SOME POINTS IJ^ INSECT MECHANICS. 



113 



matically the typical aithropod leg. The leg is supposed to be 

 extended into a straight line, though in most cases the natural 

 limitations of the motion of the joints would prevent such an 

 extension being actually accomplished. In insects with folding 

 wings two degrees of freedom are obtained in the same way by 

 two closely approached joints at the base of the wing. 



The locomotive muscles of insects offer many points of 

 mechanical interest. As far as the legs are concei-ned, the same 

 forms of muscles and muscular attachments are found not only 

 in insects but in all arthropods with well-developed joints, and 

 this can be readily examined in the legs of the large crustaceans ; 

 but in insects which have a head, thorax, and abdomen, the 

 chief locomotive muscles are all contained in the thorax, which 

 may be compared with the engine-room of a ship, the head corre- 

 sponding to the conning tower, and the abdomen to boiler-room, 

 stokehold, and repair outfit. 



Text-liffure 2. 



Diagram of typical Arthropod leg : 

 (rt) coxa, (6) trochanter, (c) femur, (d) tibia, (e) tarsi. 



Ic was said at the beginning of this note that the mechanical 

 details of animals with external skeletons rai-ely ofiered characters 

 adapted for the purj^oses of classification. 



Amongst insects, however, the mechanism of the Aving-muscles 

 does definitely separate one small order, viz., the dragon-fiies, 

 from all the others. Dragon-flies or their allies are, I believe, 

 geologically the oldest form of insect known, and the action of 

 their wing-muscles is the simplest. It is represented diagram- 

 matically in text-fig. 3. 



Each of the group of muscles which act on the wing is attached 

 at its lower end to processes of the thorax, and is capped by a 

 chitinous cone, rather like an extingiiisher, terminating in a fine 

 ligament. These ligaments lead respectively to points on either 

 side of the wing-joint; thus the contraction of one set of muscles 

 (represented by A in the figure) raises the wing, while the other 

 group^ B, depresses it. 



In all other Orders of flying insects the arrangements are much 

 Proc. Zool. Soc— 1919, No. VIII. 8 



