M.H. Rathke on the Respiratory Process in Insects. 87 
thinner and softer. The respiratory movements are of the same 
kind as in Panorpa but stronger, and extend throughout the whole 
length of the abdomen; in some species they are more distinctly 
visible in the anterior, in others in the posterior part of the 
abdomen. 
§ 9. In Blatia the abdomen is broad and flat in proportion to 
its length. The upper halves of the segments are of the same 
size as the lower ones, and both extend to the rather acute late- 
ral margins of the abdomen. Between them there is on each 
side a moderately wide space occupied by a soft skin, in which 
_ a-series of constantly open stigmata is visible. 
During respiration there is an alternate mutual approximation 
and removal of the upper and lower plates; but the respiratory 
movements do not take place rapidly. 
§ 10. In the Dragon-flies it was observed that, when they are 
in perfect repose, that part of the abdomen on which the central 
nervous cord rests alternately rises and falls, although but 
slightly. This movement is most remarkable in the genera 
Aischna and Libellula, and weakest in Agrion; in the Aischne 
it is strongest in the posterior broader half of the abdomen. 
In 4éschna and Libellula the alternate contraction and expansion 
of the abdomen is repeated eighty or ninety times in a minute. 
With more powerful respiration the abdomen is also alternately 
contracted and expanded at the sides, and then the lateral margins 
of the abdomen bend a little downwards during the contraction. 
This is very strikingly the case in the posterior part of the ab- 
domen in the species of the genus A’schna. These respiratory 
movements are rendered possible partly by the peculiar struc- 
ture of the abdominal segments, and partly by their tissues and 
the presence of peculiar muscular bundles in their interior. 
Hach segment, except the first one in male individuals, consists 
essentially of two very elastic plates or shields of hardened 
epidermis, of which one forms the lateral and dorsal walls of 
the segment, and the other, which is much smaller, the ventral 
wall; these plates are separated at both sides by a space, occu- 
pied by a softer skin. At some distance from the posterior ex- 
tremity of the lower plate there issues on each side a slender 
muscular bundle, which becomes somewhat broader as it passes 
upwards and outwards to the other plate of the segment, to 
which it is attached at some distance from the lower margin. 
When these bundles contract a little, the lower plate is moved 
inwards, and the cavity of the abdomen is contracted only from 
below. If they contract more strongly, not only is the lower 
plate drawn further in, but the parts of the upper plate which 
represent the lateral walls are somewhat drawn together and 
their lower margins pushed over the lower plate, by which the 
