100 M.H. Rathke on the Respiratory Process in Insects. 
2. In those, on the contrary, which possess air-sacs as well as 
trachex, or in which some trachcz are very wide in proportion 
to their length, the walls of the abdomen cause the inspiration, 
either by their proper elasticity or, as in the Hymenoptera, by 
muscular power. 
§ 21. It is well known that insects when fully fed, and re- 
quiring a greater amount of oxygen for the purpose of digestion, 
respire far more frequently than when their alimentary canal is 
empty ; the respiratory process also appears, at least in the 
greater number of insects, to take place more completely and 
powerfully. The movements of the abdominal plates and the 
extension of the soft skin uniting them are then far greater than 
when the alimentary canal is empty. The muscles moving these 
parts will be more stretched than usual, and therefore will contract 
to a far greater extent; so that the difference between the size of 
the abdominal cavity in inspiration and exspiration is increased, 
the air-passages are more strongly compressed, the air contained 
in them more completely evacuated at each exspiration, and a 
greater quantity of fresh air is taken in during inspiration, than 
under other circumstances. This at least must be the case with 
those insects which possess only ramose trachee. It probably 
applies also to these insects at the time when their generative 
organs, especially in the female, are much dilated and the abdo- 
men inflated thereby ; but it is not known whether at this time, 
if the alimentary canal be empty, the respiration goes on more 
rapidly than before, when the sexual organs had attained no 
great size. ‘The same theory cannot apply to those insects which 
possess vesicular trachez or large air-sacs, as these vesicles are 
not so elastic as to be capable of dilating again by their own 
powers on the cessation of pressure. Perhaps these insects, 
under the above circumstances, breathe more rapidly than the 
others. The author considers that many of his observations are 
in favour of this view. 
§ 22. Is that movement of the abdomen of insects which is 
connected with respiration a voluntary or an involuntary act? 
This question is to be answered in the same way as the similar 
one relating to the respiration of man. 
After the decapitation of insects (Grylli, Scarabei, Tabani, and 
Wasps) the abdomen has been seen to continue for a time con- 
tracting and dilating in the same way as before the injury, from 
which the author concludes that in these the respiratory move- 
ments may go on quite involuntarily. But these movements 
not unfrequently cease for a longer or shorter time in uninjured 
insects, or they are limited to one or two segments; and the con- 
tractions take place in these with unusual exertion, although the 
remainder of the segments which otherwise act in respiration 
