M. H. Rathke on the Respiratory Process in Insects. 838 
rally progressive, advancing rapidly from before backwards, 
rarely commencing in the middle of the abdomen and spreading 
both forwards and backwards. 
The muscles by which these movements are producedare ex- 
ceedingly simple. Hach segment which takes part in it is fur- 
nished only with a single pair of muscles for this purpose, each 
of which is attached by one end to the lateral wall and by the 
other to the dorsal wall or plate, in both places close to the soft 
part of the cutis. All these muscles, therefore, run transversely 
across the softer and more flexible parts of the abdomen. When 
they contract, the dorsal wall of the segment to which they belong 
is drawn downwards more or less according to the extent of the 
soft interspace, by which the cavity of the abdomen is more or 
less diminished and the softer skin of the interspace more or less 
folded. The respiratory movements are stronger in some Beetles, 
weaker in others. In those which need strong respiratory 
movements the above-mentioned muscles are more coarsely 
fibrous, thicker, and generally larger in proportion to the size of 
the entire body than in those in which weaker movements are 
sufficient ; but in all cases they form only short bundles, which 
are either of nearly equal thickness and breadth throughout or 
become narrower and thinner from above downwards. Besides the 
muscles just described, several other muscular layers occur in the 
abdomen, both on the dorsal and ventral walls. Between each 
pair of the upper plates which are moveable upon one another 
there is, on each side, a broader or narrower muscular layer, 
passing from the anterior margin of the posterior plate to the 
inner surface of the anterior one, and adhering at a greater or 
less distance from the anterior margin of the latter. Similar 
muscular bands, situated and attached in the same way, occur 
also on all the inferior plates which are mutually moveable. 
These lower bands are usually far thicker and more coarsely 
fibrous than the upper ones. Usually these two kinds of muscles 
only serve to bend the abdomen downwards and straighten it 
again ; but, if they both act together, they may also shorten the 
abdomen a little, and if, as is usually the case, the upper wall is 
likewise drawn towards the lower one, diminish its cavity, and 
thus increase the quantity of air expelled. It must, however, be 
observed that such a universal contraction of the abdomen occurs 
but seldom, and not always even when the beetle is much alarmed. 
The movement of the dorsal wall is not comparatively equal 
in all Beetles. The movement is greatest in those whose trachez 
possess many vesicular dilatations, such as Cetonia and Scara- 
beus, and less in those whose trache run uninterruptedly with- 
out dilatations so as to produce a shrub-like appearance ; it is 
least in those in which the tracheze appear to be narrowest in 
G* 
