296 ROBERT S. BREED AND ELSIE F. BALL. 



mesothorax. There is no interlocking of the elytra with the 

 scutellum in this beetle. 



4. The elytra are not fastened along the lateral faces of the 

 metathorax, although a rudimentary hook (Fig. 7) is found along 

 the anterior lateral edge of each elytron. A corresponding ridge 

 hooking downward (Fig. 8, loph.) is found on the meta-episternum, 

 a short distance below the origin of the musculus episternalis 

 {e'stii.). 



The probable method by which this hook would be operated 

 is shown in Fig. 8. The dotted lines show the position of the 

 movable parts when the muscle is contracted. The ridge repre- 

 sented in cross section at / is drawn up to the position /', and the 

 flexible band k is straightened. The meta-sternum ;// is drawn 

 upward and outward to the position ;//', thus forcing the ventral 

 edge n of the episternum outward. A dissection of the muscular 

 system of several beetles has shown that the lower attachment of 

 the musculus episternalis is along a straight line, while the upper 

 attachment is arched somewhat like the gable of a house. This 

 form gives firmness and affords reason for believing that it is the 

 lower attachment of this muscle which moves when the muscle 

 contracts. Further proof of this is furnished by the triangular 

 form of this muscle in longitudinal section (Fig. 8, c'stn.). Thus 

 the dorsal end of this muscle serves as origin, while the ventral 

 end is insertion. The movement of the ventral edge of the 

 episternum outward would cause the slant of the ridge {loph.) to 

 change slightly. If there were a functional hook present on the 

 edge of the elytron, the change in position due to the contraction 

 of this muscle would be sufficient to release the elytron or to 

 allow it to return to its place. 



However, since the hook present on the elytron is rudimentary, 

 this muscle is apparently functionless though it must have been 

 functional in some ancestral form. The muscle has not degene- 

 rated as completely as have the chitinous structures in connection 

 with it. The degeneration of the lateral hooking mechanism 

 may be accounted for by the highly developed interlocking 

 mechanisms in the other parts of the body. The dorsal suture, 

 the meta-thoracic groove, and the fastening under the scutellum 

 furnish ample means for holding the elytra firmly in place. 



