98 M.H. Rathke on the Respiratory Process in Insects. 
sequence of the structure and distribution of the respiratory ap- 
paratus. 
1. In the abdomen itself. When its cavity is diminished, the 
trachee, and the air-sacs when they occur, must be somewhat 
compressed and a portion of the air contained in them driven 
out through the stigmata. But when the cavity of the abdomen 
enlarges again, and the pressure exerted upon the air-passages 
contained in it is removed, these must furnish an entrance for the 
atmospheric air, partly because the pressure of the external air 
overcomes that upon the outer surface of the air-vessels, and 
partly because the trachez at least may again dilate by their own 
elasticity. 
2. As regards the head and thorax, which, if we except the 
thorax of the Staphylini and Carabidae, cannot expand and contract, 
A portion of the air contained in the abdomen will he driven into 
them through the air-vessels when the abdomen contracts. A 
portion of this air then flows out through the stigmata of the 
thorax (as is seen in the Locuste), but another portion inflates 
the air-passages of the thorax and probably those of the head to 
a greater or less extent. When the pressure on the abdomen 
ceases, the trachez and air-sacs of the thorax and head contraet 
by their proper elasticity, and again expel a portion of their con- 
tained air, probably in part through the thoracie stigmata, at 
least in those insects whose stigmata are constantly open, and in 
part back into the abdomen. 
The same applies probably to the movement of the air to and 
from the extremities. 
§ 19. In all the inseets examined, except the Hymenoptera, 
only the contractions, and not the dilatations, of the abdominal 
cavity are effected by the action of muscles. The means by 
which the cavity is enlarged and the inspiration effected, appear 
to be various in different insects and even in the same insect, 
1. In those insects which only possess shrub-like trachee, the 
latter appear to be a principal means of the dilatation of the 
abdominal cavity during inspiration. The trachez, like the 
arteries of the Vertebrata, possess a high degree of elasticity, 
and, even after the insect’s death, regain their volume, after 
being compressed, as soon as the pressure is removed. As they 
are undoubtedly compressed, directly or indirectly, by the walls 
of the abdomen during exspiration, it is to be expected that, 
when this pressure ceases, they will again expand, react upon 
the walls of the abdomen, and push these somewhat apart. This 
applies also to most of those insects which possess vesicular as 
well as ramified tracheze, and especially to those in which the 
former are not too wide in proportion to their length. It is less 
applicable to those whose vesicular trachez are very wide in pro- 
