INTERLOCKING MECHANISMS IN COLEOPTERA. 299 



The hooks are so pronounced that they make a very perfect 

 joining, which holds the edges of the elytra together along the 

 dorsal suture. This arrangement is made more effective by rows 

 of minute chitinous teeth [o, />) which in the cross section re- 

 semble saw teeth. These are so directed that they keep the 

 elytra from slipping laterally when they are being drawn together, 

 by interlocking at the points q and q' they assist in keeping the 

 wing covers perfectly co-adapted when at rest. 



3. The interlocking of the elytra of Thynialns along their 

 anterior edges is very much as it is in LacJinostcrna. The prin- 

 cipal difference is that the inner anterior corners of the elytra 

 slip under the scutellum more than in the June beetle and the 

 pronotum fits against the anterior edges more firmly, 



4. The method of fastening the elytra to the meta-episternum 

 is shown in Fig. 9. This figure is copied from Breed (:o3. Fig. 

 13, Plate 6). A description of the mechanism which it illustrates 

 is to be found on page 332 of the paper referred to. There is a 

 ridge (///.) on the face of the episternum, lying between the 

 attachments of the musculus episternalis [e'stn.). The contrac- 

 tion of this muscle causes the ridge (///.) to take the position 

 indicated by the dotted line, thus freeing the ridge (loph.) on the 

 elytron and allowing the latter to be raised. In like manner the 

 musculus episternalis contracts when the elytron returns to rest 

 and its immediate relaxation causes the latter to be securely 

 fastened in its place. At the point marked by a star (*) there is 

 found on the inner face of the elytron a row of minute teeth 

 directed upward. A similar series of teeth pointing downward 

 is found on the body wall opposite. These teeth interlock and 

 assist in keeping the elytron in place. A few small teeth are 

 also present on the inner face of the ridge (///.). 



The dorsal suture is thus kept intact by the combined working 

 of the dorsal metathoracic groove and this lateral hooking 

 arrangement. The combination of these two devices produces 

 the stress which holds the mid- dorsal edges of the elytra in the 

 positions shown in Figs. 10 and ii. 



