88 M.H.Rathke on the Respiratory Process in Inseets. 
cavity of the abdomen is contracted from below and also from 
the right and left. The subsequent dilatation of the abdomen is 
not effected by any peculiar muscles, but only by the elasticity 
of the skin. 
This rhythmical contraction and expansion takes place even in 
the second abdominal segment of the male Libellule ; and by 
this means the curious sexual apparatus occurring in this seg- 
ment is set in motion. 
The trachez have both stems and branches of considerable 
diameter, and are present in great numbers. Besides these air- 
vessels, several vesicles of considerable size, which collapse when 
pricked, belong to the respiratory system: the majority of these 
are placed in the abdomen. 
$11. In the Grylli and Acridia the skin of nearly all the 
segments of the abdomen is likewise hardened into two separate 
plates, one of which forms the lateral and dorsal walls; the 
other, which is much smaller, belongs to the ventral wall. The 
latter piece is wanting in the first segment, which forms only a 
half ring; on the last segment it is present, but formed differ- 
ently from the same part in the intermediate segments, and im- 
plicated in the sexual organs. On these intermediate segments 
there is on each side, between the upper and lower plates, a very 
considerable space in which the epidermis is soft and thin, and 
which possesses great extensibility, as may be seen in gravid 
female Gry. In the genus Gryllus the ends of the lower plates 
reach only to this interspace; but in the Acridia the ends of 
these plates form longer and shorter processes, according to the 
different sezments to which they belong, running upwards over 
the inner surface of the above-mentioned soft parts, and applying 
their free ends against the mner surface of the upper plates. 
The muscles attached to different parts of the abdominal skin, 
and serving for the most part to produce the respiratory move- 
ments, are very different both in number and attachment in 
Gryllus and Acridium. In Acridium, where they are most sim- 
ple, we have,—1, a pretty strong muscle on each side springing 
from the base of the outer surface of the process into which each 
inferior plate is produced, passing upwards and somewhat back- 
ward to attach itself to the upper plate of the same segment ; 
when these muscles contract, the abdomen is narrowed from 
below, the ventral wall being drawn a little upwards; 2, two 
other smaller muscles on each side, passing from the anterior 
margin of the lower plate of each segment thus furnished, 
forward to the next preceding lower plate; these muscles seem 
to shorten the ventral wall, and appear to have no essential 
part in the respiratory process; 38, a great quantity of muscular 
fibres forming a long and narrow band between every two 
