332 bulletin: museum of comparative zoology. 



of their attachments being such that they lie parallel and close together. 

 The conditions iu Synchroa and Melolontha agree with those in 

 Thymalus. 



Musculus episternaliii. 

 {Muscle expirateur dans le metatliorax of Straus-Durckheira ; Expira- 



tionsmuskel of Luks.) 



This is a muscle of which no trace can be found in the larva or young 

 pupa. Therefore it is probably a muscle of new formation in the pupa. 

 In the imago (Plate 4, Figure 9, e'stn.') it is found just beneath the 

 episternum. Its origin is near the dorsal edge of the episternum, from 

 which it extends obliquely downward and mediad to attach to the ven- 

 tral edge of the episternum. It was described and figured by Straus- 

 Durckheim ('28), who ascribed to it the function of an expiratory muscle. 

 In his own words (p. 164), "It is only by conjecture that I regard this 

 muscle as acting in respiration, not being able to ascribe to it any other 

 function." Also (p. 165), "This muscle, being placed between two 

 pieces of the case which forms the thorax, does not appear to act either 

 in flight or in the movements of the legs, and, as it compresses the tho- 

 racic cavity, and so necessarily compresses the ti-achea, I believe it ought 

 to be regarded as an expiratory muscle." Luks adopts these views with- 

 out comment. 



That this is not the function in Thymalus, is shown by a cross section 

 of the thorax in the region of this muscle (Plate 6, Figure 13). Here 

 the elytron (ely.) is shown hooked into a fold {21U.) on the episternum 

 by means of a ridge (loph.) on the inflexed edge of the elytron. The 

 elytron after being hooked into the fold is held firmly in place by the 

 interlocking of the teeth along the inner surfoce of the elytron with 

 those on the outer sui'face of the metathorax at the place indicated by a 

 star (i^) and by the teeth on the inner side of the fold {]'U.). This 

 fold extends antero-posteriorly along the episternum as far as the muscle 

 reaches. Tlie contraction of the muscle releases the elytra by bringing 

 the cuticula into the position shown by the dotted lines. This muscle 

 is aided in its action by a pull on the bases of the elytra by their exten- 

 sor muscles. The contraction of this muscle would be necessary in re- 

 placing the elytra, as it would depress the fold for the reception of the 

 ridge. 



The episternal muscle is present in all of the beetles examined, as also 

 in Melolontha and Dytiscus. Yet the elytra of some of these species do 

 not lock into a fold when closed, so that in such cases the muscle is 

 probably functionless. 



