96 M.H.Rathke on the Respiratory Process in Insects. 
each of the stiginata placed above the intermediate legs separated, 
indicating that a portion of the air contained in the thorax was 
driven out through them. ‘The same phenomenon was observed 
in the stigmata above the posterior pair of legs in Gryllus ver- 
rucivorus. 
§ 18. Distinct movements of the walls of the body are not ~ 
seen in every perfect insect, even when observed for a long time ; 
and hence it might be supposed that the trachez and air-saes in 
many (or perhaps in all) insects may be capable of contracting 
and dilating themselves by the tone and elasticity of their tissue, 
Elasticity is certainly possessed in a high degree by all those 
trachez which are not too wide in proportion to their length, or 
too thin-walled in proportion to their width, as may easily be 
seen in any such vessel, even of a recently killed insect, by com- 
pressing it and then removing the pressure. All parts of the 
respiratory system also possess a greater or less degree of physi- 
cal contractility according to their different nature. It is 
greatest in the traches, as these, when extended in width, or 
still more in length, contract again to their previous dimensions ; 
it is least in the air-sacs, which, when dilated, contract but little 
and scarcely perceptibly on the removal of the cause of the ex- 
tension. A tone, or organic contractility, seems on the con- 
trary to be entirely wanting in all these parts. This view the 
author supports by the following observations. Of several living 
insects, especially Grylli, Acridia, Scarabeus stercorarius, and 
Carabus granulatus, he opened the abdomen, and observed par- 
ticular parts of their respiratory systems under the action not 
only of the air, but also of cold water, concentrated sunlight, 
and mechanical irritants, sometimes with a very strong lens and 
sometimes with the microscope, without ever remarking a con- 
traction which indicated any tone of these parts. Nor could he 
observe any such contractions in the trachez of Tenebrio molitor, 
by examining them through the transparent parts of the skin of 
that insect. Hence he considers that we are justified in stating 
that respiration (and especially exspiration) is never effected in 
any insect by the proper powers of the traches and air-sacs 
alone, but that they require a pressure exerted upon them by 
other surrounding structures. This pressure can be exerted only 
by the wall of the abdomen, or in some insects by that of the 
metathorax; for although some of the viscera by their own 
activity may be capable of exerting a pressure upon them, this 
must be much less than that of the wall of the body. If no 
respiratory movement of the wal! be observed in a perfect insect, 
the cause of this may be various. 
1. The insect may be too small, and the movement too slight, 
to allow its observation ; 
