joints with one degree of freedom only. 



The jointed parts in vertebrates are kept in position by what 

 may be compared to an elastic stocking composed of ligaments, 

 and the working surfaces of the bones are kept from actual 

 contact by a thin cushion of cartilage. The elasticity of the 

 connection makes the "degree of constraint" (i.e., the limits 

 which the motion, other than that appropriate to the degree of 

 freedom, cannot exceed) rather lax, and for this reason the verte- 

 brate-joint can bear accidental strains without the injury which a 

 more rigid constraint would induce. 



The animals whose skeleton is external are not so favourably 

 circumstanced in this respect. None of their joints have more 

 than two degrees of freedom, and in general only one. The 



