OF RESPIRATION. 389 



from the air streaming in and out of the thorax during flight. We 

 find also the motion in the wings of insects even at rest during their 

 chirping and crying, for instance, of the great grasshopper, to harmonise 

 with this opinion, for without the air streaming out of the thorax upon 

 the fluttering wings, not a tone could be produced. Therefore, the 

 voice of all insects is no mechanical friction of portions of the skeleton, 

 but in them, as elsewhere, it stands in immediate connexion with the 

 respiratory apparatus and its outlets. 



229. 



The spiracles themselves participate somewhat in the pulsations of 

 the entire body, at least in the larger ones which lie exposed upon the 

 surface of the body on opening and shutting of them, synchronal with 

 the in- and ex-piration has been observed. We also know, from 

 the preceding description of all the forms of these spiracles, that only 

 those which lie exposed are supplied with a peculiar apparatus for 

 the opening and closing of their lips, whereas those which are 

 concealed beneath portions of the skeleton exhibit either none or 

 only a partially closing margin. Such spiracles consequently do not 

 appear to be able to be closed, but the air seems constantly to pass in 

 and out with each breath. Other writers, on the contrary, maintain a 

 complete closing of the spiracle in some insects by means of extraneous 

 substances which lay in front of it. Reaumur was the first to observe 

 this closing of the spiracles in a pupa by means of a viscous substance, 

 and Sprengel * confirmed it. If now such a substance shall have 

 been observed in insulated cases, which may not be doubted, from the 

 positive assertion of Sprengel, it can occur only as an exception, per- 

 haps, in consequence of the diseased state of the caterpillar ; or it was 

 perhaps a peculiar secretion which was separated around the spiracle, 

 and at a moment of danger, for instance, upon being touched, flowed 

 in front of the spiracle, to prevent the application of something preju- 

 dicial 5 subsequently, however, when the caterpillar no longer feared 

 the presence of its enemy, was again absorbed, or mechanically removed ; 

 perhaps also the substance may have got there by accident. In all 

 cases, however, free respiration would be impeded by it, and this 

 stoppage could not last long without becoming prejudicial to the insect. 

 It appears, therefore, probable to me, that all pupa in which such 



* Comment, de Partib. 4. 



