302 ROBERT S. BREED AND ELSIE F. BALL. 



along the outer surface of the region zv and the surface x show 

 a better development in the section figured than in most of the 

 sections. 



There are at least two reasons for thinking that the condition 

 figured is one which represents the contracted or nearly contracted 

 state of this muscle. These reasons are : (i) the cross striations 

 of the muscle appear as they do in contracted fibers, (2) the posi- 

 tion of the parts affected. A dotted outline has been drawn 

 (not shown satisfactorily in the figure) which indicates the prob- 

 able position of these parts when the muscle relaxes. This re- 

 la.xation would cause the teeth at s and s' to grip each other 

 firmly if the elytron had previously been brought near enough 

 the body for these surfaces to touch each other. 



The most effective of the interlocking devices in this beetle are 

 the mid-dorsal metathoracic groove, the slipping of the corners 

 of the elytra under the scutellum, and the interlocking of these 

 teeth along the meta-episternum. 



{d) Comparison. 



A comparison of the devices which are used by these beetles 

 in holding their elytra in place reveals a close similarity in all 

 cases, except in the interlocking of the lateral edges with the 

 meta-episternum. Here the most striking dissimilarity exists. 

 In all of the beetles which we have examined, a muscle has been 

 found which originates along the dorsal edge of the episternum, 

 and is inserted on the suture which marks the ventral boundary 

 of this plate. This has been called the musculus episternalis. In 

 one case {Lacluiostenid) this muscle is apparently functionless since 

 the chitinous structures which it operates are so degenerate that 

 they no longer interlock. In Thymalns this muscle operates an 

 upward hooking ridge, in Tcnchrio a series of downward hook- 

 ing teeth. 



This dissimilarity of structure with its consequent differences 

 in the method of operation is made possible by varying flexi- 

 bilities of the chitinous cuticula. 



It would be interesting to know how many other variations of 

 these structures may be present among beetles. The three species 

 examined were chosen at random and it does not seem possible 



