M. H. Rathke on the Respiratory Process in Insects. 99 
portion to their length, as is the case in the Acridia: these 
tracheze, when compressed and emptied of air, do not completely 
recover their original diameter by the agency of their proper 
elasticity. The air-sacs, which exist in greater or less number 
in many insects in connexion with the trachez, usually consist 
of an extremely delicate membrane, in which no trace of a spiral 
fibre-can be detected: when strongly dilated with air, they may 
certainly contract a little by their proper contractility, if the 
pressure from within ceases; but when compressed from without, 
they cannot dilate themselves again by virtue of any inherent 
elasticity. Consequently in those insects which have air-sacs or 
very wide vesicular trachez in the abdomen, these structures 
cannot contribute to the dilatation of the ventral cavity. 
2. In those insects in which during exspiration the extre- 
mities of the upper halves of several abdominal segments are 
bent somewhat inwards (as in the Grylli, Acridia, Libellula, 
Tabani, some Beetles [Carabus granulatus] and many Lepido- 
ptera), these extremities, when the muscles cease to act upon 
them, move apart again by their own elasticity, and thus assist 
in enlarging the abdominal cavity. 
8. In those insects in which the softer skin between the upper 
and lower halves of the abdominal segments possesses consider- 
able thickness and elasticity, and folds inwards during exspira- 
tion, it appears, during inspiration, to press outwards again by 
its own elasticity, recovering the plane in which it is stretched 
during repose, and pushing apart the upper and lower plates of 
the segments. In those, on the contrary, in which this skin is 
comparatively thin and does not fold during exspiration (as in 
Tabanus, Musca, Tipula, and the Beetles), it probably does not 
act in the above way in the dilatation of the abdominal cayity, 
For the contractility of this portion of the skin is very con- 
siderable, and it may therefore be supposed that when it has been 
somewhat compressed during exspiration, it will again extend 
itself on the cessation of the pressure. 
§ 20. From the statements contained in the preceding para- 
graphs we may understand how the process of inspiration goes 
on in perfect insects. 
1. In those which possess only shrub-like trachez, these ex- 
pand by their own elasticity as soon as the direct or indirect 
pressure of the abdominal walls producing exspiration ceases, and 
by their proper force cause the atmospheric air to place itself 
in equilibrium with the air contained in them and penetrate 
through the stigmata, If the abdominal walls also expand by 
their own elasticity after the completion of exspiration, this dila- 
tation can only be regarded as a removal of the obstacles to the 
independent dilatation of the tracheze, 
7* 
