INTERLOCKING MECHANISMS 1\ COLEOPTERA. 



301 



The triangular area embraced between the straight dotted lines 

 shows the form of the episternal muscle projected on the plane 

 of the section. The origin of the musculus episternalis is along 

 the entire dorsal boundary of the meta-episternum. Most of its 

 fibers take their origin from the thickened ridge v. The insertion 

 is by means of a tendon which attaches to the anterior portion of 

 the ventral edge of the episternum. The entire muscle is thus 



Fig. 12. Lateral portion of a cross section of the metathorax ot Tenebrio, show- 

 ing the parts affected by the contraction of musculus episternalis. X 75- 



somewhat fan-shaped in side view. It is only in sections anterior 

 to the one shown that the fibers are cut so as to show their full 

 length in any one section. 



It is difficult to determine the exact method by which the 

 muscle operates. The structure of the suture to which this mus- 

 cle attaches is remarkably complex and entirely unlike that found 

 in the two beetles just described. The structure shown in Fig. 1 2 

 is typical for all of the sections of this suture The teeth shown 



