332 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of their attachments being such that they lie parallel and close together. 
The conditions in Synchroa and Melolontha agree with those in 
Thymalus. 
Musculus episternalis. 
(Muscle expirateur dans le métathorax of Straus-Dirckheim ; 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- 
Diirckheim (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 trachea, 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 ( pli.) on the episternum 
by means of a ridge (Joph.) 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 surface of the elytron with 
those on the outer surface of the metathorax at the place indicated by a 
star (*) and by the teeth on the inner side of the fold (pilz.). This 
fold extends antero-posteriorly along the episternum as far as the muscle 
reaches. The 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. 
i 
; 
